MatLab: Error handling of integer input

Question

How do I in MatLab catch the error that occours when the user enters letters and other things that aren't numbers in the input:

width = input('Enter a width: ');

I have played around for a while with the try/catch command:

width = 0;
message = '';

% Prompting.
while strcmp(message,'Invalid.') || width < 1 || width ~= int32(width)

  try
     disp(message)
     width = input('Frame width: ');
  catch error
     message = 'Invalid.';
  end

end

But with no luck (the above doesn't work). As shown I would like a simple message like "Frame width: " for the user the first time he has to enter his choice. But if an error is caught I want the message for him to be "Invalid. Try again: " fx everytime an error occours.

I have also tried the error() but I don't know how to place that correctly. Since the error() doesn't take the input command, where the error happends, as an argument, it must detect it in another way, which I can't figure.

Any help would be appreciated.

Gunther Struyf · Accepted Answer · 2012-06-20 23:19:27Z

4

width = input('Frame width: ');
while(~isInt(width))
    width = input('Invalid. Try again: ');
end

and you'll have to have the following function somewhere (or another implementation of it)

function retval = isInt(val)
    retval = isscalar(val) && isnumeric(val) && isreal(val) && isfinite(val) && (val == fix(val));
end

edited Jun 20, 2012 at 23:19

answered Jun 20, 2012 at 23:13

Gunther Struyf

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8 Comments

tmpearce Over a year ago

+1. I'd note that you could just toss an isscalar in there rather than the if/else statement.

Steeven Over a year ago

It doesn't work when I enter a letter... The error happens at the line with width = input('Frame width: ');. It seems that when a letter (or any string) is entered, an error happens here because the input() did not ask for a string. Then the program never reaches the while loop. Instead it breaks at this line.

tmpearce Over a year ago

@Steeven If you enter a string with ' ' around it, what happens? If you don't enclose the string, it will think you're trying to enter a variable from the workspace and evaluates that variable. If an error occurs, it should just prompt again though rather than breaking entirely

Steeven Over a year ago

So, if I want to avoid the 's' argument to the input(), I have to add apostrophes around the input string? That wouldn't be user friendly for the user. Is it then not possible to catch the error that occours because a string is sent?

Gunther Struyf Over a year ago

only apostrophes for strings, just like in your normal code: 4 is integer, 4.5 is double, a is a variable, 'b' is a string. You can always use the 's' parameter to define that your input is always a string.. but since you're avoiding that, I guess we're stuck..

|

kol · Accepted Answer · 2012-06-20 23:38:40Z

2

answer = input('Frame width: ', 's');
[width, status] = str2num(answer);
while ~status || ~isscalar(width) || width ~= floor(width)
  answer = input('Invalid. Try again: ', 's');
  [width, status] = str2num(answer);
end
disp(width);

(status is 0 if the conversion failed. Without the isscalar test, an input like [1 2; 3 4] would also be accepted. The last test ensures that width must be an integer.)

edited Jun 20, 2012 at 23:38

answered Jun 20, 2012 at 23:16

kol

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4 Comments

tmpearce Over a year ago

You may also want to make sure that fractional parts aren't allowed (the title suggests integer inputs are desired).

Steeven Over a year ago

Unfortunately I need to avoid using the 's' argument in the input() command. I am looking for a solution to catch the error when this argument is not used.

kol Over a year ago

Interesting. Why do you need to avoid the 's'? It's needed to handle the input as a string. It's a usual practice to receive user input as a string, and try to convert it and check the result of conversion in a second step.

Steeven Over a year ago

I have to give in it seems. I am convinced. Your answer did the trick, when the 's' couldn't be avoided.

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