Cannot return value of a variable

Hello guys, I am making a program to search for square root of a number, such as x^2 mod p (searching the value of x). However, I cannot return the value of x. Here's my program :

 a= 20;
p=29;
r=0;
b=0;
g=0;
%a == mod(a,1)double(uint64(a));
if (mod (a,1) ~= 0) || ~isprime(p)||p <=2
    disp('kondisi tdk memenuhi');
elseif (mod (a,1) == 0) && isprime(p)&&p>2 && gcd(a,p)==1
    if mod(a^((p-1)/2),p) == mod(-1,p)
        disp('tidak ada square root');
    elseif mod(a^((p-1)/2),p) == 1
        e = 1; 
        s = (p-1)/(2^e);
        while (mod (s,2) == 0) 
            e=e+1;
            s = p-1/(2^e);
        end
        n = 1;
        while mod(n^((p-1)/2),p) ~= 1 || mod(sqrt(n),1) == 0
            n = n+1;
        end    
        x = a^(s+1)/2;
        b = a^s;
        g = n^s;
        r = e;
        m = 0;
        while (m == 0)
            m = 0;
            while mod(b^(2*m),p)~=1
                m = m+1;
            end
            if m > 0
                x = x*g^(2^(r-m-1));
                b = b*g^(2^(r-m));
                g = g^(2^(r-m));
                r = m;
            elseif m < 0
                disp('tak ada square root');
            end    
        end
    end  
end    
disp(x);

Anyone know what's wrong with my code? because the matlab only says I have an error at "disp(x)". Thanks for your help :D

Answer 1

[Ken Atwell]

'x' is only assigned if certain conditions are met, so you will get an error with some combinations of 'a' and 'p'.

However, I see what might be an error in logic beginning at 'while (m == 0)': 'm' is next set to 0, so b^(2*m) == b^0, which is always one. So, the 'while mod...' condition is never satisfied and 'm' is never incremented. It would seem that m would remain zero and therefore the while loop would never exit.

I recommend stepping through this code with the debugger to understand what is happening.