Check if input is a 4 element vector

I'm trying to write an error check in a function code, which should check if the input is specifically a 4-element vector.

All i have so far is:

if ~isvector(x) 
  error('Inputs must be vector') 
end

Is there a way to write a code in order to check if the input, x, is a 4-element (row) vector.?

I'd like it to throw an error if the input x is for example a 2-element row vector, and of course if it is not a vector.

Thanks

Answer 1

[Guillaume]

The simplest way to do this kind of checks is to use validateattributes which will give you consistent comprehensive and localised error messages and check for a lot more properties. In your case,

validateattributes(x, {'numeric'}, {'row', 'numel', 4});
%or
validateattributes(x, {'numeric'}, {'row', 'numel', 4}, mfilename, 'x'); %for an even better error message

You may want to add 'finite' to the list of required attributes.

[OblongPea]

Many thanks, this also works!

[Guillaume]

Not only does it work, but as I explained, it's way better than the answer you've accepted. It will generate the same kind of error message that matlab issues, so the user is familiar with the format, the error message will be in the language that the user uses as well, and it's trivial to add more properties to check, properties that you probably haven't even thought to check, e.g. does your code work with NaN or Inf? sparse vectors? complex vectors? If not, it's trivial to add to the list of attributes:

validateattributes(x, {'numeric'}, {'row', 'numel', 4, 'finite', 'real', 'nonsparse'}, mfilename, 'x'); %for an even better error message

and it makes it also a lot easier to understand what properties are required since they're spelled out in words.

Answer 2

[Turlough Hughes]

if size(x,2)~=4 || ~isvector(x) || ~isnumeric(x)
    error('Input must be a row vector of length 4')
end

[OblongPea]

Thanks for this, but when i test it with for example a 2 element row vector, i get the message 'Index exceeds matrix dimensions'.

It then highlights that the next line in the code has an error. As the next line uses all the elements of the input vector x.

Cheers

[OblongPea]

Thanks, this now works a treat!

[Stephen23]

Simpler to use assert:

assert(isnumeric(x) && isrow(x) && numel(x)==4, 'Input must be a row vector of length 4')