Études for Erlang

Étude 7-2: List Comprehensions and Pattern Matching

Is it possible to use pattern matching inside a list comprehension? Try it and find out.

Presume you have this list of people’s names, genders, and ages:

People = [{"Federico", $M, 22}, {"Kim", $F, 45}, {"Hansa", $F, 30},
{"Tran", $M, 47}, {"Cathy", $F, 32}, {"Elias", $M, 50}].

Part One

In erl (or in a module, if you prefer), write a list comprehension that creates a list consisting of the names of all males who are over 40. Use pattern matching to separate the tuple into three variables, and two guards to do the tests for age and gender.

Part Two

When you use multiple guards in a list comprehension, you get the moral equivalent of and for each condition you are testing. If you want an or condition, you must test it explicitly. Write a list comprehension that selects the names of all the people who are male or over 40. You will need one guard with an or; you may also use orelse.

Note

Because or has higher priority than comparison operators like < and ==, an expression like X > 5 or X < 12 will generate an error, as Erlang interprets it to mean X > (5 or X) < 12. Use parentheses to force the correct evaluation: (X > 5) or (X < 12). If you use orelse, which has a lower priority than the comparison operators, you don’t need the parentheses, though it doesn’t hurt to have them. Another advantage of orelse is that it doesn’t do any unnecessary comparisons.