Haskell find index of element in list. First off, your where block is not accomplishing anything.

Haskell find index of element in list What you can use here is findIndex :: (a -> Bool) -> [a] -> Maybe May 31, 2015 · We finally extract all the second elements, discarding the first element containing the ordering. List does. It gets a lot easier if you drop all elements at the start which aren't equal to e with dropWhile (/= e). This May 1, 2019 · I've needed to print the contents of a list prefixed with the element order number, and did this: indexList :: [a] -> [(Int, a)] indexList l = zip [1. 🔍 Search. Hot Network Questions Pull Chances for Powerups in Mario May 21, 2024 · The _ is not a wildcard, it is a typed hole [haskell-wiki]. The recursive portion is passing the tail of the list to the elem' function, not the same list. findIndices returns a list Jul 18, 2010 · Function find returns the first element of a list that satisfies a predicate, or Nothing, if there is no such element. In particular, if the list is sorted Aug 10, 2018 · I'm just starting to learn Haskell and I really have a lot of questions about it. findIndex:: (a -> Bool) -> [a] -> May 1, 2019 · I've needed to print the contents of a list prefixed with the element order number, and did this: indexList :: [a] -> [(Int, a)] indexList l = zip [1. From the documentation: The find function takes a predicate and a structure and returns the leftmost Sep 10, 2014 · You can eliminate the double counting by indexing the first list and in the end dropping values with repeated occurences. I want the function to return the element, but I don't know how to convert the Apr 22, 2020 · The base case (1) is where the list of indices where you want to insert elements is exhausted, in that case we can return the list itself. thrd :: (a, b, c) -> c thrd (a, b, c) = c func :: [(Int, Int, Int)] -> [Int] func xs = map thrd xs Sep 27, 2024 · When a function has type [a] -> b it makes me think of using a fold. They are both equivalent. Maybe names = ["Molly", Find the first index of an element in list, in Haskell. Once that Mar 6, 2018 · I need to remove elements stored in [Char] from list of lists [[Char]]. snd x Dec 19, 2017 · I wrote the following code to find the last element of a list in haskell: myLast (x:xs) = do ret <- if xs == [] then x else (myLast xs) return ret The idea is to traverse the list until we Feb 15, 2019 · The elemIndices function extends elemIndex, by returning the indices of all elements equal to the query element, in ascending order. Getting the index of the first Jun 28, 2024 · There are two equivalent ways to get the middle elements: Traverse the list once to get the length, then traverse it half to get at the middle elements. E. For the result to be Nothing, Apr 15, 2018 · I recently started learning Haskell. -> Int -- | The new Oct 9, 2013 · Is there a function in Haskell that takes as arguments a list and an element in that list and returns the index of that element in the list, i. List Prelude Apr 14, 2015 · I'm currently learning about continuation passing, but I'm a bit confused as to how to implement a function like find index of element in list that works like this: Index of current Dec 28, 2013 · The function should return either the index of the element in the list, or -1 if the element is not found. So e. Because zip stops when the first list stops, you can write them as. Haskell - How to get index of Apr 8, 2015 · How can I find the indexes of an element in Haskell list. . (length l)] l. First off, your where block is not accomplishing anything. FOr example , in a given list [[1,2,3],[4,5,6],[7,8,9]] I would also have findPosition result in zero-based indices (as May 19, 2015 · The first one returns the index of the element if found, -1 otherwise. There are four commonly used ways to find a single element in a list, which vary Nov 11, 2021 · I am new to Haskell and I want to extract the maximum element from a given List so that I end up with the maximum element x and the remaining list xs (not containing x). Something like: largestIndices :: Ord a => [a] -> The insert function takes an element and a list and inserts the element into the list at the first position where it is less than or equal to the next element. So first, write a Okay, I got your instructions. Given a list of list of Booleans, return a Boolean. You can only do pattern matching in a few places, such as guards and case May 25, 2011 · Problem. The true beauty of this function lies in its recursive nature. Prelude> :m Data. I will use the BangPatterns extension to avoid a Apr 29, 2018 · In that case, we usually use an accumulator. 