What does "0x" mean at the beginning of hexadecimal numbers?

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17

I realized that when it comes to hexadecimal numbers sometimes one is placed 0x in front.

For example, 0xA1B2C3 instead of A1B2C3.

What is that 0x means?

hexadecimal
  • 1

    Languages use the prefix 0x as a way of differentiating hexadecimal values from other values (int, string, etc.).

4 answers

20

0x or 0X is a prefix that was initially used by AT&T Assembly compilers in the late 1960s to represent hexadecimal numerical values.

Bell Laboratories, at the time a subsidiary of AT&T, was the first to adopt the standard. It is also known for creating the operational environment UNIX, where it made wide use of this notation. Several syntactic descendants of *NIX (C, C#, Java, Javascript and others) propagated the use to the present day.

The 0 (initial zero) indicates that the value is a numerical constant; x is phonetically similar in English to 'Hexa''.

  • 3

    +1 for including a history lesson in the answer :)

  • 1

    @Renan Thanks! I found it relevant, and it is always interesting to learn a little more about what you like =)

19


Has been answered by Lucas that "the prefix 0x identifies the number that follows as a hexadecimal constant", but I want to complement the answer with some relevant details:

P: Why use a prefix?
R: To differentiate from a decimal, because it is perfectly normal a hexa without any letter. 0x99, for example, it is 153 decimally, but if you write int x = 99, the compiler could not guess his intention. o 0x99 already makes explicit that is hexa.


Curiosities:

This is a real trick of the rascal: in some languages, the 0 (without the x) before numbers, indicates "octal", thus, x = 032 is the same thing as x = 26. This easily confuses anyone who has no experience with prefixes.

I’ve seen certain BASIC dialects using &b or 0b to indicate binary, for example 0b00001011 to represent the decimal 11, as in some cases the &h for hexadecimal (in MSX, &h8000 is the usual way to represent the beginning of the available memory for writing after a normal boot).

One of the languages I use a lot (Harbour), uses 0d for dates. For example, dNascimento := 0d20010527.

  • 1

    Is there a list of all the prefixes, or is it just these?

  • 2

    @Patrick the most common are the 0x for hexadecimal and only 0 for octal, but it is an individual feature of the languages, not a standard. It is worth consulting the specific documentation of the same language.

  • 1

    Some languages realized that 0 for octal was not a good idea (it took, huh?) and are in the process of transitioning to remove this prefix or exchange it for 0o (with the letter "o" of "octal"). For example: Javascript, Python, Scala, D.

  • @(half-late) I didn’t understand the problem of using 0 for octal.

  • 1

    @Piovezan The first reason is in the answer: 032 (Oct) is 26 (Dec). Are you going to say that it’s not confusing that only a zero in the front changes the whole value of the number? That’s not how it should be... And if you are working with months (from 1 to 12) and want to put 0 forward to align, will give error in 08 and 09 (invalid octal numbers). Not to mention asymmetry: because hexadecimal uses 0x and octal is only 0? It must be from the time when octal was more used than hexadecimal.

8

The prefix 0x identifies the number it follows as a hexadecimal constant, and the prefix 0 as a number in octal (base 8).

For example,

#include <iostream>
using namespace std;

int main() {
    cout << 0xF << endl;
    cout << 010 << endl;
    return 0;
}

Will print 15 and 8 on the screen.

6

Some languages define 0x as prefix to a hexadecimal number, it is basically a sign for the compiler/interpreter that the number should be treated on another basis(16).

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