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stringerlangstring-matching
David S
David S

Reputation: 187

Erlang: Matching strings in guard statement

Started working with erlang quite recently and ran into the problem above, how do you go about comparing two strings in a guard statement? Tried the string:equal(x,y) method but couldn't get it to work inside a guard.

Upvotes: 2

Views: 6687

Answers (3)

legoscia
legoscia

Reputation: 41568

You don't need the function string:equal/2 to compare strings; you can use the operators == or =:=, which are allowed in guard tests. For example:

foo(A, B) when A =:= B ->
    equal;
foo(_, _) ->
    not_equal.

Though in most cases you'd want to use pattern matching instead, as described in the other answer.

NB: As of Erlang/OTP 20.0, string:equal(A, B) is no longer equivalent to A =:= B. string:equal/2 now operates on grapheme clusters, and there are also string:equal/3 and string:equal/4 that can optionally ignore case when comparing and do Unicode normalisation. So you need to understand what you mean by "equal" before settling on a comparison method.

Upvotes: 5

mpm
mpm

Reputation: 3584

You could use pattern matching like this:

are_the_same(A, A) ->
  true;
are_the_same(_, _) ->
  false.

In first clause both arguments are named A which will result in them being pattern matched against each other. Or to be exact first argument will be bind to A variable with use of = operator, and than second argument will be bind to A variable with = operator, but since A is bound already it will be treated as "comparision". You can read more about this in docs.

And of course you could write write first clouse with use of guard like:

are_the_same(A, B) when A =:= B ->

Upvotes: 9

Soup in Boots
Soup in Boots

Reputation: 2392

The functions you can use in guards are limited because of the nature of Erlang's scheduling; specifically, Erlang aims to avoid side-effects in guard statements (e.g., calling to another process) because guards are evaluated by the scheduler and do not count against reductions. This is why string:equal does not work.

That being said, you can use Erlang's pattern matching to match strings. Please bear in mind the use of strings as lists, binaries, or iolists (nested lists/binaries) in Erlang, and make sure you're testing/passing strings of the right type (iolists are particularly hard to pattern match and are usually best handled with the re module, or converting them to binaries via iolist_to_binary).

For example, say we want a function that tests to see if a string begins with "foo":

bar("foo" ++ _Rest) -> true;
bar(<<"foo", Rest/binary>>) -> true;
bar(_Else) -> false.

If you just want to test for a particular string, it's even easier:

bar("foo") -> true;
bar(<<"foo">>) -> true;
bar(_Else) -> false.

Upvotes: 4

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