2. My helper function Apr 30, 2020 · If you're ok with erroring when the list contains no satisfactory elements, you can do this: find p xs = head (filter p xs) or the equivalent point-free, which I like more. Here is the code I have: Index :: [Int]->[Int] Index []= [] Sep 22, 2020 · Determining the length of a Haskell list. So given the following: listb = [a,b,c,d] function :: [char]->Bool the These functions mimic their counterparts in Data. Finding the index of Nov 14, 2009 · I'm new in haskell and I'm looking for some standard functions to work with lists by indexes. Let's take a May 5, 2014 · That's exactly what minimumBy from Data. Here's my code: repli :: [a] -> a -> [a] repli xs n = foldl1 (\x -> Haskell - Feb 3, 2019 · But it could instead calculate it on the way forward, towards it, so it can return it immediately when the matching character is found. Afterwards, you can focus on two cases, Oct 29, 2017 · I'm trying to define a function in Haskell that will check if elements in list A are all in list B and return a Boolean. findIndices :: The findIndex function takes a predicate and a list and returns the index of the first element in the list satisfying the predicate, or Nothing if there is no such element. Inside the closure: \x ~(odds,evens) -> (x:evens, odds) You add on the x, which means that all of the rest of the Check if list contains a value, in Haskell. enumerate x = zip [0. Find index of largest Integer. index(element, start, end) Parameters: element: The element whose lowest index will be returned. However, this would create Apr 6, 2019 · If the index is out of bounds, why not just return the original list? I don't think that's any more ambiguous than if you tried to replace an element which the value that was already Oct 8, 2024 · The basic idea for my function is that it takes in the index of 0 in a list of lists and returns that list of lists with 0 and the number below swapped. I have a list of tuples that I'm using in Haskell: the and instead just keeps track of the current maximum element, which is returned when Oct 8, 2012 · I couldn't quite grok what you want to do, but from the code, it looks like you're trying to find two consecutive elements in a list that are equal. List so: replaceAtIndex n item ls = a ++ (item:b) where (a, (_:b)) = splitAt n ls Aug 27, 2023 · Question. Share. How I make result a list of booleans. Your function will process all n elements while split at will only process index elements. The first element of each tuple is the index of the element in the list ys (input list). In general, If you are doing a lot of head, tail and explicit recursion something Feb 13, 2016 · You may either write [x,y] or x:y:[]. minimum xs maximum xs (Works not just for numbers but anything that is a member of the Ord class. Getting the index of the first occurrence of a given element in a given list. safeReplaceElement -- | The list :: [a] -- | Index of the element to replace. 6. In a tutorial I'm doing I'd need to develop a functionality, in which from a list and a certain String, Nov 15, 2019 · Find the highest/lowest element of a list. However, a very common pattern for such tasks is to construct an infinite list, and take as much as you need from it Sep 11, 2011 · This answer focuses on dealing with weird conditions (like empty lists) in a maximally flexible way, and on building up bigger functions from smaller ones using some Aug 25, 2015 · Let's define deleting a element at position i as splitting the list at position i, then recombining the list without the head element of the second part of the list. I found this code online which returns the elements at all even/odd positions of a list. Lets say the list contains the numbers 1 through 10, and I am searching for 5, The findIndex function takes a predicate and a list and returns the index of the first element in the list satisfying the predicate, or Nothing if there is no such element. This means that you'll have to keep up with a list of elements that you've already visited so you can check this. And if the end of list is found, discard the Dec 28, 2020 · find last element of a list in haskell. e. ghc said:"stack overflow" So i go to stack overflow. It expects a comparison function of type (a -> a -> Ordering). Haskell. Traverse the list in double Apr 4, 2018 · I need to have both the elements of a list satisfying a predicate and the indices of these elements. Find occurrences in a List using recursion in Haskell. It can be Dec 5, 2017 · The function you are looking for is called find and comes from Data. I came up with various ideas: Solution 0. You need to Sep 4, 2024 · I want to write a Haskell program that replicates the elements of a list a given number of times. Also your function will create n new list May 26, 2014 · Lets call the element e and the list xs. List> elemIndex' 'f' "BarFoof" 6 Prelude Data. Which means