Restrict user string inputs

Hey guys, just a quick question, how could I ban all characters inputed by the user execpt 'i' and 'm'.

Thanks

 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 ~strcmp(y,'i') | ~strcmp(y,'m')%Need help in this line of code
     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 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
 end
    
 
 


        switch y
            case 'i'
                fahrenheit = (z*9/5)+32
                
            case 'm'
                celsius = (z-32)*5/9
        end
        

[Adam]

y = input('Please enter the letter ''i'' for an imperial output or ''m'' for a metric output: ', 's');
y = validatestring( y, [ "i", "m" ] );

Depends how stupid you have to treat your users though, by giving them new messages repeating exactly what the first message asked them for if they enter something else!

Answer 1

[M]

The problem comes from your conditions:

~strcmp(y,'i') | ~strcmp(y,'m')

This is never true, even if you enter 'i' or 'm'. What you want is to stop when you enter 'i' or 'm', so the criteria to continue the while loop should be the opposite, not 'i' and not 'm':

~strcmp(y,'i') && ~strcmp(y,'m')

This while loop will also stop when you enter 'i' or 'm'