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How would I find the distance between two elements in a list?

Gap function that returns the integer distance between first appearance of two elements in a list using either foldl or foldr.(Haskell)Getting started with HaskellBasic Structure of a Haskell ProgramCheck if two lists have the same elementsHow to find the node that holds the minimum element in a binary tree in Haskell?How to optimize filtering of a listChecking if two lists are equal at Nth positionIntuitively, how does Haskell find the length of a list without using a standard length function?Haskell mix two listsGap function that returns the integer distance between first appearance of two elements in a list using either foldl or foldr.(Haskell)Testing if one list is a sublist of another without testing equality between elements

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;

0

I have to find the distance between first appearance of two elements in a list. The program we are using is Haskell. I've been online for HOURS trying to look for help on how to start or direction to solve it. Please help!

This is how we are defining it:
gap :: (Eq a) => a -> a -> [a] -> Maybe Int

Here are some examples:

> gap 3 8 [1..10]
Just 5

> gap 8 3 [1..10]
Nothing

> gap 'h' 'l' "hello"
Just 2

> gap 'h' 'z' "hello"
Nothing

haskell

share|improve this question

edited Mar 26 at 2:41

Alec

25.6k44 gold badges4949 silver badges9696 bronze badges

asked Mar 26 at 2:13

Lola MartinezLola Martinez

1444 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  have a look at the Data.List library hackage.haskell.org/package/base-4.12.0.0/docs/Data-List.html , there are many different approaches to problems like this that depend on how much you want/have to write your own functions from scratch
  
  – Lorenzo
  Mar 26 at 2:19
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also remember you can look for function by types on Hoogle hoogle.haskell.org, so in this case you would want a function that, given an element and a list, returns its index: a -> [a] -> Maybe Int
  
  – Lorenzo
  Mar 26 at 2:21
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  What would gap 'a' 'a' "a" be? Just 0 or Nothing? How about gap 'a' 'a' "aa"? Just 0 or Just 1?
  
  – Joseph Sible
  Mar 26 at 2:25
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Possible duplicate of Gap function that returns the integer distance between first appearance of two elements in a list using either foldl or foldr.(Haskell)
  
  – Joseph Sible
  Mar 26 at 3:10
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @AaditMShah, I would reach for elemIndex for the second part.
  
  – dfeuer
  Mar 26 at 3:40
  

  
  
  
  

 | 
show 4 more comments

0

I have to find the distance between first appearance of two elements in a list. The program we are using is Haskell. I've been online for HOURS trying to look for help on how to start or direction to solve it. Please help!

This is how we are defining it:
gap :: (Eq a) => a -> a -> [a] -> Maybe Int

Here are some examples:

> gap 3 8 [1..10]
Just 5

> gap 8 3 [1..10]
Nothing

> gap 'h' 'l' "hello"
Just 2

> gap 'h' 'z' "hello"
Nothing

haskell

share|improve this question

edited Mar 26 at 2:41

Alec

25.6k44 gold badges4949 silver badges9696 bronze badges

asked Mar 26 at 2:13

Lola MartinezLola Martinez

1444 bronze badges

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  have a look at the Data.List library hackage.haskell.org/package/base-4.12.0.0/docs/Data-List.html , there are many different approaches to problems like this that depend on how much you want/have to write your own functions from scratch
  
  – Lorenzo
  Mar 26 at 2:19
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also remember you can look for function by types on Hoogle hoogle.haskell.org, so in this case you would want a function that, given an element and a list, returns its index: a -> [a] -> Maybe Int
  
  – Lorenzo
  Mar 26 at 2:21
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  What would gap 'a' 'a' "a" be? Just 0 or Nothing? How about gap 'a' 'a' "aa"? Just 0 or Just 1?
  
  – Joseph Sible
  Mar 26 at 2:25
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Possible duplicate of Gap function that returns the integer distance between first appearance of two elements in a list using either foldl or foldr.(Haskell)
  
  – Joseph Sible
  Mar 26 at 3:10
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @AaditMShah, I would reach for elemIndex for the second part.
  
  – dfeuer
  Mar 26 at 3:40
  

  
  
  
  

 | 
show 4 more comments

I have to find the distance between first appearance of two elements in a list. The program we are using is Haskell. I've been online for HOURS trying to look for help on how to start or direction to solve it. Please help!

