List comprehensions and tuples in Julia

Question

I am trying to do in Julia what this Python code does. (Find all pairs from the two lists whose combined value is above 7.)

#Python    
def sum_is_large(a, b):
    return a + b > 7

l1 = [1,2,3]
l2 = [4,5,6]
l3 = [(a,b) for a in l1 for b in l2 if sum_is_large(a, b)]
print(l3)

There is no if for list comprehensions in Julia. And if I use filter(), I'm not sure if I can pass two arguments. So my best suggestion is this:

#Julia
function sum_is_large(pair)
    a, b = pair
    return a + b > 7 
end

l1 = [1,2,3]
l2 = [4,5,6]

l3 = filter(sum_is_large, [(i,j) for i in l1, j in l2])
print(l3)

I don't find this very appealing. So my question is, is there a better way in Julia?

Answer 1

Guards (if) are now available in Julia v0.5 (currently in the release-candidate stage):

julia> v1 = [1, 2, 3];

julia> v2 = [4, 5, 6];

julia> v3 = [(a, b) for a in v1, b in v2 if a+b > 7]
3-element Array{Tuple{Int64,Int64},1}:
 (3,5)
 (2,6)
 (3,6)

Note that generators are also now available:

julia> g = ( (a, b) for a in v1, b in v2 if a+b > 7 )
Base.Generator{Filter{##18#20,Base.Prod2{Array{Int64,1},Array{Int64,1}}},##17#19}(#17,Filter{##18#20,Base.Prod2{Array{Int64,1},Array{Int64,1}}}(#18,Base.Prod2{Array{Int64,1},Array{Int64,1}}([1,2,3],[4,5,6])))

Answer 2

Another option similar to the one of @DanGetz using also Iterators.jl:

function expensive_fun(a, b)
    return (a + b)
end

Then, if the condition is also complicated, it can be defined as a function:

condition(x) = x > 7

And last, filter the results:

>>> using Iterators
>>> result = filter(condition, imap(expensive_fun, l1, l2))

result is an iterable that is only computed when needed (inexpensive) and can be collected collect(result) if required.

The one-line if the filter condition is simple enough would be:

>>> result = filter(x->(x > 7), imap(expensive_fun, l1, l2))

Note: imap works natively for arbitrary number of parameters.