How do I check that input is numerical?

I'm trying to create a program that will prompt the user for a numerical input, for example: 'What age are you?'. I know how to prompt the user to ask for an input, but I want to know how to check the answer to make sure it is a numerical figure, ie to stop the user putting in the answer 'twelve'. Any help would be greatly appreciated.

Seán

[Stephen23]

More reliable than str2num is to use str2double (which does not call eval and does not evaluate arbitrary code). str2double outputs NaN for invalid input strings, so simply do this:

 val = str2double(input('What age are you?','s'));
 if isnan(val)
     ... not a number
 else
     ... was number
 end

See KSSV's answer for another example of using str2double.

Answer 1

[Adam]

str2num( inputVal )

will return the number if it is valid or empty otherwise so a

if ~isempty( str2num( inputVal ) )

test would check it is a number, or remove the ~ if you want to check for not a number and give an error message in that case.

[Seán Ó Héir]

Hi Adam, thanks for replying!

I tried implementing that and I'm getting the following error message: 'Requires character vector or array input.'

What I'm looking for is something to check the input to make sure the answer given is an integer number, and anything else will be rejected.

prompta = 'What age is Seán? ';
age = (input(prompta))

So if the user enters something silly like 'dog', the program will return something like 'Please enter an integer value next time', and it will then loop back to the original question.

[Adam]

Yeah, sorry, it was a quick answer and I chose a very bad variable name. I've edited it now. the argument to str2num was just meant to be your variable that you have extracted from the prompt, not a call to the 'input' function.

[Peter]

I'm a new user, with some help from other answers I found this solution. Option to use existing data from a file or new input; plus checking numeric input, maybe not the best solution, it works

str2num will create error message after input either i or j, compex number

doRun=true;
while doRun=true
    disp('input calc_data D1:')
    disp('Choose 1 or 2');
    disp('1= keep old data');
    disp('2= new data');
    Sip_choose=input ('Choose 1 or 2: ','s');
    option=str2double(Sip_choose);
    if(option==1)
        calc_data_D1= D1
        % old value in mat file saved
        break
    elseif(option==2)
        while doRun
            D1=input('=> input calc_data D1= ','s');
            D1t=str2double(D1);
            TF=isstrprop(D1t,'digit');
            if TF==0
                D1=D1t;
                % new value of D1
                doRun=false;
            else
                disp('no numeric input')
            end
        end
    else
        disp('wrong choose of option')
        disp('1 or 2 accepted only')
    end
end

[Stephen23]

"so a if ~isempty( str2num( inputVal ) ) test would check it is a number"

Note that STR2NUM is fundamentally buggy:

str = 'struct("x",pi)';
if ~isempty( str2num(str) )
    disp('Oh no!')
end

Oh no!

Read the STR2NUM file to understand why. The reliable approach is to use STR2DOUBLE or SSCANF or the like.

@Peter: you can simplify your code by avoiding the conversion to numeric and using STRCMPI.

Answer 2

[Thorsten]

Using

isnumeric