In the first line of your code snippet, you are defining a boolean variable 'diagonal' and set it to true.
But in the if/else conditional you are checking if the value of the variable x is diagonal or not.
For comparing strings you can use the strcmp() function.
Below is the working code of what you intended to achieve.
for index = 1:1
disp('straight , sideways , diagonal')
x = input('Which way does checker pieces move? ','s');
disp('False, Start Over!');
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