CODEWITHSUNDEEP
if-statementerlang
Abhimanyu
Abhimanyu

Reputation: 369

How to make the equivalent of a C-style if-else statement in Erlang?

How do I make the equivalent of a C-style if-else in Erlang?

 if(a == b) {
   expression1
 } else {
   expression2 
 }

Upvotes: 23

Views: 20404

Answers (4)

Raoul Supercopter
Raoul Supercopter

Reputation: 5114

if
    a == b ->
        expr1;
    true ->
        exprElse
end

There is no real else in Erlang, you just match with true. More information at the Erlang documentation.

You have to pay close attention to the ';' terminator :

if
    a == b ->
        clauseN,
        expr1;
    cond2 ->
        clause1,
        clause2,
        expr2;
    cond3 ->
        expr3;
    true ->
        exprElse
end

It's not a copy-paste friendly language.

Upvotes: 33

Hynek -Pichi- Vychodil
Hynek -Pichi- Vychodil

Reputation: 26121

if
    A =:= B ->
        expr1;
    true ->
        expr2
end

or

case A =:= B of
    true -> expr1;
    false -> expr2
end

or

case A of
    B -> expr1;
    _ -> expr2
end

or

  foo(A,B), ...

...
foo(X,X) -> expr1;
foo(_,_) -> expr2.

Upvotes: 15

marcc
marcc

Reputation: 12399

Another option:

case a of
    b -> expr1;  %% true
    _ -> expr2   %% false
end.

Upvotes: 8

Jonas
Jonas

Reputation: 19642

Pattern matching is one of Erlangs many strengths and it can often be used instead of if statements. It is often more readable to pattern match on function arguments or use a case expression (which also is pattern matching).

Suppose you have a function some_function which takes two arguments. If the arguments are equal you evaluate expression1, otherwise you evaluate expression2. This can be expressed in erlang as: 

some_function(A, A) -> expression1;
some_function(A, B) -> expression2.

It is quite amazing how much you can achieve with this kind of simple pattern matching. With guard clauses you can express more complicated patterns, e.g.,

some_function(A, B) when A < B -> expression3.

Another possibility is to use a case .. of expression. Your particular example would translate to

case a == b of
    true -> expression1;
    false -> expression2
end.

(In this case expression2 would always be evaluated since a and b are atoms and never equal to each other. Variables in Erlang are upper case.)

You can read more about Erlang expressions here.

Upvotes: 19

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