Why are hexadecimal numbers prefixed with 0x?
Why are hexadecimal numbers prefixed as 0x?
I understand the usage of the prefix but I don't understand the significance of why 0x was chosen.
Answer
Short story: The 0 tells the parser it's dealing with a constant (and not an identifier/reserved word). Something is still needed to specify the number base: the x is an arbitrary choice.
Long story: In the 60's, the prevalent programming number systems were decimal and octal — mainframes had 12, 24 or 36 bits per byte, which is nicely divisible by 3 = log2(8).
The BCPL language used the syntax 8 1234 for octal numbers. When Ken Thompson created B from BCPL, he used the 0 prefix instead. This is great because
- an integer constant now always consists of a single token,
- the parser can still tell right away it's got a constant,
- the parser can immediately tell the base (
0is the same in both bases), - it's mathematically sane (
00005 == 05), and - no precious special characters are needed (as in
#123).
When C was created from B, the need for hexadecimal numbers arose (the PDP-11 had 16-bit words) and all of the points above were still valid. Since octals were still needed for other machines, 0x was arbitrarily chosen (00 was probably ruled out as awkward).
C# is a descendant of C, so it inherits the syntax.