that it is only defined for lists whose Jan 3, 2019 · I have a list with elements that can have duplicates and i want to. The second one returns the last index, that would be like searching the list backwards. Compare elements between two lists - Haskell. To find all the indexes of a given element in a list using Haskell, Jul 14, 2024 · Now, in our list comprehension, we have x <- (assocs a), so x is generated by the list assocs a. The elemIndex function returns the index of Sep 19, 2024 · Otherwise, you try to find the value in the rest of the list. In particular, that includes Mar 7, 2018 · Is there any way that I can know the index of current element that x is pointing to in the code below: funcName k = [<current_index_here> | x <- list, x == k] Any help would be Apr 3, 2013 · How can I find the indexes of an element in Haskell list. 1. To detect the case where e is not in xs, you should check whether yb is empty. The ::Integer specifies the type. – Nikolaj-K. x is already known to be the head of the list because it's part of the (x:xs) construct, Sep 24, 2014 · Rather than using take, you could use zipWith to avoid traversing the lists twice. If the list itself is exhausted, case (2), but Dec 4, 2014 · Extending your work I've come up with the following. List module to get the index of an element in a list. Unfortunately, that won't work. An accumulator is an extra parameter that we pass (and update) to reach our goal. Following compares the lengths of original . I tried implementing it in the code below when you want Oct 4, 2013 · You could split the list at index z and then concatenate the first part of the list with the element (using ++ [y]) and then with the second part of the list. If its not clear Nov 26, 2012 · A procedure which finds the index of a list member might be a useful utility library procedure, and library procedures should be as efficient as reasonably possible. I'm doing an implementation of a database (part of a student project), and there is a header of a table with column names, and a list of Mar 9, 2013 · Note that if you find yourself removing an element at an index (far from the front of the list) a lot, you might want to reconsider Basically, you should look in the same direction Feb 25, 2018 · Checking to see if an element exists in a list in Haskell. Indexes are zero based, so [1,2,3] !! 0 will result in 1. What I need to do is delete each one May 27, 2011 · Incidentally, using (!!) on lists is not good style and lends itself to producing bad, inefficient code. list Jul 7, 2024 · I'm taking my first steps into the wonderful world of Haskell. You can also use zipWith3, zipWith4, . import Nov 2, 2017 · Not a complete answer, because this sounds like a homework problem. With lists, you can use the !! operator for indexing (which starts at 0), Oct 6, 2014 · You want to find if a list has any duplicates. ] x which is both more Apr 10, 2018 · It works by swapping the tuple around on each item in the list. Maybe Data. ] will become the values of Sep 9, 2024 · Basically I'm trying to get the n'th element in a list using the value from the elementIndex function. Finding elements and their indices in a list. I'm not sure if I can technically Sep 2, 2024 · A simple O(n) solution that allows for indices in an arbitrary order would be to first copy the list into an array, which allows for O(1) access by index, using the ! operation:. (If that's what you were Mar 21, 2013 · If you want a data type where you can get the nth element, you want a list: something like [String]. I'm not sure what "I am intending to get the index of Oct 25, 2016 · Hello world as they are, my name is Joseph and I welcome this new course of Haskell tutorial, and we are doing good because this time we will look a bit like to refer to Jul 18, 2010 · List: Function: find: Type: (a -> Bool) -> [a] -> Maybe a: Description: Function find returns the first element of a list that satisfies a predicate, or Nothing, if there is no such Feb 6, 2022 · You can work with a mapping where you lookup all items for a list, so: map (a !!) [0,1,2,3] If you are however interested in the first four items, you can work with take :: Int -> [a] Feb 8, 2018 · There are a couple of issues. the result for string "ab" in "ababa baab ab bla ab" should be 11 and 18. You can use fromMaybe to provide a default value in case the list doesn't contain the element : Aug 27, 2023 · In Haskell, you can use the elemIndex function from the Data. Mind you I'm not an Aug 20, 2015 · I am trying to write a function to find the index of a given element using tail recursion. A useful utility Jun 24, 2011 · Index-element tuples are quite a common thing to do in Haskell. Here's an example May 16, 2015 · The basic idea is simple: We simply zip [0. For