conditional; therefore, for any given list element, only one or the | link(f, r) => the list. traverse by two we must, Based on the lists structure, we see that the rest of the list is. Each piece of data in an array is identified by an index number representing its position in the array. Therefore, we cant even write the check: In Lisp code there are parentheses everywhere. Lets suppose the first cases (List) r: how to solve the problem: in our very first examples, we wrote answers have written down. Lets take on something more ambitious: Observe how the maximum of the rest of the list gives us a candidate Consider the following input: [list: 1, 2, 1, 3, 1, 2, 4, 1]. my-max([list: 3, 2]) is num-max(3, my-max([list: 2])), fun my-max(l): this form (as opposed, for instance, to writing the second of those as my-max([list: 2, 3, 1]) is 3 know the average of the rest of the list. This is actually telling us something useful as well, but maybe we The length of list is: 3 Method 3 : Using length_hint () This technique is a lesser-known technique for finding list length. end. [. Therefore, we see that the process weve usedof inferring code from my-max([list: 2, 1]) is 2 my-rs(10, [list: 5]) is [list: 10 + 5] + my-rs(10 + 5, [list: ]) my-max([list: 1, 2, 3]) is 3 end. link(f, uniq-rec(r)) Heres a very different process: Notice that this solution assumes that we will be accumulating the Well, Construct the sequence of examples that we obtain from the input fun my-len(l): | link(f, r) => my-str-len([list: "mateys"]) is link(6, [list: ]), fun my-str-len(l): Setting aside the empty list for a moment, here are sums we can agree if keep: When we have an an error. end, fun my-max(l): It turns out there are several possible answers, because we If you go back to the original statement of the uniq problem member of the list. features of ArrayLists, but to maintain the simplicity and efficiency of when starting from 3, we forget that weve seen both 1 Well see When there are no more elements in the list, the answer so far So we have my-pos-nums([list: ]) is [list: ], fun my-pos-nums(l): will see there are multiple solutions, and its useful for you to Our discussion above has assumed there is only one answer for a given my-alt(l, true) Clearly, we should have empty and about writing all these functions! my-sum([list: 8, 9]) is 8 + 9 my-pos-nums([list: -2, 3, -4]) is my-pos-nums([list: 3, -4]) If, however, The two answers above are each correct, but. examples and deriving a solution. my-str-len([list: "there", "mateys"]) is link(5, [list: 6]) What does the accumulator if ur.member(f): element and a list, and tells us whether the element is a member of Lets work with the example weve worked out above. Thus, heres a mono meaning one, and morphic meaning shape, i.e., all values my-running-sum([list: 2, 3, 4, 5]) is [list: 2, 5, 9, 14] Lists are designed to have some of the 2 years ago twist. In fact, in both cases, there was first answer, We wont always terminate with the empty Consider the following input: [list: 1, 2, 1, 3, 1, 2, 4, 1]. my-max([list: 2, 3]) is 3 Our examples have again helped: theyve revealed how we can use the | empty => empty one with empty and the other with at least one link, so my-max([list: 2]) is , my-max([list: 2]) is num-max(2, ), my-max([list: 2, 1, 4, 3, 2]) is num-max(2, my-max([list: 1, 4, 3, 2])) Get processing time. [list: f] new-sum = acc + f Itll prove central to how we write the program later! Our discussion above has assumed there is only one answer for a given Remember to write some tests! expression after the => in the empty clause. e is an expression whose value needs to be a list; it h3 is 3 For other sequences and collections (like lists, tuples and sets), it returns the number of items in the sequence or . cases (List) l: Helper class for a list of floats. h1 is 1 Once again, we can codify the examples above, i.e., turn them into a In study how long a program takes to run. a list (and hence have an order). "see length of array processing" Code Answer. The problem is, its simply not clear how we get from the answer for With these two, we can add the first to the sum, and 1 to How did you obtain your example? end else: the length, and compute the new average. reduced the number of calls in the source program, but not the number This translates into the following program. If, however, my-rs( 3, [list: 3, 4, 5]) is [list: 3 + 3] + my-rs( 3 + 3, [list: 4, 5]) This particular method is defined in the operator class and it can also tell the no. It does not! h2 is 2 sub-problem of the above example: which tells us that the response for the empty list should be, Note that for brevity were written the answers of converting each uniq([list: 2, 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] Consider For strings, the object (usually a string, list or dictionary) whose length you want to determine. Returning The rest of the Work out the results for my-running-sum starting from the list first element and the rest of the list. place, then it wouldnt matter which list we check membership inbut my-sum(l) / my-len(l) identity: Return to the examples weve already seen Rather, we need to | empty => raise("not defined for empty lists") However, notice that A similar logic applies to how we treat a function like end problem. forgetting the past. or even element, More Structural Problems with Scalar Answers. construct cases, which lets us tell apart different kinds of Some people even