When use || and | in if?

Hello,

when can I use and | in command if?

For example:

if a||b 
if a| b

Thank you

Answer 1

[Azzi Abdelmalek]

a||b will return 1 if the first expression a is true, without evaluating the second expression b

Example

2==2 || hhh   % even hhh is not defined Matlab will not evaluate it, because the first expression 2==2 is true

a&&b will return 0 if the first expression a is false without evaluating the second expression b

[john]

OK Azzi,

what about a|b? What is different?

[Azzi Abdelmalek]

a | b and a||b should give the same result, the difference is: Matlab when evaluating a|b, a and b are both evaluated, try

2==2 | h

In this case the first expression is 2==2, which is true, we know that if one of the two expression is true, then the result is true, without evaluating the second expression, but by using | Matlab will evaluate the second expression which is h, in our case h is not defined, that's why you will get an error.

[john]

ok, thank you

[john]

but result :

if 2==2 || h
   fff=1
end

is the same like:

if 2==2 | h
   fff=1
end

but for this I get error:

if h | 2==2
       fff=1
  end

[Azzi Abdelmalek]

Because Matlab, in both case (| or | | ) evaluate the first expression h which is not defined.

[Azzi Abdelmalek]

It's better to use | |, this can make your code faster, when the first expression is true, Matlab doesn't need to evaluate the second one.

Answer 2

[dpb]

Depends entirely on the purpose...the double logical operators short-circuit and return only a scalar whereas the single ones are point-by-point operators over the full dimension of the two operands and return a matrix of the same size.

doc relop

has further details and info

[john]

a=3;
b=2;

What is better ?

if a==3 || b==2
end;

Or

if a==3 | b==2
end;

[Jan]

@John: When a and b are scalars, both versions are equivalent. But the first one || is slightly faster (nano-seconds for scalar operands...), when the first expression is true already. When a and/or b is a vector, you need the , which is equivalent to |or(a==3, b==2). But then the vector expression in the if command is tricky, because implicitly this is performed:

expr = or(a==3, b==2);
if all(expr) && ~isempty(expr)  ...

This is at least confusing or can even be a bug, if this behavior is not intended.