0177 versus 177 and their complements

Question

In c, why is that ~177 yields -178, while ~0177 yields -128?

I tried printing out the values before and after, but couldn't discern anything. I also couldn't anything explaining this topic. I am reading "The C Programming Language".

http://stackoverflow.com/questions/791328/how-does-the-bitwise-complement-operator-work — msc, May 16 '17 at 17:53

score 3 · Answer 1 · answered May 16 '17 at 17:21

3

The leading zero indicates that 0177 is an octal number (127).

answered May 16 '17 at 17:21

ikegami

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score 2 · Accepted Answer · answered May 16 '17 at 18:01

The constants 177 and 0177 are not the same value. The former is decimal which the latter is octal.

From section 6.4.4.1 of the C standard:

3 A decimal constant begins with a nonzero digit and consists of a sequence of decimal digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal digits and the letters a (or A) through f (or F) with values 10 through 15 respectively.

4 The value of a decimal constant is computed base 10; that of an octal constant, base 8; that of a hexadecimal constant, base 16. The lexically first digit is the most significant.

The octal constant 0177 is equal to 127 in decimal. As a 32-bit hex value, it is represented as 0x0000007f. Using the ~ operator on this value gives you 0xffffff80. Assuming 2's complement representation for negative numbers, this is -128 in decimal.

0177 versus 177 and their complements

2 Answers2