c++cprintfcoutunsigned-long-long-int

Why do cout and printf produce different results?

I'm learning C/C++ and started to try out programing with big numbers but when printing them I get a strange error.

Here is my example code:

unsigned long long shiftresult = 1ULL;
for (int i = 0; i < 64; i++) {
    shiftresult = (1ULL << i);
    printf("%lld << %d = %lld\n", 1ULL, i, shiftresult);
    cout << 1ULL << " << " << i << " = " << shiftresult << endl;
}

The problem being that when the loop reaches i = 63 printf prints -9223372036854775808 while cout prints 9223372036854775808 which is the "real" output.

Could someone explain why this happens?

Solution

  • You're using the wrong format specifier for printf.

    The %lld format specifier expects a long long argument, but you're passing an unsigned long long argument, so the unsigned number is being interpreted as a signed number.

    You need to use %llu to print an unsigned long long.