How to restrict user inputs?

Hey everyone, just wondering how I could restrict certain user inputs. I want my program to accept two inputs, in this case being 'i' and 'm' and if the user were to enter a number or a different string, my script would keep looping the input question until the user enters the correct value. (Essentialy I just need help on where I commented)

 y = input('Please enter the letter ''i'' for an imperial output or ''m'' for a metric output: ', 's');
        
 while isempty(y)
       disp('Blank is an invalid input you are required to enter a number from the options above!')
       y = input('Please enter the letter ''i'' for an imperial output or ''m'' for a metric output: ', 's');
        
 end
 
while %Need a function that if the user enters a number or string other than i or m, then this loop activates.
    disp(' Invalid input,please enter the letter ''i'' for an imperial output or ''m'' for a metric output')
       y = input('Please enter the letter ''i'' for an imperial output or ''m'' for a metric output: ', 's');
       
       while isempty(y)
       disp('Blank is an invalid input you are required to enter the letter ''i'' for an imperial output or ''m'' for a metric output!')
       y = input('Please enter the letter ''i'' for an imperial output or ''m'' for a metric output: ', 's');
      
       end
end
        switch y
            case 'i'
                fahrenheit = (z*9/5)+32
                
            case 'm'
                celsius = (z-32)*5/9
        end

Answer 1

[Adam Danz]

inputOK = false;
while ~inputOK
    y = input('Please enter the letter ''i'' for an imperial output or ''m'' for a metric output: ', 's');
    if ~isempty(y) && ismember(y, {'m', 'i'})
        inputOK = true; 
    end 
end

Another option is to use a question dialog box:

answer = questdlg('Please select output type', mfilename, 'imperial', 'metric', 'quit', 'quit'); 

[Jan]

A simplification of:

if ~isempty(y) && ismember(y, {'m', 'i'})
    inputOK = true;
end 

is

inputOK = any(ismember(y, {'m', 'i'}); 

[Adam Danz]

Smart! Thanks for the improvement.

(add one more closed-parenthesis to avoid error).