Erlang equivalent to if else

问题:

I have 2 parts of code I want to execute. Both are conditionals

if Value1 < N do something 

else if Value1 >= N do something

if Value2 < N do something 

else if Value2 >= N do something

I want at one statement of each to execute.

How does the if work in erlang? there is no else. I use multiple guards, but that looks like I have 4 if statements. in groups of 2.

if some condition   
code;

if other condition  
code

end.

I get a syntax error.

回答1:

The form for an if is:

if
    <guard 1> -> <body1> ;
    <guard 2> -> <body2> ;
    ...
end

It works trying the guards in if-clauses in top-down order (this is defined) until it reaches a test which succeeds, then the body of that clause is evaluated and the if expression returns the value of the last expression in the body. So the else bit in other languages is baked into it. If none of the guards succeeds then an if_clause error is generated. A common catch-all guard is just true which always succeeds, but a catch-all can be anything which is true.

The form for a case is:

case <expr> of
    <pat 1> -> <body1> ;
    <pat 2> -> <body2> ;
    ...
end

It works by first evaluating and then trying to match that value with patterns in the case-clauses in op-down order (this is defined) until one matches, then the body of that clause is evaluated and the case expression returns the value last expression in the body. If no pattern matches then a case_clause error is generated.

Note that if and case are both expressions (everything is an expression) so they both must return values. That is one reason why there is no default value if nothing succeeds/matches. Also to force you to cover all options; this is especially important for case. if is just a degenerate case of case so it inherited it. There is a bit of history of if in the Erlang Rationale which you can find on trapexit.org under user contributions.

回答2:

Erlang doesn't allow you to have an if without a true statement option. Whether or not this is something that is a true statement or an actual true is up to you, but it is commonplace to have your true be the else in other languages.

if 
    some_condition -> some_code;
    some_other_condition -> some_other_code;
    true -> else_code
end.

See the "What the If?" section on this page for more on this.

回答3:

Remember if in Erlang has a value to return, and it's an expression. It's not that if like in C or Java.

If you want to do something for a value, the code should be something like this;

if
  % do something and get the value
  X >= Val -> Something;
  % for doing something otherwise and get the value
  true -> Else_than_the_Something 
end.

See Section for the if expression of Erlang Reference Manual for the further details.

回答4:

First of all, I recommend you to get used to use 'case' statement, because of 'if' conditions restricted to guard expressions:

case custom_call(A) of
  1 -> do1(A);
  2 -> do2(A)
end.

There is one more way to do conditional execution besides 'if' and 'case' that works starting from R13:

  1> N =10.
  10
  2> ((N > 10) andalso more).      
  false
  3> ((N == 10) andalso equals).
  equals