What's the error with this xor

Question

Why, when using this xor in C++,

int main() {
    bitset<4> a=1100, b=0110,c;
    c = a ^ b;
    cout << c;
    return 0;
}

is the result 0100?

Answer 1

Those constants aren't in binary, that's why. 1100 decimal is 10001001100 binary. 0110 octal is 1001000 binary. (Why octal for the second one? Because a constant that starts with a leading zero and consists only of digits 0..7 is octal in C++.)

When you truncate both to 4 bits, you get 1100 binary XORed with 1000 binary, which gives 0100 binary.

Try this instead (assuming your compiler supports the nonstandard prefix 0b for binary literals):

int main() {
    bitset<4> a=0b1100, b=0b0110,c;
    c = a ^ b;
    cout << c;
    return 0;
}

Alternately, specify your constants in hexadecimal:

int main() {
    bitset<4> a=0xC, b=0x6,c;
    c = a ^ b;
    cout << c;
    return 0;
}

or as string constants: (C++11)

int main() {
    bitset<4> a( "1100" ), b( "0110" ),c;
    c = a ^ b;
    cout << c;
    return 0;
}