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I've seen the following line in a source code written in C:
printf("%2$d %1$d", a, b);
What does it mean?
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In C language, %n is a special format specifier. It cause printf() to load the variable pointed by corresponding argument. The loading is done with a value which is equal to the number of characters printed by printf() before the occurrence of %n.
%s refers to a string %d refers to an integer %c refers to a character. Therefore: %s%d%s%c\n prints the string "The first character in sting ", %d prints i, %s prints " is ", and %c prints str[0].
The %a formatting specifier is new in C99. It prints the floating-point number in hexadecimal form. This is not something you would use to present numbers to users, but it's very handy for under-the-hood/technical use cases. As an example, this code: printf("pi=%a\n", 3.14); prints: pi=0x1.91eb86p+1.
In C printf(), %n is a special format specifier which instead of printing something causes printf() to load the variable pointed by the corresponding argument with a value equal to the number of characters that have been printed by printf() before the occurrence of %n. The above program prints “geeks for geeks 10”.
It's an extension to the language added by POSIX (C11-compliant behaviour should be as described in an answer by @chux). Notation %2$d means the same as %d (output signed integer), except it formats the parameter with given 1-based number (in your case it's a second parameter, b).
So, when you run the following code:
#include <stdio.h> int main() { int a = 3, b = 2; printf("%2$d %1$d", a, b); return 0; } you'll get 2 3 in standard output.
More info can be found on printf man pages.
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Per the C spec C11dr 7.21.6.1
As part of a print format, the first % in "%2$d %1$d" introduces a directive. This directive may have various flags, width, precision, length modifier and finally a conversion specifier. In this case 2 is a width. The next character $ is neither a precision, length modifier nor conversion specifier. Thus since the conversion specification is invalid,
... the behavior is undefined. C11dr 7.21.6.1 9
The C spec discusses future library directions. Lower case letters may be added in the future and other characters may be used in extensions. Of course $ is not a lower case letter, so that is good for the future. It certainly fits the "other character" role as $ is not even part of the C character set.
In various *nix implementations, $ is used as describe in Linux Programmer's Manual PRINTF(3). The $, along with the preceding integer defines the argument index of the width.
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