# Comprehensions in Julia

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One of Haskell’s features that I really liked was list comprehensions, so I was very pleased to discover how nice Julia’s comprehensions are!

I was curious how they compared, so I went through “Chapter 5: List Comprehensions” in Graham Hutton’s “Programming Haskell” book. I think you’ll agree that Julia’s comprehensions compare favorably.

[ x^2 | x <- [1..5] ] [ x^2 for x = 1:5 ]
[ (x,y) | x <- [1, 2, 3], y <- [4, 5] ] [ (x,y) for x = 1:3 for y = 4:5 ]
[ (x,y) | x <- [1..3], y <- [x..3 ]] [ (x,y) for x = 1:3 for y = x:3 ]
concat xss = [ x | xs <- xss, x <- xs ] concat(xss) = [ x for xs in xss for x in xs ]
firsts ps = [ x | (x,_) <- ps ] firsts(ps) = [ x for (x,_) in ps ]
length xs = sum[ 1 | _ <- xs ] length(xs) = sum(1 for _ in xs)
factors n = [ x | x <- [1..n], n `mod` x == 0 ] factors(n) = [ x for x = 1:n if n % x == 0 ]
prime n = factors n == [1, n] prime(n) = factors(n) == [1, n]
primes n = [ x | x <- [2..n], prime x ] primes(n) = [ x for x = 2:n if prime(x) ]
find k t = [ v | (k', v) <- t, k == k' ] find(k,t) = [ v for (k1,v) in t if k == k1 ]
pairs xs = zip xs (tail xs) pairs(xs) = zip(xs, xs[2:end])
sorted xs = and [x <= y | (x,y) <- pairs xs ] sorted(xs) = all(x <= y for (x,y) in pairs(xs))
positions x xs = [i | (x',i) <- zip xs [0..n], x == x']
where n = length xs - 1
positions(x,xs) = [ i for (x1,i) in zip(xs, 1:(length(xs)))
if x == x1 ]
lowers xs = length [x | x <- xs, isLower x ]

lowers(xs) = length([x for x in collect(xs)
if islowercase(x)])
count x xs = length [x' | x' <- xs, x == x'] count(x,xs) = length([x1 for x1 in collect(xs) if x == x1])