This is how we are defining it:
gap :: (Eq a) => a -> a -> [a] -> Maybe Int

Here are some examples:

> gap 3 8 [1..10]
Just 5

> gap 8 3 [1..10]
Nothing

> gap 'h' 'l' "hello"
Just 2

> gap 'h' 'z' "hello"
Nothing

haskell

share|improve this question

edited Mar 26 at 2:41

Alec

25.6k44 gold badges4949 silver badges9696 bronze badges

asked Mar 26 at 2:13

Lola MartinezLola Martinez

1444 bronze badges

> gap 3 8 [1..10]
Just 5

> gap 8 3 [1..10]
Nothing

> gap 'h' 'l' "hello"
Just 2

> gap 'h' 'z' "hello"
Nothing

share|improve this question

edited Mar 26 at 2:41

Alec

25.6k44 gold badges4949 silver badges9696 bronze badges

asked Mar 26 at 2:13

Lola MartinezLola Martinez

1444 bronze badges

share|improve this question

edited Mar 26 at 2:41

Alec

25.6k44 gold badges4949 silver badges9696 bronze badges

asked Mar 26 at 2:13

Lola MartinezLola Martinez

1444 bronze badges

edited Mar 26 at 2:41

Alec

25.6k44 gold badges4949 silver badges9696 bronze badges

edited Mar 26 at 2:41

Alec

25.6k44 gold badges4949 silver badges9696 bronze badges

asked Mar 26 at 2:13

Lola MartinezLola Martinez

1444 bronze badges

asked Mar 26 at 2:13

Lola MartinezLola Martinez

1444 bronze badges

2 Answers
2

active

oldest

votes

3

An answer which is expected to run fast:

import Data.List (elemIndex)


gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x y ls = elemIndex y $ dropWhile ((/=) x) ls 

If you do not want to rely on Data.List then you have to write and add your own elemIndex'.

After Daniel Wagner's feedback an alternative:

gap' :: Eq a => a -> a -> [a] -> Maybe Int
gap' x y ls
 | null ls' = Nothing
 | head ls' == y = Nothing 
 | otherwise = elemIndex y ls'

 where

 ls' = dropWhile (t -> t /= x && t /= y) ls

share|improve this answer

edited Mar 26 at 12:35

answered Mar 26 at 9:41

Elmex80sElmex80s

2,38011 gold badge88 silver badges1616 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Same comment as I gave dfeuer: this does not appear to comply to the spec. Consider e.g. gap 'a' 'b' "bacb" -- this reports Just 2, which is clearly incorrect. (Nothing, Just 1, or Just (-1) all might be sensible choices depending on exactly how you interpret the spec.)
  
  – Daniel Wagner
  Mar 26 at 11:46
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DanielWagner thanks, see update
  
  – Elmex80s
  Mar 26 at 12:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah nice, upvoted!
  
  – Daniel Wagner
  Mar 26 at 13:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Just 2 seems correct to me @Daniel . The second example case indicates that the second argument should appear in the list at or beyond the first argument. On "bacb" then gap 'a' 'a' is Just 0, gap 'a' 'c' is Just 1, gap 'a' 'b' is Just 2
  
  – moonGoose
  Mar 26 at 16:40
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @ATayler The second example's return value is not a Just, so I don't see how you can use it to support returning a Just in some other case.
  
  – Daniel Wagner
  Mar 26 at 17:27
  

  
  
  
  

 | 
show 3 more comments

2

Here's how I'd do it:

import Data.List (elemIndex)

gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x1 x2 xs = do
 i <- elemIndex x1 xs
 j <- elemIndex x2 xs
 if i <= j
 then Just (j - i)
 else Nothing

Credits to dfeuer for reminding me of the elemIndex function.

share|improve this answer

answered Mar 26 at 4:00

Aadit M ShahAadit M Shah

51.8k1919 gold badges116116 silver badges234234 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This is gratuitously inefficient. dropWhile was the right way to start.
  
  – dfeuer
  Mar 26 at 5:24
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @dfeuer I think it is about as efficient as can be done given the spec. If I have correctly guessed what you have in mind for dropWhile, it will get the wrong answer for gap 'a' 'b' "bacb". (Whether the code in the answer gets it right or not is not clear -- the spec is a bit ambiguous, I have to say -- but the dropWhile version will definitely get it wrong!)
  
  – Daniel Wagner
  Mar 26 at 5:27
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DanielWagner, it seems a bit ambiguous, but I see what you mean. However, it's still not the best way, because it traverses the list twice, retaining memory. You could use a slightly different dropWhile invocation, or to avoid a redundant test work up something custom.
  
  – dfeuer
  Mar 26 at 5:33
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @dfeuer I agree. Without optimizations, this code is very inefficient. However, a smart compiler could most probably perform loop fusion to traverse the list only once. Either way, the first step is to get the program right, and the next step is to perform optimizations if necessary. If it doesn't matter that this code is inefficient, then let it be. However, if you find out that it's a performance bottleneck only then should you bother optimizing it.
  
  – Aadit M Shah
  Mar 26 at 11:08
  

  
  
  
  

add a comment | 

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3

An answer which is expected to run fast:

import Data.List (elemIndex)


gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x y ls = elemIndex y $ dropWhile ((/=) x) ls 

If you do not want to rely on Data.List then you have to write and add your own elemIndex'.

