c++ - Uninitialized Error

c has an indeterminate value in your for loop here:

for(int c; x < c; c++)
        ^

you need to initialize it to a value for example:

for(int c=2; x < c; c++)

it will need to be greater than 0(perhaps 2 since you probably don't want 1 as a divisor) since you are using it in the modulus operation here:

if (x%c==0)

and modulus by 0 is undefined behavior as per the draft C++ standard section 5.6 Multiplicative operators paragraph 4 says:

[...]If the second operand of / or % is zero the behavior is undefined.[...]

It looks like you may have flipped your ending condition and it should go from c < x instead of x < c.