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<  Erlang  ~  List Comprehensions: [] gives me an unexpected result
bluefly
Posted: Sun Jan 13, 2008 5:59 pm Reply with quote
User Joined: 06 Jan 2008 Posts: 10
This may reflect my lack of comprehension of list comprehensions, but here is something I did not expect... First, a normal list comprehension:
Code:
erl> [ [X, Y] || X <- [1, 2, 3], Y <- [4] ].
[[1,4],[2,4],[3,4]]
I asked myself, what if Y was an empty list? I expect [1, 2, 3]. Instead:
Code:
erl> [ [X , Y] || X <- [1, 2, 3], Y <- [] ].
[]
Similarly, if I swap who is empty,
Code:
erl> [ [X , Y] || X <- [], Y <- [1, 2, 3] ].
[]
I tried framing my results as tuples and received the same empty set result. Why is my result []? Is this a desired behavior?
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Mazen
Posted: Wed Jan 16, 2008 9:34 am Reply with quote
User Joined: 20 Jul 2006 Posts: 164 Location: London
I don't know for a fact if this is good or bad but it is logical and since logic is considered a good thing then I guess it is desirable Smile

The number of possible combinations is LengthX * LengthY and if LengthY = 0 then I guess the whole thing is 0. If you would do the equivalent thing in a for each loop it would look somewhere along the line of this:
Code:

foreach X in ListX
    foreach Y in ListY
       Store {X,Y}
    end
end


I.e. neither the outer or inner list must be of length 0.

I hope I'm making sense hehe

/Mazen
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