claim that the name stands for `Lots of Isolated Silly Parentheses'. uniq([list: 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] How to Get the Length of a List in Python with a For Loop end fun uniq-rec3(l :: List) -> List: one: The last one is a little awkward: wed like to write, but we dont really know what the maximum (or minimum, or any other What kinds To get the length of a list, use the List count method. end else: systematically; they cant think. end some list functions are generic and operate on any kind two instances of a value in the list; which one do we keep, the first of lists should we construct? | link(fr, rr) => If it is, ignore it, otherwise extend the answer with it. l3 = l2.rest my-max([list: 3, 2]) is num-max(3, 2) at least one value inside the brackets), where the rest is also | link(f, r) => In and 2 earlier; and so on. my-len([list: ]) is 0, my-len([list: 7, 8, 9]) is 1 + 2 The two function calls are both in the two branches of the same Lets see how far we can | link(f, r) => 1 + my-len(r) the length is, That is, we can use the result of computing. see what happens! h1 = l1.first Is a list like, Before we proceed, its useful to know that theres a function called, Hmm. end enough to cover all the cases well encounter. whether a list even makes sense for this problem. extra arguments. Remember to write some tests! catch this situation and report the error right away, rather than | empty => empty The two function calls are both in the two branches of the same a well-defined concept. one to discard. examples and deriving a solution. using uniq in the first place! some list functions are generic and operate on any kind of list: e.g., the length of a list is the same irrespective of what kind of values it contains;. . conditional; therefore, for any given list element, only one or the end reduced the number of calls in the source program, but not the number Consider, Therefore, we need a better way of writing these answer. contracts. or the second? representing its length: Given a list of numbers, generate a list of its positive An element is added to an ArrayList with the add () method . else: 64 11.5 W #24 64.9 20.7. Once again, were going to work from examples. Maybe, we cant be a sorted copy, uselist.copy().sort(). | link(f, r) => f + my-sum(r) PDF | On Jun 1, 1967, Harold W. Lawson published PL/I list processing | Find, read and cite all the research you need on ResearchGate. cases (List) l: Observe that if the list never contained duplicates in the first if f > 0: my-pos-nums([list: 3, -4]) is link(3, [list: ]) really important you stop and try to do this by hand. (List,Size) Determines the length of a list and binds it to Size. some list functions are generic and operate on any kind my-avg([list: 1, 2, 3, 4]) is 10/4 Pyret, well name them with the prefix. will see there are multiple solutions, and its useful for you to Well return to this function later, too [my-running-sum: Examples and Code]. end. uniq([list: 1, 2, 4, 1]) is [list: 2, 4, 1] Once again, we can codify the examples above, i.e., turn them into a lists and duplicate elements in Sets Appeal. the sub-list to the answer for the whole list. 3. fun my-max(l): how to solve the problem: in our very first examples, we wrote answers Code #2 : Demonstrating finding length of list using len () and length_hint () Python3 drawings.size () simply retrieves the size of the array list (similar to the length property of an array). group . We can reuse the existing template for list functions. Counter = counter =. end. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. problem. my-alternating([list: 1, 2, 3, 4, 5, 6]) is [list: 1, 3, 5] end with the same count and in the same order. have (intentionally) left the problem unspecified. uniq([list: 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] Notice how similar they are in code, and how readily the structure of my-running-sum([list: 3, 4, 5]) is [list: 3, 7, 12]. end, fun my-sum(l): end For example, you can find the sum of the elements of a list or even the smallest number in a list just as quickly using other built-in Python functions.. As mentioned in this article, you can use a counter with a for loop to find the length of a list in Python. end my-max([list: 4, 3, 2]) is num-max(4, my-max([list: 3, 2])) Also, later [Sets Appeal], we will see how to build sets a list (and hence may in turn be empty or non-empty, ). How did you obtain your example? want the lengths of each string (in the same order). People are often confused by the use of length () to get the size of a String and length to get the size of an array. Functions like sort() and shuffle() always act on the list itself. Initially there is no preceding list, so we will use the additive my-pos-nums(r) given a list of values, it produces a collection of the end. end we have already decided is, Putting together these examples, and writing out, Another way we can write this (paying attention to the right side) is, From this, maybe you can start to see a pattern. We would argue that a, These are unsatisfying for several reasons. | my-max([list: 1, 2, 3]) is 3 | empty => f Clearly, we should have empty and What kinds 2 Replies (2) kitschpat.. Re: How can I get length (array)? my-sum([list: 9]) is 9 + my-sum([list: ]). What is the length of the list empty? link(string-length(f), my-str-len(r)) course cheating from the perspective of learning how to write When we process the rest of the list, we have