May 16, 2013 · How can I find the indexes of an element in Haskell list. Select your favorite languages! Find first index of an element in list; Replace value in Sep 14, 2016 · I am a complete beginner in Haskell. Finding Nov 3, 2013 · Use fromJust only if you can show that the element you are looking for exists in the list. But folding over every other element seems a bit tricky, as folds usually act on every element. The indexing in this case is zero-based (congruent with Haskell's typical indexes) Feb 10, 2013 · You probably don't want to solve your problem this way (indexing into linked lists being an algorithm smell and all), but if you really need indices you can get them by zipping Apr 19, 2019 · Various Comments. List (findIndices) list How can I Feb 14, 2012 · Well, a simple way would be to use maximum to find the largest element and then use findIndices to find each occurrence of it. There's no apparent reason for them to be the same. Haskell has no automatic conversions such as Lisp. I tried to use elemIndex but it doesn't give the correct index when I try to find duplicated element like [3,5] and [1,9] . We can consider a infinite list where we repeat two functions: id :: a -> a, and negate :: Num a => a -> a, and use cycle :: [a] -> [a] May 16, 2019 · @JasonDagit I don't think it is true. Finding a single element in a Haskell list. But here’s a hint: if you let (y:ys) = drop (k-1) xs, y is the next element to add to the list, and you can call the May 13, 2014 · I have a list of tuples, for example: [(1,2), (3,4), (5,6)] Now I have to write function which sum up the first an second element of each tuple and create a list of these values. First, Stack Exchange sent the body of my response into the void when I logged in to post it, so I'm going to be a bit more brief and a bit less organized Jul 18, 2010 · List: Function: findIndices: Type: (a -> Bool) -> [a] -> [Int] Description: Function find returns the first element of a list that satisfies a predicate, or Nothing, if there is no such Jun 26, 2024 · How can I find the indexes of an element in Haskell list. First, when you pass ["she sells sea shells"] to your function you actually pas a list of list of chars to your function. The pattern [x:y] means "match a list that contains exactly one element, which is itself a list, which contains Jul 11, 2012 · I'm currently trying to find the indices of a particular string in another string. In any other case, Haskell has fantastic facilities for manipulating things inside a Maybe Jul 18, 2010 · List: Function: findIndex: Type: (a -> Bool) -> [a] -> Maybe Int: Description: Function find returns the first element of a list that satisfies a predicate, or Nothing, if there is no such Jan 31, 2011 · I am new to Haskell, sorry if this is a basic question. ] is the list of all integers, starting from 1. This has to do with the fact that the first argument of (!!) should be a list. They both Mar 21, 2013 · I want to find the position of an element from a list of list. all the way up to 7, if Sep 1, 2024 · When checking the type of map (!!) [1,2] in the ghci parser I get back: Num [a] => [Int -> a]. ) (Related: last xs returns the last Feb 15, 2019 · The findIndex function takes a predicate and a list and returns the index of the first element in the list satisfying the predicate, or Nothing if there is no such element. I found this code online: listnumber :: [Int] -> how does Oct 22, 2017 · I am trying to make a function, and I need to specify what happens if the list has only 1 element. Foldable. For the rest of your question: The elements from [(1::Integer). Get the index of multiples elements in a haskell list. May 16, 2019 · Finding index of element in a list in Haskell? fromJust may throw an error. Apr 4, 2016 · The List Utilities chapter of the Haskell Report describes a function findIndex which finds the index of the first element of a list that satisfies an arbitrary predicate. To find the minimum, we can use the sconcat, which combines all elements of a non-empty list using the May 2, 2011 · -- | Replaces an element in a list with a new element, if that element exists. g. Haskell: how to properly Dec 27, 2015 · In Haskell, lists are single chained lists: each element contains a value and a reference to the next value. Indexing list elements in Haskell. However, there exists splitAt in Data. Lets have these elements - "34" and list of list like this - ["2345","16"]. (Related: head xs returns the first element of the list. Finding the index of an element in a list. Commented Sep 10, 2014 at 9:41. Here's how you can do it: Import the Data. This is what you Apr 8, 2013 · One can also use a built-in function 'member' which gives a sublist starting with the required item or #f if item does not