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Topic: Very strange thing when using if else structure
Replies: 1   Last Post: Feb 27, 2013 11:59 AM

 Steven Lord Posts: 18,038 Registered: 12/7/04
Re: Very strange thing when using if else structure
Posted: Feb 27, 2013 11:59 AM

"Manalap " <mujyhane@gmail.com> wrote in message
news:kgl8cs\$2o0\$1@newscl01ah.mathworks.com...
> I encountered a very strange problem when using if else structure.
>
> Simply put, this problem is like the following.
>
> If I code
>
> if x >3
> ....
> elseif x<=3
> ....
> end
>
>
> then I get a wrong result, but where as if I code
>
> if x>3
> ...
> else
> ...
> end
>
>
> then I get the correct result that I expected through symbolic reasoning.
>
> Aren't these two the same thing?

Not if x is nonscalar or x is NaN.

> Can these two be different?

Yes.

x = [1 2 3 4 5];
x > 3
x <= 3

http://www.mathworks.com/help/matlab/ref/if.html

"An evaluated expression is true when the result is nonempty and contains
***all*** nonzero elements (logical or real numeric). Otherwise, the
expression is false." [Emphasis added.]

In the first case, neither the IF condition nor the ELSEIF condition are
satisfied for the x I gave above, since some elements are greater than 3 and
some are less than or equal to 3. In the second case, the IF condition is
not true but the ELSE is unconditional; it WILL execute if the IF does not.

--
Steve Lord
slord@mathworks.com
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