How to error check a number for strings and blank inputs?

I have some issues with a unit converter program I'm writing. Currently I have error checking code for negative values but I also want it to check for string and blank inputs. what if statments should I add to achieve this? Any other improvements I can make?

I've added a condensed version of my code below, the full version has 6 different conversions.

clear, clc
n = 1;
m = 1;
o = 1;
fprintf('1. Celsius ↔ Fahrenheit \n2. Millilitre ↔ Fluid Ounce')
menu = input('Enter a number to choose a conversion: ')
while n == 1
    if (menu < 0)
    % string and blank error checking here
    elseif menu == 2
        disp('Millilitre ↔ Fluid Ounce Converter')
        disp('1. Millilitre to Fluid Ounce')
        disp('2. Fluid Ounce to Millilitre')
        unit = input('What conversion should be performed?: ')
        while m == 1
            if unit == 1
                mil = input('Input a volume in millilitres: ');
                while o == 1
                    % string and blank error checking here
                    if (mil < 0)
                        disp('A negative number has been entered')
                        mil = input('Input a volume in millilitres: ');
                    else
                        floz = (mil/29.5735);
                        fprintf('The volume in fluid ounces is %1.2f\n', floz)
                        n = 0;
                        m = 0;
                        o = 0;
                    end
                end

            elseif unit == 2
                floz = input('Input a volume in fluid ounces: ');
                while o == 1
                    % string and blank error checking here
                    if (floz < 0)
                        disp('A negative number has been entered')
                        floz = input('Input a volume in fluid ounces: ');
                    else
                        mil = (floz*29.5735);
                        fprintf('The volume in millilitres is %1.2f\n', mil)
                        n = 0;
                        m = 0;
                        o = 0;
                    end
                end

            else % string and blank error checking here
                disp('An invalid input has been entered')
                unit = input('Please enter a valid number: ')
            end
        end 
    end
end

Answer 1

[Daniel Pollard]

Are you looking for validateattributes or assert? For example,

assert(menu == 1 || menu == 2)

will throw an error if menu is not a number equal to 1 or 2.

[Sourav Anilkumar]

No I don't think I can use those. This is for an entry level class so I can only use basic functions that we've covered. Would rather use an if statement if possible.

[Daniel Pollard]

In that case, could you use something like

if isstring(menu)
    % Whatever you want to do if menu is a string
elseif isempty(menu)
    % Whatever you want to do if the user inputs a blank variable, or hits
    % enter without typing anything
end

Is that in line with what you were after? If you can't use isstring or isempty then someone else may have to leave an answer because I'm not sure where to go from there.

[Sourav Anilkumar]

I think those should work. I've turned off my computer for the night so I can't try it out right now. I'll let you know how it goes tomorrow!!

[Sourav Anilkumar]

Ok so isempty works fine when implemented like this

elseif isempty(menu)
    disp('Error: An invalid number has been entered')
    menu = input('Please enter a valid number: ');

However, when I change isempty to isstring, I get red error messages, if I input the character w for example, from the command window and the

disp('Error: An invalid number has been entered')

line I added doesn't run, though the input line keeps looping.

[Daniel Pollard]

What do the red error messages actually say? Can you copy and paste them here?

[Sourav Anilkumar]

Yeah sure:

Error using input
Unrecognized function or variable 'w'.

Error in s3784513_Milestone_2 (line 6)
menu = input('Enter a number to choose a conversion: ')

Pretend thats all red haha

[Daniel Pollard]

Ok, so according to the documentation, by default, MATLAB thinks that an input is numerical. Give it something like w and it says "hey! w isn't a number!"

You can tell it to read the input in as a string by default by writing something like

menu = input("Your input here: ", 's')

Then, you can test whether the input can be converted into a number by using code from the top answer to this question.

If it can, convert it to a number using str2num. From there you can check it has other properties you want using commands like isempty.