How to generate one lazy sequence from a nested for loop in Clojure

Question

This is a question about how to properly collect the results from a nested for loop in Clojure. Suppose you want to create a sequence of all vectors [i j] where 0<=j<i<4

The following code

(for [i (range 1 4)] 
  (for [j (range i)] 
     [i j]
  )
)

produces

(([1 0]) ([2 0] [2 1]) ([3 0] [3 1] [3 2]))

but what I really want to get is

([1 0] [2 0] [2 1] [3 0] [3 1] [3 2])

What is the right way to do this?

Notice that I'm not interested in this specific sequence. My purpose here is to learn how to collect results from a nested for loop, which I need for a more complex problem.

FYI for in Clojure is not like for in C or Java. Having a for nested in an other for is rather unusual. Clojure for is a list comprehension function, it helps you create a list. See Python list comprehension for exemples. — kawas44
– kawas44, Commented Feb 2, 2016 at 8:29
Thanks. I tried the approach you are suggesting and it it really cleaner. — Adi Levin
– Adi Levin, Commented Feb 2, 2016 at 8:30

Oin · Accepted Answer · 2016-01-31 22:43:01Z

6

Don't nest two loops, rather use one loop with two iterators:

(for [i (range 1 4)
      j (range i)] 
   [i j])

answered Jan 31, 2016 at 22:43

Oin

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Comments

Community · Accepted Answer · 2017-05-23 11:45:13Z

3

Assuming you have to use nested for loops, apply concat is the preferred way to flatten a sequence by one level

=> (apply concat 
         (for [i (range 1 4)] 
           (for [j (range i)] 
             [i j])))

([1 0] [2 0] [2 1] [3 0] [3 1] [3 2])

@Oin's solution is always preferable, unless the inner loop depends on the outer loop.

edited May 23, 2017 at 11:45

CommunityBot

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answered Jan 31, 2016 at 22:42

munk

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1 Comment

Adi Levin Over a year ago

Thanks. This is exactly what I was looking for.

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