"Int" multiplication in c++ with "long long" result

Question

I am trying to find the square of a int. My code looks like below:

long long sqr=0;
int num=77778;
sqr= num*num;

The result should have been 6049417284 But when I check the output it shows 1754449988. What is the mistake I am doing? long long should be able to store the result but why am I getting a different value?

Answer 1

Intermediate result type is the same as first argument type in this case. So this code puts wrong value into sqr (because here you have integer overflow). Try this:

long long num = 77778;
long long sqr = num * num;

Another way with casting:

int num = 77778;
long long sqr = (long long) num * num;

To understand it better you can check these lines:

int ia = 1, ib = 2;
double da = 1.0, db = 2.0;
double ic = ia / ib; // c == 0.0, because intermediate result of division is 0
double dc = da / db; // c == 0.5, expected result