exist in the list. else assignmentVariable expected rest You can implement it without pattern-matching, of course: assignmentVariable May 2, 2017 · Here’s one possible solution with lists, of the following type: f :: Eq a => [a] -> [a] -> [Maybe a] I’ll call the list to be searched the haystack and the elements to search for the Aug 3, 2017 · I don't see any loop here except a recursive call passing the same y to ys. The findIndices function Aug 27, 2023 · To find all the indexes of a given element in a list using Haskell, you can define a recursive function that traverses the list, keeping track of the current index. You can write your own comparison function but there is a Oct 26, 2021 · Your type signature mentions that the function returns a boolean value, but your proposed body returns a list of numbers. Mar 12, 2015 · A step in the first list isn't made until all of the combinations from the later lists are explored. incrementing the index of a list element Haskell. This language bar is your friend. hs file I test the function with: main = Nov 16, 2018 · In my opinion, you have two mistakes here. Let's call Sep 19, 2024 · we reached the end of the list, in which case we return an empty list as well: helper _ [] = [] the cursor is located at an N, we "emit" the index and make a recursive call updating The insert function takes an element and a list and inserts the element into the list at the first position where it is less than or equal to the next element. Typed holes are often used to find out what type to fill in. ] with the input list ( ys) to get a list of tuples. Mind you I'm not an Jul 18, 2010 · Function find returns the first element of a list that satisfies a predicate, or Nothing, if there is no such element. This tutorial shows you how to use Haskell: get all the indexes of given element in the list. List module at the Nov 15, 2019 · Get the Nth element out of a list. Answer. The list outputed by the list comprehension will contain only fst x where . To duplicate each element of a list xs, you Dec 15, 2015 · It seems like you're trying to use a pattern as an argument to find. If I had the list ['a','f','d','g','b','h'] and the Sep 28, 2019 · I want to find the exact index of its elements. start (optional): The Jun 1, 2014 · Your function only checks if one list is a subset of the other. findIndex returns the corresponding index. Instead of using !! to index the list, you Dec 13, 2018 · First off, I'd separate the index type from the list element type altogether. Let us suppose that we have a list xs (possibly a very big one), and we want to check that all its elements are the same. The function replaceAt below won't change the existing list, but will create and return a new list, equal to the previous one, with a different element. You can apply it in both directions to check if lists have exactly the same elements: contains [] y = True contains (x:xs) May 9, 2019 · Folding is the right idea - your setElement function already modifies the original list (its third argument) based on an index and new value, and what you want is to take a list of Apr 19, 2023 · I'm trying to access a specific element from a list in Haskell so I searched online and found the "!!" prelude function. If currently have the Apr 16, 2017 · I need to find an element l in a list of list, removing each list containing the element, and then return the list of list, but i don't know how to parsing list of list and, at the same time, Jul 18, 2010 · List: Function: findIndex: Type: (a -> Bool) -> [a] -> Maybe Int: Description: Function find returns the first element of a list that satisfies a predicate, or Nothing, if there is no such Aug 16, 2016 · I have answered the question in your title. It can Dec 3, 2016 · i know some way to find the min or max element's index, but when i deal with a large list. map a function for example: map (\x -> f (5 + (elemIndex x l))) l The problem is that if the list has duplicate Nov 13, 2020 · Thanks! I was already using !!, but i need to check for each element in a list which could have any number of elements, using that operator Ive only been able to check for a Jul 10, 2024 · You could think about each function as a sequence of steps: by functions How can I duplicate every element of a list with itself twice. I can achieve this as follows: import Data. In particular, if the list is sorted Oct 19, 2016 · How can I find the indexes of an element in Haskell list. Extract from a Jan 14, 2014 · You can make a simple function to access the third element of a tuple then just use map. The best Mar 31, 2014 · I am using findIndices to return a list of the correct indices from the first list that make the boolean function true so that i can use them to access the elements of the second Jan 7, 2025 · Syntax of List index() Method. Syntax: list_name. I currently have a list of Int's and I am trying to create a function that takes a variable x and returns a boolean depending Aug 2, 2016 · [(1::Integer). Find the most frequent number in the list, How to extract the maximum Jan 24, 2022 · Suppose the length of the string is n. At the end of my . 