C programming/stdlib.h/itoa

The itoa (integer to ASCII) function is a widespread non-standard extension to the standard C programming language. It cannot be portably used, as it is not defined in any of the C language standards; however, compilers often provide it through the header <stdlib.h> while in non-conforming mode, because it is a logical counterpart to the standard library function atoi. void itoa(int input, char *buffer, int radix) itoa takes the integer input value input and converts it to a number in base radix. The resulting number (a sequence of base-radix digits) is written to the output buffer buffer. Depending on the implementation, itoa may return a pointer to the first character in buffer, or may be designed so that passing a null buffer causes the function to return the length of the string that would have been written into a valid buffer. For converting a number to a string in base 8 (octal), 10 (decimal), or 16 (hexadecimal), a Standard-compliant alternative is to use the standard library function sprintf. K&R implementation The function itoa appeared in the first edition of Kernighan and Ritchie's The C Programming Language, on page 60. The second edition of The C Programming Language ("K&R2") contains the following implementation of itoa, on page 64 [for Spanish editions go to page 47]. The book notes several issues with this implementation, including the fact that it does not correctly handle the most negative number −2wordsize-1.For the solution to this exercise, see "K&R2 solutions" on clc-wiki.net. /* itoa: convert n to characters in s */ void itoa(int n, char s[]) { int i, sign; if ((sign = n) < 0) /* record sign */ n = -n; /* make n positive */ i = 0; do { /* generate digits in reverse order */ s[i++] = n % 10 + '0'; /* get next digit */ } while ((n /= 10) > 0); /* delete it */ if (sign < 0) s[i++] = '-'; s[i] = '\0'; reverse(s); } The function reverse used above is implemented two pages earlier: #include <string.h> /* reverse: reverse string s in place */ void reverse(char s[]) { int i, j; char c; for (i = 0, j = strlen(s)-1; i<j; i++, j--) { c = s[i]; s[i] = s[j]; s[j] = c; } } Other appearances An itoa function (and a similar function, ftoa, that converted a float to a string) was listed in the first-edition Unix manual."Unix Programmer's Manual", November 3, 1971. Section "Library routines". Unlike the versions given above, the Unix library version had an interface roughly equivalent to void itoa(int input, void (*subr)(char)) and would invoke the callback routine subr on each character in the output string, thus eliminating the need for a buffer big enough to hold the entire string. References External links itoa() implementations with performance tests lexical_cast - C++ alternative, part of the boost libraries modp_numtoa - C/C++ alternative for converting integers and floats to char buffers. Good old Integer To Ascii conversion: itoa - Another fast implementation of itoa for various datatypes, plus some boost-style wrapping in the form of boost::lexical_cast template specializations. deep