Need some assistance with "if" command

I'm trying to get my program to stop completely when given an invalid response after entering the side lengths. I've created the following:

a=input('Enter value for side a: ');
if a < 0 
    disp('Not a valid input.')
end
b=input('Enter value for side b: ');
if b < 0 
    disp('Not a valid input.')
end
c=input('Enter value for side c: ');
if c < 0 
    disp('Not a valid input.')

Answer 1

[Geoff]

If you don't have a function or anything to return from, there's a sneaky little trick you can use. This can be useful for other things too...

ok = false;
while 1
  a=input('Enter value for side a: ');
  if a < 0, break; end
  b=input('Enter value for side b: ');
  if b < 0, break; end
  c=input('Enter value for side c: ');
  if c < 0, break; end
  ok = true;
  break;
end

if ok 
  % Do your thing here....
else
  disp('Not a valid input.');
end

Err... Bear in mind this isn't the best of programming practices. =)

Answer 2

[Geoff]

Oh, the better way is to use the proper MATLAB error function:

if a < 0
  error( 'Not a valid input.' );
end

Answer 3

[Walter Roberson]

To get your program to stop completely you are going to need to use the command "quit", as anything else you can voluntarily do has the risk of being intercepted by a different level, such as by a try/catch block. "quit" will exit MATLAB.

Honestly, even "quit" might not be good enough, as I think "quit" would still end up running any registered onCleanup() routines rather than stopping the program completely. So possibly what you should do is write a little mex routine that deliberately dereferences an invalid pointer so that MATLAB crashes.