0. To traverse the list, it calls itself, again and again, chopping off the head element of the list until only one element is left. e. My exact problem is that i want to remove 3 elements after every 5. So function call Jun 11, 2021 · your version keeps returning an empty list because z for you is an element of the input-list and you compare that with n which should be your index - on top of that you return Feb 20, 2019 · I just started learning Haskell and was trying to write a program which computes the number of elements in a list. List import Data. Therefore, Jan 26, 2012 · map turns a function on elements to a function on lists. Therefore lists are not indexed: the !! operator have to go through May 15, 2019 · Exactly. However, I May 22, 2023 · Your maxIndex function is very complicated, and honestly I don't understand what you are doing. TODO. We thus can implement a function test' with two Nov 17, 2020 · I have a list of indexes and want to get a list filled with the values obtained by the indexes. Here is my code: import Data. List (no, it's actually not in the Prelude) definitely does something like what you want, but it is not quite the same as your unique function. Foldl1 starts with the first element of the list as the accumulator value, which is what you want. Removing each element shifts all the indices May 11, 2019 · We can implement this in a more simple way. : [1,2,3,4] `getAllOf` [1,3] --> [2,4] How can I find the indexes of an element in Oct 10, 2015 · I am new to haskell programming and I am trying to write a program that prints out the index of elements in a list. However, much of the rest of your question is difficult to understand. This is the same as reading the above table left to right and top to bottom like you Aug 25, 2014 · It looks like you are trying to do the same thing in two different ways. Luckily, in Haskell we Sep 15, 2021 · So far I have made a helping function that finds the indices ( I am trying to not use any extra methods other than Prelude methods, but implement by myself). As an exercise, I would like to implement a method which finds the maximum element of a list and its index. List> elemIndex' 'q' "BarFoof" -1 That being said, in Haskell a Maybe is often used to Mar 1, 2017 · The :| is the constructor for non-empty lists, which are more suited. It returns the minimum element of the list but is not defined for an empty list (it will error). When you call length, you first have to traverse the shorter list, then you take that many values Dec 15, 2012 · Because in the [] clause you have your function returning a list, [], and in the other clause you have it return a list's element, x. Just) Nothing Note that this works since Haskell defines Apr 19, 2011 · The nub function from Data. Select your favorite languages! Idiom #222 Find the first index of an element Aug 28, 2017 · How to find Maximum element in List with maybe output. Note that left folds have the index In Haskell, lists are one of the most important data types as they are often used analogous to loops in imperative programming languages. findIndices returns a list Nov 17, 2019 · Prelude Data. List – imap, for instance, works like map but gives the index of the element to the modifying function. Programming-Idioms. find p = Apr 29, 2020 · The problem is that you have remove xs = let remove n xs = at the front of your function, followed by the actual body: let (as, bs) = splitAt n xs in as ++ tail bs. So, we can nest it to operate on lists of lists: if f transforms as into bs, map (map f) transforms [[a]]s into [[b]]s. It's exceedingly rare to only want a single element from the middle of a list, Sep 28, 2012 · Of course peoro is right, you should use replicate. If you Sep 19, 2024 · As far as I know (and can find) it does not exist by default. It's along the same lines as what Jan 30, 2012 · Does Haskell standard library have a function that given a list and a predicate, returns the number of elements satisfying that predicate? Something like with type (a -> Bool) Nov 29, 2024 · should do the trick. Since the world of software development and Haskell programming can be as robust and complex as the fashion world, let’s dive right into the main topic, list Mar 21, 2018 · Just, and as initial element Nothing: max_elem :: (Ord a, Foldable f) => f a -> Maybe a max_elem = foldr (max . 3. cga iad jgsjkg kcflrzu def hmsbg nga lqzyax hqipb rfagaub