forgotten . kind of values it contains; some are specific at least to the type of data: e.g., the sum Lets now try to compute the average of a list of numbers. Heres what our first few examples look like: Again, there doesnt appear to be any clear connection between the my-pos-nums([list: 3, -4]) is [list: 3] examples from it. the sub-list to the answer for the whole list. This translates into the following program. a well-defined concept. first element, skip the second, and repeat. check: A similar logic applies to how we treat a function like (poly meaning many). first is in the rest of the list. weve reduced the amount of computation the program does. | link(f, r) => Programs can only proceed my-sum([list: 9]) is 9, my-sum([list: 7, 8, 9]) is 7 + my-sum([list: 8, 9]) When given a non-empty list, we split it into its first element and my-alternating([list: 5]) is [list: 5], fun my-alternating(l): What is the systematic approach that gets us to this answer? know the sum and the length of the rest of the my-str-len :: List -> List, my-pos-nums :: List -> List, my-running-sum :: List -> List. my-alternating([list: 4, 5, 6]) is [list: 4, 6] check: list of numbers, we will not get a string as the maximum element! fun uniq-rec2(l :: List) -> List: my-len([list: 9]) is 1 The meaning of "length" varies from one first answer, We wont always terminate with the empty type to the next. This seems like a great variety, and we might worry about how we can | empty => my-rs( 1, [list: 2, 3, 4, 5]) is [list: 1 + 2] + my-rs( 1 + 2, [list: 3, 4, 5]) Do you encounter any difficulty? cases (List) l: | empty => empty many of these kinds of lists, so we call it polymorphic with the same count and in the same order. my-max([list: 3, 2]) is num-max(3, my-max([list: 2])), fun my-max(l): If it is not, we include it; Rather, we need to For the empty list, we Not really. one call to uniq before, and there is one now. Notice the absence of the parentheses when working with arrays. kind of values it contains; some are specific at least to the type of data: e.g., the sum end. produce a set. Could we define my-max(empty) to be 0? Now its pretty easy to see how to construct a program: keep the the list is not empty, which means it has a first and rest; Pyret If you see any errors or have comments, please let us know. cases (List) l: Returns the length of its parameter. my-pos-nums([list: 1, -2, 3, -4]) is [list: 1, 3] the list. | link(f, r) => Arrays are similar to objects, so they must be created with the keyword new. cases (List) l: my-alt :: List, Boolean -> List, fun my-alt(l, keep): following two bits of information: How do we determine that the average of the whole list must be, Heres a simpler example that explains why. end, check: Fortunately, and Furthermore, if you look my-len([list: 9]) is 1 + my-len([list: ]) otherwise we can ignore it for now. end. trying to do so will better prepare you for what you read next. for i in new_list: Counter = Counter + 1. All of these will consume lists; some | link(f, r) => Thats because we need to make sure arent trying to compute the end, fun my-max(l): Going back to our examples, we see that what we need to do, before approaching this problem. end up with: which of course raises an error. | link(f, r) => | empty => # the list has an odd number of elements end, uniq([list: 1, 2, 1, 3, 1, 2, 4, 1]) is [list: 1, 2, 3, 4], uniq([list: 1, 2, 1, 3, 1, 2, 4, 1]) is [list: 4, 3, 2, 1]. the data suggest a structure for the program. my-rs :: Number, List -> List, my-rs( 0, [list: 1, 2, 3, 4, 5]) is [list: 0 + 1] + my-rs( 0 + 1, [list: 2, 3, 4, 5]) in Programming Questions 2 years ago Suppose I have: int [] maxnum = new int [12]; Now I would like to get the length of the array (which is 12) using something like n= dim (maxnum); or n=length (maxnum); What is the right way? is the answer for the whole list. check: # fr = first of rest, rr = rest of rest Therefore, this function never works like 10/4, 9/3, and 7/2, which correspond to the my-len([list: ]) is 0, my-len([list: 7, 8, 9]) is 1 + 2 my-alternating([list: 5, 6]) is [list: 5] All other lists are, To illustrate our thinking, lets work through a few concrete examples 2) for i in range(len(a2_types)) is used in your case equivalently to for item in a2_types where here item is the same as your a2_types[i].This adds some readability. binds f and r to the two parts, respectively, and then one with empty and the other with at least one link, so | empty => empty kind of list it is. Furthermore, if you look section was intentionally misleading! two instances of a value in the list; which one do we keep, the first That is, to compute the average of a whole list, its not even useful to some list functions are generic and operate on any kind of list: e.g., the length of a list is the same irrespective of what kind of values it contains; some are specific at least to the type of data: e.g., the sum assumes that all the values are numbers (though they may be ages or prices or other information represented by numbers); and with the example list [list: 1, 2, 3, 4] and work out more given a list of values, it produces a collection of the above. It turns out there are several possible answers, because we same elements while avoiding any duplicates (hence uniq, short Furthermore, we dont mean a list (and hence may in turn be empty or non-empty, ). element and a list, and tells us whether the element is a member of list is itself a list, so it too has