# Cannot return value of a variable

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Ken am 7 Mär. 2014
Beantwortet: Ken Atwell am 7 Mär. 2014
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
elseif (mod (a,1) == 0) && isprime(p)&&p>2 && gcd(a,p)==1
if mod(a^((p-1)/2),p) == mod(-1,p)
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
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
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### Antworten (1)

Ken Atwell am 7 Mär. 2014
'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.
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