CODEWITHSUNDEEP
if-statementerlangboolean
Chathura Jayawardane
Chathura Jayawardane

Reputation: 47

ERLANG + IF + BOOLEAN

Boolean x = true;
int y = 1;
int z = 1;
if(y ==1 && x == true){
    z++;
    x = false;
}
else{
    z--;
    x = true;
}

I want to do this in erlang.. How Can I do this? (Please be noted that this is a example code. What I want to do are two conditions in one if statement & this boolean functionality). Any help is welcomed.Actually z-- & z++ are not needed.

Upvotes: 1

Views: 1873

Answers (3)

7stud
7stud

Reputation: 48649

Boolean x = true;
...
    x = false;

That's never going to happen in erlang. Erlang variables can only be assigned to once, which also means that you can't do var++ and var-- in erlang.

You can use what are called guards in the head of a function clause to employ boolean filters on the function arguments. In a guard, a comma acts like && in other languages and a semi-colon acts like ||.

-module(my).
-compile(export_all).

            guard: Y==1 && X
         +----------+
         |          |  
         |          |
go(X, Y) when Y==1, X ->
      false;
go(_, _) ->
      true.

go_test() ->
    false = go(true, 1),
    true = go(false, 1),
    true = go(true, 20),
    all_tests_passed.

In the shell:

~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.3  (abort with ^G)

1> c(my).       
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}

2> my:go_test().
all_tests_passed

3>

Per Wotjek Surowka, go/2 can be written more simply without guards. Because there is only one combination of arguments where the result is false, while all other combinations of arguments produce a true result, you can write:

go(true, 1) ->
    false;
go(_, _) ->
    true.

Upvotes: 2

Ming L.
Ming L.

Reputation: 401

If you program functional language, especially Erlang, please avoid even thinking about if statement, although Erlang case supports that.

Always think of pattern match.

Upvotes: 2

Eugen Dubrovin
Eugen Dubrovin

Reputation: 916

You may use case statement

*this example will always returl tuple with two elements {Z, X}

case_statement() ->

  X  = true,
  Y  = 1,
  Z  = 1,

  case {Y, X} of
    {1, true} ->
      {Z + 1, false};
    _ ->
      {Z -1, true}
  end.

If you need to use exact 'if' statement here is the example

if_statement() ->

  X  = true,
  Y  = 1,
  Z  = 1,

  if
    Y =:= 1 andalso X =:= true ->
      {Z +1, false};
    true ->
      {Z -1, true}
  end.

Upvotes: 3

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