If-statements won't break when conditions aren't met

Hi there!

I am working on a mainscript in Matlab and I have troubles with determining why my code doesn't break when it does not meet the conditions that I have placed within the if statement. Anyone who has got an idea of this?

I hope there are someone out there who has an answer for this!

Best regards

Kristian

[Walter Roberson]

http://www.mathworks.co.uk/matlabcentral/answers/43073-a-guide-to-tags

[James Tursa]

@Kristian: It is an abuse of this site and a disservice to the readers and contributors for you to edit out the content of your question after you have received answers.

Answer 1

[Walter Roberson]

You are using "if" with a vector of values. "if" is considered true if all of the values in the vector are true. The condition

if vector_of_values

is equivalent to

if all(vector_of_values)

What you probably want in your situation is to "break" if any of the values are true. To achieve that, recode as

if any(vector_of_values)

For example change

if beamLength < loadPositions | loadPositions <= 0 | ~isnan(loadPositions);

to

if any(beamLength < loadPositions | loadPositions <= 0 | ~isnan(loadPositions))

You should also be considering using any() on the individual parts of the construction, and also considering short-circuiting the test:

if any(beamLength < loadPositions) || any(loadPositions <= 0) || any(~isnan(loadPositions))

However, your code appears to be attempting to validate that the values are all valid, and to "break" if they are all valid. In such a case, you do not want to or the portions together, you want to and them together, and you would want to use all() either explicitly or implicitly. I do not recommend using all() implicitly as it then is not clear whether you specifically want the all() behaviour or if you did not know about it. I suspect what you want for your code is

if all(beamLength < loadPositions & loadPositions <= 0 & ~isnan(loadPositions))

You could also write that with explicit short-circuiting using

if all(beamLength < loadPositions) && all(loadPositions <= 0) && all(~isnan(loadPositions))

Is it correct that the user is to input strictly negative values for beamLength and non-positive values for loadPositions? It is possible and perhaps meaningful for your purpose, but it would be much less common than expecting the values to be >= 0 instead of <= 0

[James Tursa]

See my answer.

[Walter Roberson]

I might be misunderstanding your code. My reading of your code, with all the repeated input() and so on, was that your code is attempting to detect bad input and to re-prompt and re-input until you received valid input, and only then do the calculation. If I am correct then when you receive bad input you do not want to break(), as continuing on is the method that you use to prompt and fetch the repaired values.

The multiple "while true" are confusing me. You need to clean up the indentation on the code so I can figure out which parts are nested and which are not.

The usual structure for code that validates input is to loop,

set GotValidInput false
while not GotValidInput
   prompt
   get some input
   if the input shows the user asked to cancel, break the loop
   validate the input
   if validation SUCCEEDS then set GotValidInput to True and break out of the loop
   you had bad input if you got here. Complain
 end of loop

   if GotValidInput is not set at this point then the user asked to cancel, so do something appropriate and get out of here

   You only got here if input was valid. Run the calculation. Or start a new loop concerned with fetching and validating another input value.

Answer 2

[James Tursa]

This line does no do what you think it does:

if beamLength < positions(:) | positions(:) < 0 | ~isnan(positions)

If positions is a vector, then beamLength < positions(:) will be a vector result, not a scalar result. So using it as the argument to the if test isn't appropriate. Maybe you meant this instead?

if any(beamLength < positions(:)) || any(positions(:) < 0) || any(isnan(positions(:)))

Similar comments apply to your other if tests as well.