# Difference between revisions of "99 questions/Solutions/24"

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< 99 questions | Solutions

Remi.berson (talk | contribs) (Last solution modified to return IO [Int] and using global number generator) |
Amackillop (talk | contribs) (Provided solutions using nub may not terminate.) |
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+ | Note: With the above solutions using nub, the function will not terminate if m < n. I suggest adding a guard to protect against this. |
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[[Category:Programming exercise spoilers]] |
[[Category:Programming exercise spoilers]] |

## Latest revision as of 00:09, 7 April 2019

Lotto: Draw N different random numbers from the set 1..M.

```
import System.Random
diff_select :: Int -> Int -> IO [Int]
diff_select n to = diff_select' n [1..to]
diff_select' 0 _ = return []
diff_select' _ [] = error "too few elements to choose from"
diff_select' n xs = do r <- randomRIO (0,(length xs)-1)
let remaining = take r xs ++ drop (r+1) xs
rest <- diff_select' (n-1) remaining
return ((xs!!r) : rest)
```

The random numbers have to be distinct!

In order to use randomRIO here, we need import module System.Random.

As can be seen, having implemented problem 23, rnd_select, the solution is trivial.

```
diff_select n to = rnd_select [1..to] n
```

Alternative solution:

```
diffSelect :: Int -> Int -> IO [Int]
diffSelect n m = do
gen <- getStdGen
return . take n $ randomRs (1, m) gen
```

(Note that this doesn't really solve the problem, since it doesn't generate *distinct* numbers).

Using nub from Data.List:

```
diff_select :: Int -> Int -> StdGen -> [Int]
diff_select n m = take n . nub . randomRs (1, m)
```

Or without giving StdGen as argument and returning IO [Int]:

```
import System.Random
import Data.List
import Control.Applicative
diff_select :: Int -> Int -> IO [Int]
diff_select n m = take n . nub . randomRs (1, m) <$> getStdGen
```

Note: With the above solutions using nub, the function will not terminate if m < n. I suggest adding a guard to protect against this.