Matlab Switch-case

Question

I'm trying to write a function that puts the class, length, and value of each of the things in a cell array into a struct, but I keep getting an error with the switch statements

function [ out, common ] = IDcell( cA )
%UNTITLED Summary of this function goes here
%   Detailed explanation goes here
cl={};
val={};
len={};
for x=1:length(cA)
    switch cA(x)
        case isnum
            cl(x)='double';
        case ischar
            cl(x)='char';
        case islogical
            cl(x)='logical';
        case iscell
            cl(x)= 'cell';
    end

val=[val cA{x}];
len=[len size(value(x))];
end

out=struct('value', val, 'class', cl, 'length', len);


end





[out]=IDcell(cA)
SWITCH expression must be a scalar or string constant.
Error in IDcell (line 8)
switch cA(x)

Solution

isnum is not a Matlab function. isnumeric may be what you were thinking of, but it isn't what you typed. Which means your code is seeing case isnum and it has no idea what the heck isnum is, so it is telling you whatever it is, if you want to use it there you need to make it something that evaluates to a number (what it means by scalar) or to a piece of text (what it means by string constant).

Further, ischar is a matlab function, but you are not using it the right way. You must use it as ischar(cA(x)) for example, will then evaluate to true if cA(x) is a string or snippet of text, will evaluate to false if cA(x) is anything else.

While it would be lovely if switch worked this way, it doesn't. You can't put a thing in the switch part, and then just list functions that need to be evaluated on that thing in the switch part.

The kind of thing you can do is this:

switch class(x)
    case 'double'
        fprintf('Double\n');
    case 'logical'
        fprintf('Logical\n');
end

Here I have used the class function the way it needs to be used, with an argument in it. And I then switch my cases based on the output of that function, class outputs a string.

Comments

[Ben Voigt]

of course, the Matlab-provided class function replaces almost his entire code