C++ cout output explanation please [duplicate]

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Closed 9 years ago.

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why is initializing an integer in VC++ to 010 different from initialising it to 10?

This got me very confused, and I hope one of you can answer my question. How come this code will produce the output "116"?

#include <iostream>

int main()
{
    std::cout << 0164 << std::endl;
    return 0;
}

The code has been compiled with MSVC++ 2010 and g++ under Mac OS X. "cout" can print '0' alone and '164' alone, but as soon '0' is the first digit in the number the output changes.

Answer 1

Because the 0 in front makes the number be interpreted as octal.

0164 = 
 4 * 1 +
 6 * 8 + 
 1 * 64
 = 116

Or, via binary:

 0164 =
   0   1   6   4 = 
 000 001 110 100 =
 1110100 = 
 116

The same goes for hexadecimal numbers, you write them as 0x1FA for example.

Answer 2

In C and its brethren, a number with 0 on the front is octal, not decimal.

Hence your number is 1 * 82 (1 * 64 = 64) plus 6 * 81 (6 * 8 = 48) plus 4 * 80 (4 * 1 = 4) which equates to 116.

See here for a large treatise on what hexadecimal and octal are in C.