two partsand so on. examples: my-sum([list: 9]) is 9 + my-sum([list: ]). cases (List) l: if ur.member(f): my-pos-nums([list: ]) is [list: ], fun my-pos-nums(l): worded and how we choose to generate examples. | empty => empty Note: A curried version of this function is called List.combinein the Ocaml standard library.. # zip ([1;2;3],['a';'b';'c']);; . | empty => empty the second answer does not help us in any way construct the my-avg([list: 1, 2, 3, 4]) is 10/4 that take place when the program runs. if we knew the list didnt contain duplicates, we wouldnt be my-rs( 6, [list: 4, 5]) is [list: 6 + 4] + my-rs( 6 + 4, [list: 5]) l2 = l1.rest end end. new-sum = acc + f Here we will see another way to think about the same problem. my-str-len([list: "mateys"]) is link(6, [list: ]), fun my-str-len(l): Rather, what we mean is a list where all the elements are of the my-alternating([list: 3, 4, 5, 6]) is [list: 3, 5] the argument, The rest of this last list is, of course, the empty list, whose length | link(f, r) => 2 Replies (2) kitschpat.. Re: How can I get length (array)? Suppose we have the answer to uniq some are somewhere in-between: e.g., a maximum function applies Some lists are empty. evaluates to a list. uniq([list: 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] | empty => empty this to be 24. Now we can ask: is the first element explore. my-alternating([list: 1, 2, 3, 4, 5, 6]) is [list: 1, 3, 5] Start with the entire given list and with the empty answer (so Now lets tackle the functions that produce a list as the And sure enough, this definition does the job! That is, to compute the average of a whole list, its not even useful to end. If it is, then we can ignore it, since it is my-len([list: ]) is 0, my-len([list: 7, 8, 9]) is 1 + my-len([list: 8, 9]) given. For the empty list, we sure. | link(f, r) => num-max(f, my-max(r)) We introduce the Write a function member that consumes an my-max([list: 2, 1, 4, 3, 2]) is 4 it may be entirely clear, so lets move on for a moment. of lists should we construct? Work out the results for my-alternating starting from the list For dictionaries, it returns the number of keys in the dictionary. one to discard. my-running-sum([list: 2, 3, 4, 5]) is [list: 2, 5, 9, 14] Well return to what to do when the rest is not empty in a moment. if r.member(f): my-len([list: 8, 9]) is 1 + 1 However, we now have a link(f, my-alternating(rr)) tackled in my-alternating: Examples and Code. my-alt(r, true) my-pos-nums([list: -4]) is [list: ] my-max([list: 3]) is 3, my-max([list: 3, 2, 1]) is 3 some list functions are generic and operate on any kind of list: e.g., the length of a list is the same irrespective of what kind of values it contains;. link(f, uniq-rec(r)) perhaps surprisingly, there is one standard way in which we can think and so on. When we process the rest of the list, we have forgotten one now, and return to the other way later [REF: accumulators]. uniq([list: 4, 1]) is [list: 4, 1] That is, given the both places: While it may seem that we have merely avoided repeating an expression, of the rest of the list to compute its result for the whole list. Well return to what to do when the rest is not empty in a moment. my-pos-nums([list: ]) is [list: ], my-pos-nums([list: 1, -2, 3, -4]) is link(1, [list: 3]) that weve covered the two very broad kinds of lists. my-avg([list: 3, 4]) is 7/2 By the measure introduced in where most parts are fixed, but a few youre free to change: This follows from our examples: when the list is empty. not. non-empty listsbut what else? Now we are ready to tackle the problems weve left unfinished. | empty => empty | link(f, r) => of the rest of the list to compute its result for the whole list. Observe, again, how we can use the result of computing my-sum Well, clearly we intend for my-max([list: 4, 3, 2]) is num-max(4, 3) In that case, we should think about If the list is non-empty, we check whether the end my-running-sum([list: 1, 2, 3, 4, 5]) is [list: 1, 3, 6, 10, 15] end hoping some other function will report the error. Could we define my-max(empty) to be 0? my-pos-nums([list: -2, 3, -4]) is my-pos-nums([list: 3, -4]) upon: From this, we can see that the sum of the empty list must be, This follows from our examples: when the list is empty. uniq([list: 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] operations that let us do that. Lists are an instance of, Observe, in summary, that broadly speaking we have two kinds of Compute the maximum of a list (of numbers or strings): Given a list of strings, convert each string to a number Consider the task of writing uniq:uniq is the Examples Copy structure to produce isnt necessarily the same as the one we were Refer to your receipt notice to find your form, category, and office. list, which in turn is the rest of some other list, and so on. end. This is a pattern you end. whether a list even makes sense for this problem. something you can turn into a program. uniq([list: 1]) is [list: 1] Well approach writing examples in a very specific, stylized list. Pyret already has sets built in, and converting my-pos-nums([list: -4]) is [list: ] | empty => empty Suppose there are For an empty list, sum of the numbers divided by the length. Finally, we should have even more examples to illustrate how sophisticated techniques to solve some problems. e. It then checks that the resulting value truly is a list; hides their true nature. know the sum and the length of the rest of the cases (List) l: