CODEWITHSUNDEEP
c
Kevin Dong
Kevin Dong

Reputation: 5359

Meaning of leading zero in integer literal

I'm studying (ANSI) C by The C Programming Language (2nd Edition).

This is a code snippet from 2.10 Assignment Operators and Expressions:

1  /* bitcount() counts the number of 1-bits in its integer argument */
2  int bitcount(unsigned x)
3  {
4      int b;
5      for (b = 0; x != 0; x >>= 1)
6          if (x & 01)
7              b++;
8      return b;
9  }

I am confused why x & 01 is written in line 6 rather than x & 1 or x & 0x1? Is 0 before 1 necessary?

Upvotes: 7

Views: 2130

Answers (3)

Mahonri Moriancumer
Mahonri Moriancumer

Reputation: 6003

The 01 could also be written 1, or 0x1.

01  = Octal Constant  Base 8 (1 Decimal)
1   = Decimal Constant. Base 10
0x1 = Hexadecimal Constant. Base 16 (1 Decimal).

When the book was written, Base 8 (octal) was pervasive in existing computer programming.

Upvotes: 12

Ryan Haining
Ryan Haining

Reputation: 36872

A leading 0 makes a constant into an octal (base 8) value.

In this case it's no different from 1 or 0x1 because they will all have the same binary representation of 000...001. People often use hex constants to distinguish that a value is being used as a bitmask or other bitwise value today. In the past, octal constants were more often used for the same purpose.

0x1  = hex constant = 1
0X1  = hex constant = 1
01   = octal contant = 1
1    = decimal constant = 1
1U   = unsigned decimal constant = 1

Upvotes: 8

Brian Cain
Brian Cain

Reputation: 14619

Indeed "01" is distinct from 1 and 0x1 in principle. "01" is encoded in octal. Kernighan/Ritchie probably value the ease in which one can convert from octal to binary for this particular problem.

Upvotes: 2

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