How to make a repeat process for a statement in a while loop when condition is not met?

Right now I am stuck as to how do I return z to asking for the conversion method if the 'C to F' is not entered correctly or nothing is entered. Help please.

 clear,clc
x =[];
while isempty(x)
    x = input('Make a choice(imperial or metric): ','s');
    if strcmp(x,'imperial')
        y = input('Please enter a value: ');
        z = input('Conversion method (C to F): ','s');
        if strcmp(z,'C to F')
            convert(x,y,z)
        end
    elseif strcmp(x,'metric')
        y = input('Please enter a value: ');
        z = input('Conversion method (C to F): ','s');
    else 
        x =[];
        disp('Please make sure the spelling are correct!')
    end
end

Answer 1

[Walter Roberson]

z = '';
while ~strcmp(z,' C to F')
 z = input(...)
 if strcmp(...)
   Convert...
 else
  Complain 
  z = '' 
 end 
end