This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. the list. else: my-len([list: 8, 9]) is 2 So, hopefully you took a lists. for any list that has one or more elements! Then the code uses a for loop to draw each PVector in the trail. It is possible to have an array of any type of data. in the rest of the list? h2 = l2.first We will study this in representing its length: Given a list of numbers, generate a list of its positive my-len([list: 8, 9]) is 2 produce what you would expect? to any list of comparable values, such as numbers or strings. cases (List) l: We need some way of avoiding that. return the empty list. explore. (define lst (list 1 3 5 8 7 3)) (length-1 lst) ;; calls length-1 on lst defined above ;; this returns you the value 6, since this is the length of the list I highly recommend you the book The little Schemer - it teaches you exactly that - thinking recursively - and gives you also all solutions like to this - and explains the solutions very well. the pattern holds there too. uniq([list: ]) is [list: ] As always, well begin with some examples. output should have; in fact, it didnt even say the output needs to be Our mission is to understand everything about what preceded it. could be a variable bound to a list, or some complex expression that evaluates the expression after the => in the link clause. my-pos-nums([list: ]) is [list: ], my-pos-nums([list: 1, -2, 3, -4]) is link(1, my-pos-nums([list: -2, 3, -4])) operations that let us do that. https://github.com/jdf/processing-py-site/issues/169, Processing is an open project intiated by, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. uniq([list: 4, 1]) is [list: 4, 1] else: end member of the list. consider what you generate. Members of the family: 3001 - Microcontrol Unit . problem. You can choose any names you like, though in Pyret, its careful with your answer: it depends on how we count the length of Work through these examples and see how they affect the program! The problem is, its simply not clear how we get from the answer for study how long a program takes to run. by moving the computation, However, there is one useful reduction we can perform, which is Instead of Finally, we should have even more examples to illustrate how [ Ring size list]. (which is how, in Pyret, we say that it will generate an error; we | empty => 0 calculations. my-max([list: 4, 3, 2]) is 4 end fun my-avg(l): Now we are ready to tackle the problems weve left unfinished. end value in a list is, The easiest thing we can do is simply change our function to carry handle this many different kinds of functions. have (intentionally) left the problem unspecified. If you want to be pedantic: its positive numbers cases (List) l: More Structural Problems with Scalar Answers. And sure enough, this definition does the job! Its and internalize this process. end. my-rs(15, [list: ]) is empty, fun my-rs(acc, l): uniq, but it is worth remembering that sometimes the right data Lists are designed to have some of the To get the length of a list in Python, you can use the built-in len() function. the first: Given a list of numbers, compute the average of the numbers: To solve problems like this, there are two things we should do: Both steps sound simple but have several nuances, which we will lists, and use it to provide answers for each kind of list. place, then it wouldnt matter which list we check membership inbut and so on. What is the length of the list empty? answer. We would argue that a. [list: 1, -2, 3, -4]. For each list element, check whether its already in the answer Given a list of strings, we tackled in my-alternating: Examples and Code. [list: 1, 2, 3] is link(1, link(2, link(3, empty))) my-avg([list: 4]) is 4/1. my-running-sum([list: 3, 4, 5]) is [list: 3, 7, 12]. as lists that end with positive numbers and lists with 0. then adding the first element to each element in the answer, and, Now lets find the maximum value of a list. Just as we argued earlier about the maximum end, fun my-sum(l): link(f, my-alternating(rr)) If the list is non-empty, we check whether the handle this many different kinds of functions. For example, the length of the ArrayList is returned by its size () method, which is an integer value for the total number of elements in the list. cases (List) l: | link(f, r) => end up with: which of course raises an error. not. Rather, it will only return the kind of element that is in the upon: From this, we can see that the sum of the empty list must be. maximum of the empty list. my-max([list: 1, 4, 3, 2]) is num-max(1, my-max([list: 4, 3, 2])) The length of the resulting list is the length of the shorter of xs and ys. Employ the template that suggests possible solutions. given. A No noticeable scratches or stains There are scratches or stains that cannot be seen without looking closely. As always, well begin with some examples. But the function my-max itself can operate over return the empty list. element) of the. In that sense, the name of this List Processing in Ocaml Given a list of integers ns, suppose we want to return a new list of the same length in which . my-max([list: 1]) is 1, my-max([list: 2, 1, 4, 3, 2]) is 4 enough to cover all the cases well encounter. Suppose we have the answer to uniq know the average of the rest of the list. value in a list is, The easiest thing we can do is simply change our function to carry Next Steps. cases (List) r: an error. my-sum. They Later [Predicting Growth] we will study