After Daniel Wagner's feedback an alternative:

gap' :: Eq a => a -> a -> [a] -> Maybe Int
gap' x y ls
 | null ls' = Nothing
 | head ls' == y = Nothing 
 | otherwise = elemIndex y ls'

 where

 ls' = dropWhile (t -> t /= x && t /= y) ls

share|improve this answer

edited Mar 26 at 12:35

answered Mar 26 at 9:41

Elmex80sElmex80s

2,38011 gold badge88 silver badges1616 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Same comment as I gave dfeuer: this does not appear to comply to the spec. Consider e.g. gap 'a' 'b' "bacb" -- this reports Just 2, which is clearly incorrect. (Nothing, Just 1, or Just (-1) all might be sensible choices depending on exactly how you interpret the spec.)
  
  – Daniel Wagner
  Mar 26 at 11:46
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DanielWagner thanks, see update
  
  – Elmex80s
  Mar 26 at 12:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah nice, upvoted!
  
  – Daniel Wagner
  Mar 26 at 13:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Just 2 seems correct to me @Daniel . The second example case indicates that the second argument should appear in the list at or beyond the first argument. On "bacb" then gap 'a' 'a' is Just 0, gap 'a' 'c' is Just 1, gap 'a' 'b' is Just 2
  
  – moonGoose
  Mar 26 at 16:40
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @ATayler The second example's return value is not a Just, so I don't see how you can use it to support returning a Just in some other case.
  
  – Daniel Wagner
  Mar 26 at 17:27
  

  
  
  
  

 | 
show 3 more comments

3

An answer which is expected to run fast:

import Data.List (elemIndex)


gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x y ls = elemIndex y $ dropWhile ((/=) x) ls 

If you do not want to rely on Data.List then you have to write and add your own elemIndex'.

After Daniel Wagner's feedback an alternative:

gap' :: Eq a => a -> a -> [a] -> Maybe Int
gap' x y ls
 | null ls' = Nothing
 | head ls' == y = Nothing 
 | otherwise = elemIndex y ls'

 where

 ls' = dropWhile (t -> t /= x && t /= y) ls

share|improve this answer

edited Mar 26 at 12:35

answered Mar 26 at 9:41

Elmex80sElmex80s

2,38011 gold badge88 silver badges1616 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Same comment as I gave dfeuer: this does not appear to comply to the spec. Consider e.g. gap 'a' 'b' "bacb" -- this reports Just 2, which is clearly incorrect. (Nothing, Just 1, or Just (-1) all might be sensible choices depending on exactly how you interpret the spec.)
  
  – Daniel Wagner
  Mar 26 at 11:46
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DanielWagner thanks, see update
  
  – Elmex80s
  Mar 26 at 12:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah nice, upvoted!
  
  – Daniel Wagner
  Mar 26 at 13:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Just 2 seems correct to me @Daniel . The second example case indicates that the second argument should appear in the list at or beyond the first argument. On "bacb" then gap 'a' 'a' is Just 0, gap 'a' 'c' is Just 1, gap 'a' 'b' is Just 2
  
  – moonGoose
  Mar 26 at 16:40
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @ATayler The second example's return value is not a Just, so I don't see how you can use it to support returning a Just in some other case.
  
  – Daniel Wagner
  Mar 26 at 17:27
  

  
  
  
  

 | 
show 3 more comments

An answer which is expected to run fast:

import Data.List (elemIndex)


gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x y ls = elemIndex y $ dropWhile ((/=) x) ls 

If you do not want to rely on Data.List then you have to write and add your own elemIndex'.

After Daniel Wagner's feedback an alternative:

gap' :: Eq a => a -> a -> [a] -> Maybe Int
gap' x y ls
 | null ls' = Nothing
 | head ls' == y = Nothing 
 | otherwise = elemIndex y ls'

 where

 ls' = dropWhile (t -> t /= x && t /= y) ls

share|improve this answer

edited Mar 26 at 12:35

answered Mar 26 at 9:41

Elmex80sElmex80s

2,38011 gold badge88 silver badges1616 bronze badges

import Data.List (elemIndex)


gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x y ls = elemIndex y $ dropWhile ((/=) x) ls 

gap' :: Eq a => a -> a -> [a] -> Maybe Int
gap' x y ls
 | null ls' = Nothing
 | head ls' == y = Nothing 
 | otherwise = elemIndex y ls'

 where

 ls' = dropWhile (t -> t /= x && t /= y) ls

share|improve this answer

edited Mar 26 at 12:35

answered Mar 26 at 9:41

Elmex80sElmex80s

2,38011 gold badge88 silver badges1616 bronze badges

answered Mar 26 at 9:41

Elmex80sElmex80s

2,38011 gold badge88 silver badges1616 bronze badges

answered Mar 26 at 9:41

Elmex80sElmex80s

2,38011 gold badge88 silver badges1616 bronze badges

2

Here's how I'd do it:

import Data.List (elemIndex)

gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x1 x2 xs = do
 i <- elemIndex x1 xs
 j <- elemIndex x2 xs
 if i <= j
 then Just (j - i)
 else Nothing

