Argument validation for cell arrays?

I am looking for a way to express in a function validation block

arguments
    % ...
end

Illegal use of reserved keyword "end".

that the argument must be a cell array of three-vectors.

The closest I came was to check that the argument is a cell array with a variable number of elements, as in this example script:

% should be accepted
example({ [0;0;0], [0;1;0], [1;1;0], [1;0;0] });

% should fail, because of the char-array
example({ [0;0;0], [0;1;0], [1;1;0], 'not allowed' });

% should fail, because of unexpected size of last vector
example({ [0;0;0], [0;1;0], [1;1;0], [1;0] });

% definition
function example(points)
    arguments
        points (1,:) cell
    end
    size(points)
end

Best regards,

Klaus

To summarize the answers

• There is no builtin easy way to make such checks.
• If needed, a custom mustBeASomething function has to be defined. Preferably with useful error messages.
• Using user-defined classes would help further.

[Mario Malic]

Hey, I am not sure if it is possible to validate your arguments within the argument block as I never used it, but here's an idea if you want to do by adding the code.

points1 = { [0;0;0], [0;1;0], [1;1;0], 'not allowed' };
test1 = string(cellfun(@class, points1, 'UniformOutput', false))

test1 = 1×4 string array
    "double"    "double"    "double"    "char"

if (numel(unique(test1)) > 1)
    error("multiple variable types")
end

Warning: multiple variable types

points2 = { [0;0;0], [0;1;0], [1;1;0], [1;0] };
test2 = cellfun(@size, points2, 'UniformOutput', false)

test2 = 1×4 cell array
    {[3 1]}    {[3 1]}    {[3 1]}    {[2 1]}

test2Arr = cell2mat(test2');
if size(unique(test2Arr, 'rows'), 1) > 1
    error ("different dimensions of points")
end

different dimensions of points

[Klaus]

@Mario Malic While it would work, I was looking for something to leverage the advantages of the arguments syntax, which guarantees, that all arguments are covered, and all information is declaratively in one place.

The result would be something like

arguments
    points(1,:) cell {mustBeListOf3Vectors(points)}
end

but if there are builtin capabilities, it would provide better readability.

If I'd design the code from scratch, I'd probably use

points(3,:) double

instead, and for the more complex structs-of-structs classdef with type-constrained properties.

Answer 1

[Stephen23]

"that the argument must be a cell array of three-vectors."

You can easily write your own argument validation function:

https://www.mathworks.com/help/matlab/matlab_prog/argument-validation-functions.html#mw_3e8f7e5b-fbb5-4725-be02-4751c675a92a

example({ [0;0;0], [0;1;0], [1;1;0], [1;0;0] });

ans = 1×2

1 4

%example({ [0;0;0], [0;1;0], [1;1;0], 'not allowed' });
example({ [0;0;0], [0;1;0], [1;1;0], [1;0] });

Error using solution>example
Invalid argument at position 1. Input must be a cell array of 3-element vectors

function example(points)
arguments
    points cell {isCell3Vec}
end
size(points)
end
function isCell3Vec(C)
X = cellfun(@isnumeric,C)&cellfun(@numel,C)==3;
assert(all(X(:)),'Input must be a cell array of 3-element vectors')
end

[Steven Lord]

While cellfun is compact, it's also sometimes cryptic. I'd also suggest a slightly more descriptive / targeted error message, and in support of that I'd use a simple loop in the validator.

For the case where one of the inputs is a numeric array with 3 elements but is not a vector (remember, by the definition codified in the isvector function a vector must be a 2-D array.)

try
    example({1:3, 4:6, reshape(7:9, [1 1 3])})
catch ME
    fprintf("Call threw error:\n%s", ME.message)
end

Call threw error:
Invalid argument at position 1. Element 3 of the input is not a vector.

Wrong length numeric vector:

try
    example({1:3, 4:7})
catch ME
    fprintf("Call threw error:\n%s", ME.message)
end

Call threw error:
Invalid argument at position 1. Element 2 of the input is not 3 elements long.

Not numeric but a 3 element vector:

try
    example({'abc', 1:3, 4:6})
catch ME
    fprintf("Call threw error:\n%s", ME.message)
end

Call threw error:
Invalid argument at position 1. Element 1 of the input is not numeric.

function example(points)
arguments
    points cell {isCell3Vec}
end
size(points)
end
function isCell3Vec(C)
for n = 1:numel(C)
    x = C{n};
    assert(isnumeric(x), "Element " + n + " of the input is not numeric.")
    assert(numel(x) == 3, "Element " + n + " of the input is not 3 elements long.")
    assert(isvector(x), "Element " + n + " of the input is not a vector.")
end
end

[Mario Malic]

Thanks Steven, very informative.