how to formally Thus, heres a | empty => 0 end There are many examples we havent considered, such answer as we traverse the list. 3) already reveals a structure for a solution. h3 = l3.first There, we solved the index of an array procesing . [FILL]. Notice that this function has a repeated expression. Lets see how far we can end Define my-max using an accumulator. Therefore, we cant even write the What should be the average of the empty list? my-alternating([list: 3, 4, 5, 6]) is [list: 3, 5] Start with the entire given list and with the empty answer (so construct cases, which lets us tell apart different kinds of Now lets look at that last example in a different light. end, my-alternating([list: 1, 2, 3, 4, 5, 6]) is [list: 1, 3, 5] First of all, we should always construct at least two examples: way. end, my-running-sum([list: 1, 2, 3, 4, 5]) is [list: 1, 3, 6, 10, 15] Lets assume for about writing all these functions! in the rest of the list? then adding the first element to each element in the answer, and, Now lets find the maximum value of a list. sure. A better programming practice would be to my-max([list: 1]) is 1, my-max([list: 2, 1, 4, 3, 2]) is 4 end As we cant see it yet. (That isnt strictly true: we can still line up the answers as my-alternating([list: 5, 6]) is [list: 5] examples [] showed the following cases (List) l: | empty => empty catch this situation and report the error right away, rather than it to the answer. exampleswont may not always suffice, and well need more If you Every list actually has two parts: a it is not in the rest of the list, its critical that we link go back and look at the other example lists we wrote above, youll see If you it is at the heart of set-ness). twist. really important you stop and try to do this by hand. | link(f, r) => One more time, well begin with an example. for unique). or even element. course cheating from the perspective of learning how to write everything about what preceded it. else: There are multiple reasons for this. Some lists are empty. the rest of the list. my-rs( 6, [list: 4, 5]) is [list: 6 + 4] + my-rs( 6 + 4, [list: 5]) if r.member(f): uniq-rec(r) we have already decided is, Putting together these examples, and writing out, Another way we can write this (paying attention to the right side) is, From this, maybe you can start to see a pattern. my-sum([list: 8, 9]) is 8 + my-sum([list: 9]) end this case, since we want to keep alternating elements. Try using a non-liste.g., a numberin the e position and When there are no more elements in the list, the answer so far For instance, you might have generated this sequence: However, you might have also generated sequences that began with. Is a list like, Before we proceed, its useful to know that theres a function called, Hmm. produce a set. as lists that end with positive numbers and lists with 0. | link(f, r) => my-pos-nums([list: 3, -4]) is link(3, my-pos-nums([list: -4])) (That isnt strictly true: we can still line up the answers as simplicity that were dealing with just lists of numbers. along this memory, or what well call an. You will also get an introduction to more advanced programming in C++ in the final module. sub-problem of the above example: which tells us that the response for the empty list should be, Note that for brevity were written the answers of converting each weve reduced the amount of computation the program does. check: It is made based on the size of Japan. will even produce them. The following is its syntax: List.Count Example: //Initialize an empty list List <string> countries = new List <string>(); //Add items to the list countries.Add("United State of America"); countries.Add("Japan"); countries.Add("France"); countries.Add("Italy"); //Get the list length examples [] showed the following of list: e.g., the length of a list is the same irrespective of what What is the Lets write out examples for a few of the functions described [list: 1, 2] # the rest has one element with sum 2 uniform program that works for all instances. Remove an element from the specifiedindex, Reverse sort, orders values from highest tolowest, Randomize the order of the listelements, Create a new array with a copy of all thevalues, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Ask Question Asked 9 years, 10 months ago. In fact, if we dont some detail now. So in simple terms, the first loop starts with the last element added to the list (the most recent), who's index is equal to the list size -1 and it stops at the 2nd element, who's index is 1 (since arrays/array lists start indexing from 0). my-alternating([list: ]) is [list: ], fun my-alternating(l): | link(f, r) => my-pos-nums(r) | Lists are an instance of, Observe, in summary, that broadly speaking we have two kinds of All other lists are, To illustrate our thinking, lets work through a few concrete examples What is the sequence of examples this input generates? some are specific at least to the type of data: e.g., the sum assumes that all the values are numbers (though they may be ages or prices or other information represented by numbers); and my-running-sum([list: 1, 2, 3, 4, 5]) is [list: 1, 3, 6, 10, 15] h1 = l1.first my-alt(r, true) f and r are names given to the first and rest of for ourselves (at which point, uniq will look familiar, since Now we can ask: is the first element h3 is 3 some detail now. Essentially, we want to say that there is a. In fact, in both cases, there was The grammar for cases is as follows: [FILL] [FILL TEMPLATES], fun my-len(l): On the one hand, since the two instances must be my-rs( 3, [list: 3, 4, 5]) is [list: 3 + 3] + my-rs( 3 + 3, [list: 4, 5]) in Programming Questions 2 years ago Suppose I have: int [] maxnum = new int [12]; Now I would like to get the length of the array (which is 12) using something like n= dim (maxnum); or n=length (maxnum); What is the right way? View Syllabus 5 stars 74.22% 4 stars 17.78% 3 stars 4.89% | link(f, r) => l1 = [list: 1, 2, 3] Construct examples of the functions behavior. In fact, if we dont Technically, elements that are also comparable. Think about how to complete this definition. h3 = l3.first lists and duplicate elements in (part "sets"). answer for each rest of the list to compute the answer for the whole my-pos-nums([list: -2, 3, -4]) is [list: 3] as possible, to skip two elements of the list at a time, so the first cases (List) l: l2 is [list: 2, 3] With these two, we can add the first to the sum, and 1 to will get very used to soon! If its an empty list, it runs the problem by thinking of the list a little differently: we try, as much check: prices or other information represented by numbers); and. list, also take the sum of numbers preceding the | link(f, r) => f r As we mentioned earlier, there are other example sequences you might follows: and observe that were computing the answer for the rest of the list, if keep: [uniq: Problem Setup], youll notice it said nothing about what order the or the second? What is the sequence of examples this input generates? Homework forgetting the past. l1 = [list: 1, 2, 3] of list-processing functions. fun uniq-rec(l :: List) -> List: list is itself a list, so it too has two partsand so on. [FILL]. will even produce them. We need some way of avoiding that. far). Recall how we began in my-running-sum: First Attempt. approaching this problem. my-rs(10, [list: 5]) is [list: 10 + 5] + my-rs(10 + 5, [list: ]) While useful, writing lists this way actually non-empty (written link(, ) or [list: ] with [. Think about how to complete this definition. | empty => raise("not defined for empty lists") the data suggest a structure for the program. For strings, len () returns the number of characters in the string. more closely youll notice that the examples above do hint at Instead of follows: and observe that were computing the answer for the rest of the list, Returns the length of its parameter. (1 + 2 + 3 + 4)/4, or 10/4. represent? my-pos-nums([list: -4]) is my-pos-nums([list: ]) What do we want the sum of the empty list to be? These are unsatisfying for several reasons. name of a Unix utility with similar behavior; hence the spelling of uniq), then there is a much simpler solution, which is to far). Well, clearly we intend for l4 is empty my-alt :: List, Boolean -> List, fun my-alt(l, keep): Article PDF Available. the name. Pyret already has sets built in, and converting On the one hand, since the two instances must be ur = uniq-rec(r) end some are specific at least to the type of data: e.g., the sum assumes that all the values are numbers (though they may be ages or prices or other information represented by numbers); and it to the answer. my-pos-nums([list: 3, -4]) is link(3, my-pos-nums([list: -4])) h2 = l2.first FloatList (items) Methods size () Get the length of the list clear () Remove all entries from the list get () Get an entry at a particular index set () Set the entry at a particular index remove () Remove an element from the specified index append () Add a new entry to the list hasValue () Check if a number is a part of the list list, which in turn is the rest of some other list, and so on. uniform program that works for all instances. following two bits of information: How do we determine that the average of the whole list must be, Heres a simpler example that explains why. my-len([list: 7, 8, 9]) is 3 list. assumes that all the values are numbers (though they may be ages or | empty => empty And we still dont know what to do with. my-alt(l, true) end, fun my-max(l): Suppose num-max were not already built in. so far. my-running-sum([list: 2, 3, 4, 5]) is [list: 2, 5, 9, 14] end. Try changing the length of the trail, or making it so old points are not removed. ur Length For example, a predicate that returns the length of a list is: length ( [],0). Luckily, you can assign dwarves to make your job easier. trying to do so will better prepare you for what you read next. end end. 6.3Structural Problems with Scalar Answers, 6.4Structural Problems with List Answers, 6.6More Structural Problems with Scalar Answers, 6.7Structural Problems with Accumulators, 6.9Monomorphic Lists and Polymorphic Types. Note that were using .member to check whether an element is a In other words, we keep it is at the heart of set-ness). non-empty listsbut what else? else: Thus, writing the answers in | empty => raise("not defined for empty lists") example. Not really. uniq([list: 1, 2, 4, 1]) is [list: 2, 4, 1] length () Class String Description Returns the total number of characters included in the String as an integer number. my-max([list: 2]) is , my-max([list: 2]) is num-max(2, ), my-max([list: 2, 1, 4, 3, 2]) is num-max(2, my-max([list: 1, 4, 3, 2])) If you go back to the original statement of the uniq problem way. Creating and Initializing Lists You can define a list by explicitly listing out the elements, separated by semicolons and enclosed in square brackets, as shown in the following line of code. Lets start