Credits to dfeuer for reminding me of the elemIndex function.

share|improve this answer

answered Mar 26 at 4:00

Aadit M ShahAadit M Shah

51.8k1919 gold badges116116 silver badges234234 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This is gratuitously inefficient. dropWhile was the right way to start.
  
  – dfeuer
  Mar 26 at 5:24
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @dfeuer I think it is about as efficient as can be done given the spec. If I have correctly guessed what you have in mind for dropWhile, it will get the wrong answer for gap 'a' 'b' "bacb". (Whether the code in the answer gets it right or not is not clear -- the spec is a bit ambiguous, I have to say -- but the dropWhile version will definitely get it wrong!)
  
  – Daniel Wagner
  Mar 26 at 5:27
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DanielWagner, it seems a bit ambiguous, but I see what you mean. However, it's still not the best way, because it traverses the list twice, retaining memory. You could use a slightly different dropWhile invocation, or to avoid a redundant test work up something custom.
  
  – dfeuer
  Mar 26 at 5:33
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @dfeuer I agree. Without optimizations, this code is very inefficient. However, a smart compiler could most probably perform loop fusion to traverse the list only once. Either way, the first step is to get the program right, and the next step is to perform optimizations if necessary. If it doesn't matter that this code is inefficient, then let it be. However, if you find out that it's a performance bottleneck only then should you bother optimizing it.
  
  – Aadit M Shah
  Mar 26 at 11:08
  

  
  
  
  

add a comment | 

2

Here's how I'd do it:

import Data.List (elemIndex)

gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x1 x2 xs = do
 i <- elemIndex x1 xs
 j <- elemIndex x2 xs
 if i <= j
 then Just (j - i)
 else Nothing

Credits to dfeuer for reminding me of the elemIndex function.

share|improve this answer

answered Mar 26 at 4:00

Aadit M ShahAadit M Shah

51.8k1919 gold badges116116 silver badges234234 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This is gratuitously inefficient. dropWhile was the right way to start.
  
  – dfeuer
  Mar 26 at 5:24
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @dfeuer I think it is about as efficient as can be done given the spec. If I have correctly guessed what you have in mind for dropWhile, it will get the wrong answer for gap 'a' 'b' "bacb". (Whether the code in the answer gets it right or not is not clear -- the spec is a bit ambiguous, I have to say -- but the dropWhile version will definitely get it wrong!)
  
  – Daniel Wagner
  Mar 26 at 5:27
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @DanielWagner, it seems a bit ambiguous, but I see what you mean. However, it's still not the best way, because it traverses the list twice, retaining memory. You could use a slightly different dropWhile invocation, or to avoid a redundant test work up something custom.
  
  – dfeuer
  Mar 26 at 5:33
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @dfeuer I agree. Without optimizations, this code is very inefficient. However, a smart compiler could most probably perform loop fusion to traverse the list only once. Either way, the first step is to get the program right, and the next step is to perform optimizations if necessary. If it doesn't matter that this code is inefficient, then let it be. However, if you find out that it's a performance bottleneck only then should you bother optimizing it.
  
  – Aadit M Shah
  Mar 26 at 11:08
  

  
  
  
  

add a comment | 

Here's how I'd do it:

import Data.List (elemIndex)

gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x1 x2 xs = do
 i <- elemIndex x1 xs
 j <- elemIndex x2 xs
 if i <= j
 then Just (j - i)
 else Nothing

Credits to dfeuer for reminding me of the elemIndex function.

share|improve this answer

answered Mar 26 at 4:00

Aadit M ShahAadit M Shah

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import Data.List (elemIndex)

gap :: Eq a => a -> a -> [a] -> Maybe Int
gap x1 x2 xs = do
 i <- elemIndex x1 xs
 j <- elemIndex x2 xs
 if i <= j
 then Just (j - i)
 else Nothing

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answered Mar 26 at 4:00

Aadit M ShahAadit M Shah

51.8k1919 gold badges116116 silver badges234234 bronze badges

answered Mar 26 at 4:00

Aadit M ShahAadit M Shah

51.8k1919 gold badges116116 silver badges234234 bronze badges

answered Mar 26 at 4:00

Aadit M ShahAadit M Shah

51.8k1919 gold badges116116 silver badges234234 bronze badges