Multiple filters in list comprehension in Erlang

Say I have a list that contains weather:

1> Weather = [{toronto, rain}, {montreal, storms}, {london, fog}, 
    {paris, sun}, {boston, fog}, {vancouver, snow}].
To get foggy places, I could do this:

2> FoggyPlaces = [X || {X, fog} <- Weather].
[london,boston]
Now I want to retrieve places that are both foggy and snowy. I tried this, but it retrieves only snowy places,

3> FoggyAndSnowyPlaces = [X || {X, fog} <- Weather, {X,snow} <- Weather].
[vancouver,vancouver]
where I was expecting [london,boston,vancouver].

How can I include multiple filters?
Last updated:5/22/2015 8:07:01 AM

1 Answers

Mayank Tripathi
Mayank Tripathi

FoggyAndSnowyPlaces = [X || {X, Y} <- Weather, (Y == fog) or (Y == snow)].
You are confusing generators (Pattern <- List) and filters (boolean conditions). Multiple generators work like nested loops in other languages, so in your 3> you get vancouver twice because the first generator produces two values.

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