Meaning of leading zero in integer literal

Question

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?

Answer 1

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.