# Difference between revisions of "Talk:99 questions/11 to 20"

From HaskellWiki

ScottWolchok (talk | contribs) (problem 20 correction?) |
(suggest modifying problem 15) |
||

Line 1: | Line 1: | ||

+ | The prototype for repli in problem 15 is |
||

+ | <haskell> |
||

+ | repli :: [a] -> Int -> [a] |
||

+ | </haskell> |
||

+ | |||

+ | Because the second parameter is the number of times to replicate, it discourages the use function composition. I mean that if you swapped the parameters you could write it pointfree: |
||

+ | <haskell> |
||

+ | repli :: Int -> [a] -> [a] |
||

+ | repli n = foldr (\ x xs -> replicate n x ++ xs) [] |
||

+ | </haskell> |
||

+ | |||

+ | This would also match the way replicate is defined: |
||

+ | <haskell> |
||

+ | replicate :: Int -> a -> [a] |
||

+ | </haskell> |
||

+ | |||

+ | So, I suggest modifying problem 15 by swapping the parameters to repli in the example and the solution. |
||

+ | |||

+ | ---- |
||

+ | |||

I made an edit to this page. I removed the following solution to problem 18: |
I made an edit to this page. I removed the following solution to problem 18: |
||

Line 11: | Line 31: | ||

Thanks to pixel for pointing this out.</haskell> |
Thanks to pixel for pointing this out.</haskell> |
||

+ | ---- |
||

The solution to problem 20 seems to be using 0-based indexing, whereas the question called for 1-based indexing in the other languages. This can be easily fixed: |
The solution to problem 20 seems to be using 0-based indexing, whereas the question called for 1-based indexing in the other languages. This can be easily fixed: |

## Revision as of 09:51, 24 April 2008

The prototype for repli in problem 15 is

```
repli :: [a] -> Int -> [a]
```

Because the second parameter is the number of times to replicate, it discourages the use function composition. I mean that if you swapped the parameters you could write it pointfree:

```
repli :: Int -> [a] -> [a]
repli n = foldr (\ x xs -> replicate n x ++ xs) []
```

This would also match the way replicate is defined:

```
replicate :: Int -> a -> [a]
```

So, I suggest modifying problem 15 by swapping the parameters to repli in the example and the solution.

I made an edit to this page. I removed the following solution to problem 18:

```
slice xs i j = [xs!!(i-1)..xs!!(j-1)]
```

Counter-example:

```
slice [1,3,6,3,1,6,7,8,3,2,4,76,8] 4 5 == []
Thanks to pixel for pointing this out.
```

The solution to problem 20 seems to be using 0-based indexing, whereas the question called for 1-based indexing in the other languages. This can be easily fixed:

```
removeAt :: Int -> [a] -> (a, [a])
removeAt k l = (elementAt l k, take (k-1) l ++ drop k l)
```

using elementAt from a previous problem.

or if you want to express that 1-based indexing is silly,

```
removeAt n+1 xs = (xs!!n,take n xs ++ drop (n+1) xs)
```