if statement with 'or' operator gives different results when conditions are swapped - matlab

Question

I'm using strfind with an 'or' comparison like so:
name='hello';
if strfind(name,'hello') | strfind(name,'hi')
disp('yes!')
end
>> yes!
The if statement must evaluate as true, since yes! is displayed.
In contrast, MATLAB doesn't return yes! if the statements are swapped:
if strfind(name,'hi') | strfind(name,'hello')
disp('yes!')
end
Why?

Answer 1

This is because short-circuiting. Short-circuited logical operators are a thing to speed up code. You can have
if veryShort | superlongComputation
so what MATLAB does is first evaluate veryShort and if it is true, then no need to evaluate the second one! The if condition is already met.
In your case strfind(name,'hello') returns 1, but strfind(name,'hi') returns [].
In the first example, as the first thing evaluated returns 1, you get to the display. However in the second case, it returns [], therefore MATLAB evaluates the second thing in the if, and returns 1. Then MATLAB applies the or operations where [] | 1 is an 0x0 empty logical array, so the if is not true.
Note, generally you want to use || to enforce short-circuiting, but | also does it, if it is inside a while or an if:
https://uk.mathworks.com/matlabcentral/answers/99518-is-the-logical-operator-in-matlab-a-short-circuit-operator

Answer 2

Both of the following conditions are empty []:
name='hello';
strfind(name,'hello') | strfind(name,'hi'); % = []
strfind(name,'hi') | strfind(name,'hello'); % = []
As referenced in Ander's answer, the | operator is using short circuiting to skip evaluation of the second condition if the first is false (or empty).
Some quick debugging will give you better understanding if we ignore short-circuiting:
strfind(name,'hi'); % = []
strfind(name,'hello'); % = 1
In both cases you are doing "if empty or non-zero", which is empty and "if []" is false (that conditional statement won't be executed).
What you want to use to be explicit is something like this:
if ~isempty(strfind(name, 'hello')) & ~isempty(strfind(name, 'hi'))
disp('yes!')
end
Here, we guarantee that everything being evaluated in the if statement is a Boolean variable, not empty or an index like strfind returns, so unexpected results are less likely.
There are simpler methods, like using strcmp or ismember if your strings should match exactly. Or contains if you have R2016b or newer:
if contains('hello', {'hello','hi'})
disp('yes!');
end