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assume there are dynamic number of nested for loop to output all combinations

in this example, there are 3 nested forloop, to generate a list of all combination such as [1,2,3], [1,3,5] etc.

if there are N nested forloop, how to use recursion to do in haskell?

pseudo code

for i from 1 to 5 do
     for j from 1 to 10 do
          if i < j then
          for k from 1 to 50 do
               if j < k then
                  list1 :: [i,j,k]

unfinished forloop has compile error

forloop :: Integer -> Integer -> [a]
forloop n m
    | n == 1 = 1
    | otherwise =  if n > m 
                     then [(forloop n-1 m)] ++ [n]
                     else []

expect a recursive version of function which can be saved in redis like Action type of .net framework

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  • FYI, your compilation error is probably caused by your syntactic confusion. Function application has the highest precedence, so forloop n-1 m is parsed as (forloop n) - (1 m). Since forloop n is not a number, and 1 is not a function, the type checker will complain. What you meant there was forloop (n-1) m. Commented Oct 5, 2014 at 18:10

2 Answers 2

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In Haskell, in general, you use the data structures to drive the flow, rather than control structures (like for-loops). In your particular example, a simple list comprehension would address the same case with a single line of code. As you can see below, this single line of code is also very clear in its intentions:

[(i,j,k) | i <- [1..5], j <- [1..10], k <- [1..50], i < j, j < k]

If you didn't want to use list comprehensions, you could still create a very succinct version using structured recursion or, as the last resort, explicit recursion.

I hope this helps.

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7 Comments

I would move i<j before k<-[1..50]. This improves efficiency and is closer to the original imperative loop.
this version need to hard code i, j, k, but it can be more for example, 5 nested, 7 nested, how about function version of it? if want to save function in redis like saving Action type of .net framework
Just define the upper boundary of the list as a variable, like [1..x], where x is defined somewhere else in your program. There is no special reason for the list boundaries to be constants. And you can define the step too, as in [start, step .. end].
how to save this into hedis ?
Is there a reason to test i<j, j<k instead of using the intended ranges, j<-[(i+1)..10], k <- [(j+1)..50]? The latter is easier to understand and also more than twice as efficient.
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Suppose you have a list of range end for each level of nesting. Consider how you are going to generate the lists, if you have a solution for the shorter list of ranges.

solve :: [Int] -> Int -> [[Int]]
solve [] _ = [[]]
solve (r:t) i = [j:s | j <- [i+1..r], s <- solve t j]
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after run let aaa3 = solve [] 5 print aaa3 it return empty list, could you show how to use this function? I have written a recursive version of function which has compile error, expect to save it in redis like Action type of .net framework
@Khovanov it looks like a homework. I thought it would help if you tried to understand how this should be used. An example use is solve [5,10,50] 0 to produce the lists for the sample problem in the post.

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