How to implement if-else

问题:

How do I implement this if-else expression in erlang?

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

回答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.

回答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.

回答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.

回答4:

Another option:

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