Itoa

Template:Lowercase The itoa 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. The book notes several issues with this implementation, including the fact that it does not correctly handle the weird number −2^wordsize-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 is implemented two pages earlier:

#include <string.h>

/* reverse:  reverse string s in place */
void reverse(char s[])
{
    int c, i, j;

    for (i = 0, j = strlen(s)-1; i<j; i++, j--) {
        c = s[i];
        s[i] = s[j];
        s[j] = c;
    }
}

An itoa function (and a similar function, ftoa, that converted a float to a string) is listed in the first-edition Unix manual. 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.

See also

• atoi

External links

• A simple itoa function - bad example with memory leak
• itoa example file
• 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. 5-20x faster than using sprintf
• 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.