CODEWITHSUNDEEP
haskellrecursion
user53073
user53073

Reputation: 51

Simple Recursion in Haskell

I am trying to understand recursion in Haskell. I want to write a simple recursive function with the following properties: if x > 50 then rec(x) = x - 5 and rec(x) = rec(rec(x+10)) otherwise. This is what I came up with:

rec x = if x > 50
    then x-5
    else rec (rec (x+10))

The not recursive part seems to work fine, but the recursion doesn't. Every number less than 50 just return 50. For example rec 60 = 55 rec 40 = 50 rec 25 = 50

Any suggestions on how to fix my code would be appreciated.

Upvotes: 1

Views: 127

Answers (1)

gallais
gallais

Reputation: 12093

For x <= 50, your function is equivalent to:

rec' x = if x `mod` 5 == 0
         then 50
         else 45 + x `mod` 5

This can be checked by running a simple test:

all (\ x -> rec x == rec' x) [0..50]

As a consequence, you need to feed an input which is not a multiple of 5 to get an answer distinct from 50.

Upvotes: 3

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