MATLAB: Halley’s Comet Error

error

I am converting a program from c++ to matlab. I have written this code, but keep getting an error that reads "undefined function or variable x_old". How an I fix this?
This is my code so far:
num = input('Enter a positive number: ')
acc = input('Enter convergence criterion: ')
approx = input('Enter initial apporiximation: ')
x = approx;
while abs(x-x_old)>acc
    x_old = x;
    y = 1/num*x*x;
    x = x/8*(15-y*(10-3*y));
end
disp(['Square root of' num2str(num) 'is' num2str(x)])

Best Answer

"How an I fix this"
By defining x_old before using it (in the while loop)?
Perhaps:
x_old = Inf;
while abs(x-x_old)>acc
    %...
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