问题
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?
回答1:
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.
来源：https://stackoverflow.com/questions/14321265/multiple-filters-in-list-comprehension-in-erlang