What is the most correct way of determining whether variable contains a number

From what i have read, isnumeric is useless since eg. nan returns numeric output. Some answers mention to use str2double but str2double(2) returns NaN as does str2double("a"). So is there a universal function that is capable to eat both strings and numbers and tell you whether the string is a number (and if supplied with a number obviously returns true as well)?

Answer 1

[Chunru]

It seems that you have to write your own function:

[t, f] = isstranum(5)

t = logical
   1

f = 5

[t, f] = isstranum('nan')

t = logical
   1

f = NaN

[t, f] = isstranum('-512.3')

t = logical
   1

f = -512.3000

[t, f] = isstranum('abc')

t = logical
   0

f =

     []

function [tf, y] = isstranum(x)
    if isnumeric(x)
        tf = true;
        y = x;
    elseif isstring(x) || ischar(x)
        if strcmpi(x, 'nan') || strcmpi(x, 'inf') 
            tf = true;
            y = str2double(x);
        else
            y = str2double(x);
            if isnan(y)
                tf = false;
                y = [];
            else
                tf = true;
            end
        end
    else
        tf = false;
        y = [];
    end
end

Answer 2

[Image Analyst]

You can use isempty() to determine if a variable has any variable of any class whatsoever in it:

if isempty(yourVariable)
    fprintf('yourVariable has nothing in it.\n')
else
    fprintf('yourVariable has something in it.\n')
end

You might also take a look at exist(yourVariable, 'var') to see if a variable by that name even exists at all (empty or not).

Answer 3

[Stephen23]

Let MATLAB do the heavy lifting for you:

fun = @(x) isnumeric(x)&&isfinite(x) || isfinite(str2double(x));
fun(2.3)

ans = logical
   1

fun(NaN)

ans = logical
   0

fun('2.3')

ans = logical
   1

fun('NaN')

ans = logical
   0

fun('abc')

ans = logical
   0