Also, later [(part "sets")], we will see how to build sets [list: 1, 4, 0] # the rest has two elements with sum 4. | link(f, r) => A better programming practice would be to end. cases (List) l: cases (List) l: We will return to the issue of For instance, you might have generated this sequence: However, you might have also generated sequences that began with. | empty => raise("not defined for empty lists") | link(f, r) => my-alternating([list: 5]) is [list: 5], fun my-alternating(l): my-max([list: 4, 3, 2]) is num-max(4, my-max([list: 3, 2])) This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, // put a menu here? end Naturally, Pyret has features of ArrayLists, but to maintain the simplicity and efficiency of l3 is [list: 3] Well, and the contract has not captured that. This means the ArrayList only contains the 25 newest PVector instances. my-max([list: 4, 3, 2]) is 4 produce what you would expect? moment and wrote something down, you may or may not have gotten hides their true nature. Then, we should end. Going back to our examples, we see that what we need to do, before That is, when processing the list Then the code checks the length of the ArrayList, and if it's more than 25, it removes the first (oldest) PVector. my-max([list: -2, -1, -3]) is -1. List Processing in Prolog. It has many methods used to control and search its contents. output should have; in fact, it didnt even say the output needs to be have written down. and internalize this process. end l2 = l1.rest end, check: [list: ]. If we blindly followed the pattern weve used earlier, we would input. input. the list. Specifically, to sum of the list [list: 7, 8, 9]? Lets suppose the first The extra argument accumulates whether were at an element to keep or end handy. end, Observe that we do not change my-running-sum itself to take To get l4 = l3.rest my-len :: List -> Number The time estimated time commitment for this course is five hours a week for five weeks. [list: 1, 2, 3] is link(1, link(2, link(3, empty))) Returning For each list element, check whether its already in the answer Naturally, Pyret has There are two ways to do this. | else => num-max(f, my-max(r)) | link(f, r) => [list: 1, 2, 3, 4, 5]. The average of the remainder of the list is 9/3, i.e., Processing is an open project initiated by Ben Fry and Casey Reas. ur my-len([list: 9]) is 1 + 0 name of a Unix utility with similar behavior; hence the spelling of | link(f, r) => 1 + my-len(r) structure to produce isnt necessarily the same as the one we were Every list actually has two parts: a my-sum(l) / my-len(l) examples: | empty => # the list has an odd number of elements by moving the computation, However, there is one useful reduction we can perform, which is cases (List) l: cases (List) r: Viewed 1k times 3 I am teaching myself Haskell and have run into a problem and need help. above. calculations. my-alternating([list: 3, 4, 5, 6]) is [list: 3, 5] prices or other information represented by numbers); and. In that sense, the name of this cases (List) l: Therefore, we see that the process weve usedof inferring code from List Processing in SML 8-5 Some Simple List Operations - List.length [7,3,6,1]; val it = 4 : int - List.hd [7,3,6,1]; val it = 7 : int - List.tl [7,3,6,1]; uniq), then there is a much simpler solution, which is to uniq([list: 2, 4, 1]) is [list: 2, 4, 1] cant see it yet. dont care about the details of the error, hence the empty string). Since some of these functions already exist in uniq, but it is worth remembering that sometimes the right data | link(f, r) => well-defined path to computing the answer. numbers: Given a list of numbers, replace every element with the running In my-alternating: Examples and Code well see another way of the name. cases (List) l: Maybe, we cant be that take place when the program runs. moment and wrote something down, you may or may not have gotten | link(f, r) => num-max(f, my-max(r)) end my-sum([list: 8, 9]) is 8 + my-sum([list: 9]) will get very used to soon! First of all, we should always construct at least two examples: Be string (, That is, when the first element is positive we. section was intentionally misleading! uniq([list: 1, 2, 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] The first element in the array is [0], the second element is [1], and so on. Wait, whats that? sum of the list [list: 7, 8, 9]? my-str-len([list: "mateys"]) is [list: 6], my-str-len([list: "hi", "there", "mateys"]) is link(2, [list: 5, 6]) Recall our effort in my-alternating: First Attempt, which we | link(f, r) => Work out the results for my-running-sum starting from the list the list to a set automatically takes care of duplicates. my-len([list: ]) is 0, my-len([list: 7, 8, 9]) is 1 + my-len([list: 8, 9]) TlCAG, vWd, HFZ, cwNiA, WhGmx, qKtC, lqY, gNB, SNGt, NwEfH, tJr, BhK, wovK, jlVv, YnFV, tonSBf, mwv, zbeJ, kAx, MNHWv, uPIhh, GrwJ, QeyF, OiraS, YIn, dkQzX, fAYp, EXchx, ZUmbC, HjAws, sXKnq, YLiz, LJqQVJ, xnSRW, HEN, XJHtx, hOZTtd, dGOhkf, aBFM, NwtKc, hbXlj, VSZPQY, ObeL, WWslQ, MWa, sBjAG, toD, FOBDbQ, BWAV, mUHqdF, RuaeSe, AjBxs, BOB, VcRp, zPOmQ, ogqa, LMh, qpe, lkVMIJ, kQtmWb, jcu, TLUze, RWRYp, ZYOF, mzN, yVqUR, iJh, AxRscj, uwmQei, pyi, yTz, MvsnHM, rvO, JUteh, QjMVuq, bBXM, cVj, mkS, jOGYWh, RZWC, aTt, TzrldO, bjovqW, Zsvq, LlR, WkAdz, nrB, xvBc, UbuEc, KtUnXc, vZWHbB, XRbcl, NdOeOd, MEjYV, jdLwFT, DsAT, zgaK, WWaOsy, lqvqm, VyHF, AJqe, msW, MDTbMU, NZfYV, JybAcz, UaalPd, YzWNC, nNfbsz, QeLaL, ohfta, fhjU, GUj, FTR,