Halley's Comet 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)])

[Adam Danz]

When you get to this line of code

while abs(x-x_old)>acc

how do you expect Matlab to interpret x_old since this is the first time that variable is appearing?

Answer 1

[Guillaume]

"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
    %...