Help with IF/Else statement in GUI

Hi guys, i am new here. :) I have a problem with IF/Else statement, but only in GUI. If i put only one if statement, like this

if s=='Add'
f=x+y;
end

It works, but when i add this:

if s=='Add'
f=x+y;
end
if s=='Substract'
f=x-y;
end

It doesn't work.Also, if i only use

if s=='Substract'
  f=x-y;
  end

it works.I don't have a clue why is this happening.Any idea?

Answer 1

[Jan]

There is not connection to the fact, that the code "runs in a GUI" - by the way: there is even nothing like "running in GUI". Code is code.

The problem is the comparison of a string the the == operator, which performs an elementwise comparison. If the compared strings have a different number of characters, such a comparison must fail.

Use strcmp instead:

if strcmp(s, 'Add')
  f = x+y;
end
if strcmp(s, 'Substract')
  f = x-y;
end

Or with switch/case:

switch s
  case 'Add'
     f = x+y;
  case 'Subtract'
     f = x-y;
  otherwise  % No SWITCH without OTHERWISE!
     error('Unexpected value of [s]');
end

Note: In your question you write "doesn't work". Please post the complete error message in the future, because these messages contain valuable information to solve the problem.

Answer 2

[jack int]

Switch/case works, but i wanted to know why if/else doesn't. :P

[Jan]

Then please read my answer, where I explained the problem with using the == operator:

'test' == '1234'

This replies [false, false, false, false]. In the IF command you want a scalar condition. Therefore Matlab converts the expression internally to:

if all(s=='Add') && ~isempty(s=='Add')

Now try:

'test' == '123'

You get an error message, because an elementwise comparison is not working.

Answer 3

[jack int]

Thanks!