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

20,262

Solution 1

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.

Solution 2

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.

Solution 3

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.

Solution 4

Another option:

case a of
    b -> expr1;  %% true
    _ -> expr2   %% false
end.
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Abhimanyu
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Updated on July 09, 2022

Comments

  • Abhimanyu
    Abhimanyu over 1 year

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

     if(a == b) {
       expression1
     } else {
       expression2 
     }
    
  • mrjohnsly
    mrjohnsly almost 15 years
    I'd also suggest using =:= instead of ==. This is a really good answer - new erlang programmers should always use case until they figure out why the if statement should have been the when statement.
  • Tommy
    Tommy about 8 years
    isn't the equality operator in Erlang =:=?
  • Talespin_Kit
    Talespin_Kit about 8 years
    Also note that it is an error if none of the conditions match, hence the true clause as the end as a fall through.
  • Electric Coffee
    Electric Coffee about 8 years
    @Tommy Erlang also has ==, == means equal to, =:= means exactly equal to, kinda like JavaScript's ===

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