while and return in a loop (Haskell) [duplicate]

Question

How to code the following pseudo-code elegantly in Haskell?

for (i from 0 to 100):
    for (j from 0 to 100):
        k=0
        while ( f(i,j,k) >0 ):
            return (i,j,k)
            k+=1

where f is an unimportant function of i,jand k.

So it should output something like this: [(0,0,0),(0,0,1)..], a list with each element a tuple formed by i,jandk. (It's fine if it were of the form [[0,0,0],[0,0,1]...] instead of tuples.)

user1804599 · Accepted Answer · 2015-10-13 18:40:34Z

3

Assuming with return you actually mean Python-like yield, otherwise the algorithm wouldn't make sense.

Using do notation, this is very straightforward:

do
    i <- [0..100]
    j <- [0..100]
    k <- takeWhile (\k -> f i j k > 0) [0..]
    return (i, j, k)

answered Oct 13, 2015 at 18:40

user1804599

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melpomene · Accepted Answer · 2015-10-13 18:42:47Z

2

Untested:

[ (i,j,k) |
  i <- [0 .. 100],
  j <- [0 .. 100],
  k <- takeWhile (\k -> f i j k > 0) [0 ..] ]

This is a list comprehension that loops over i/j in the obvious way, and uses takeWhile to limit k by the result of f.

answered Oct 13, 2015 at 18:42

melpomene

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