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
[edit | edit source]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.[1]
/* 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
[edit | edit source]An itoa
function (and a similar function, ftoa
, that converted a float to a string) was listed in the first-edition Unix manual.[2] 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
[edit | edit source]- ↑ For the solution to this exercise, see "K&R2 solutions" on clc-wiki.net.
- ↑ "Unix Programmer's Manual", November 3, 1971. Section "Library routines".
External links
[edit | edit source]- 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.