Colon input to anonymous function

If I define a function:

F = @(x,y) (x^2-y)

If I call the function with two scalar inputs it works as expected,

F(2,3) = 1,

Calling with a single colon input returns an "index exceeds matrix dimensions" error, and calling with two colon inputs

F(:,:) = 3306

evaluates the function as if each input was the seemingly arbitrary scalar 58.

Can someone explain this behavior or link me to some documentation explaining this behavior? There doesn't seem to be anything about colon inputs on the anonymous functions doc page

Thanks.

Answer 1

[Geoff Hayes]

If you create the above as a function/script, and step through the code, it will reveal something

 function [v] =  test(x,y)

 v =  (x^2-y);

Put a breakpoint at the initialization of v. In the Command Window type

 test(:,:)

Now when the debugger pauses at the first line, look at the inputs for x and y. They are 1x1 char with the colon value.

The decimal ASCII value for a colon is 58, so there is an implicit conversion from the character to the decimal/numeric equivalent before the evaluation.

Hope that this helps!

[Tommy]

Thanks!

Answer 2

[Matt J]

Colons are treated as char variables throughout MATLAB and the ASCII code for ':' is 58. In general, it is possible to do arithmetic and other numerical operations with character variables, in which case MATLAB assumes you want to do the arithmetic with the underlying ASCII values, e.g.,

>> ':'+':'

ans =

     116

Want to know what you get when you cross a dog with a cat?

>> cross('dog','cat')

ans =

          2885       -1403       -1289