How to convert an int to string in C

Question

How do you convert an int  integer  to a string  I m trying to make a function that converts the data of a struct into a string to save it in a file

User · Answer

Converting anything to a string should either 1) allocate the resultant string or 2) pass in a char * destination and size. Sample code below:

Both work for all int including INT_MIN. They provide a consistent output unlike snprintf() which depends on the current locale.

Method 1: Returns NULL on out-of-memory.

#define INT_DECIMAL_STRING_SIZE(int_type) ((CHAR_BIT*sizeof(int_type)-1)*10/33+3)

char *int_to_string_alloc(int x) {
  int i = x;
  char buf[INT_DECIMAL_STRING_SIZE(int)];
  char *p = &buf[sizeof buf - 1];
  *p = '\0';
  if (i >= 0) {
    i = -i;
  }
  do {
    p--;
    *p = (char) ('0' - i % 10);
    i /= 10;
  } while (i);
  if (x < 0) {
    p--;
    *p = '-';
  }
  size_t len = (size_t) (&buf[sizeof buf] - p);
  char *s = malloc(len);
  if (s) {
    memcpy(s, p, len);
  }
  return s;
}

Method 2: It returns NULL if the buffer was too small.

static char *int_to_string_helper(char *dest, size_t n, int x) {
  if (n == 0) {
    return NULL;
  }
  if (x <= -10) {
    dest = int_to_string_helper(dest, n - 1, x / 10);
    if (dest == NULL) return NULL;
  }
  *dest = (char) ('0' - x % 10);
  return dest + 1;
}

char *int_to_string(char *dest, size_t n, int x) {
  char *p = dest;
  if (n == 0) {
    return NULL;
  }
  n--;
  if (x < 0) {
    if (n == 0) return NULL;
    n--;
    *p++ = '-';
  } else {
    x = -x;
  }
  p = int_to_string_helper(p, n, x);
  if (p == NULL) return NULL;
  *p = 0;
  return dest;
}

[Edit] as request by @Alter Mann

(CHAR_BIT*sizeof(int_type)-1)*10/33+3 is at least the maximum number of char needed to encode the some signed integer type as a string consisting of an optional negative sign, digits, and a null character..

The number of non-sign bits in a signed integer is no more than CHAR_BIT*sizeof(int_type)-1. A base-10 representation of a n-bit binary number takes up to n*log10(2) + 1 digits. 10/33 is slightly more than log10(2). +1 for the sign char and +1 for the null character. Other fractions could be used like 28/93.

Method 3: If one wants to live on the edge and buffer overflow is not a concern, a simple C99 or later solution follows which handles all int.

#include <limits.h>
#include <stdio.h>

static char *itoa_simple_helper(char *dest, int i) {
  if (i <= -10) {
    dest = itoa_simple_helper(dest, i/10);
  }
  *dest++ = '0' - i%10;
  return dest;
}

char *itoa_simple(char *dest, int i) {
  char *s = dest;
  if (i < 0) {
    *s++ = '-';
  } else {
    i = -i;
  }
  *itoa_simple_helper(s, i) = '\0';
  return dest;
}

int main() {
  char s[100];
  puts(itoa_simple(s, 0));
  puts(itoa_simple(s, 1));
  puts(itoa_simple(s, -1));
  puts(itoa_simple(s, 12345));
  puts(itoa_simple(s, INT_MAX-1));
  puts(itoa_simple(s, INT_MAX));
  puts(itoa_simple(s, INT_MIN+1));
  puts(itoa_simple(s, INT_MIN));
}

Sample output

0
1
-1
12345
2147483646
2147483647
-2147483647
-2147483648

User · Answer

If you want to output your structure into a file there is no need to convert any value beforehand  You can just use the printf format specification to indicate how to output your values and use any of the operators from printf family to output your data

User · Answer

This is old but here s another way    include  lt stdio h gt    define atoa x   x  int main int argc  char  argv          char  string   atoa 1234567890       printf   s n   string       return 0

User · Answer

Use function itoa   to convert an integer to a string  For example   char msg 30   int num   10  itoa num msg 10

User · Answer

If you are using GCC  you can use the GNU extension asprintf function   char  str  asprintf   amp str    i   12313   free str

User · Answer

The short answer is   snprintf  str  size    d   x      The longer is  first you need to find out sufficient size  snprintf tells you length if you call it with NULL  0 as first parameters   snprintf  NULL  0    d   x      Allocate one character more for null-terminator    include  lt stdio h gt    include  lt stdlib h gt   int x   -42  int length   snprintf  NULL  0    d   x    char  str   malloc  length   1    snprintf  str  length   1    d   x        free str     If works for every format string  so you can convert float or double to string by using   g   you can convert int to hex using   x   and so on

User · Answer

You can use sprintf to do it  or maybe snprintf if you have it   char str ENOUGH   sprintf str    d   42     Where the number of characters  plus terminating char  in the str can be calculated using    int   ceil log10 num   1  sizeof char

User · Answer

EDIT   As pointed out in the comment  itoa   is not a standard  so better use sprintf   approach suggested in the rivaling answer     You can use itoa   function to convert your integer value to a string   Here is an example   int num   321  char snum 5       convert 123 to string  buf  itoa num  snum  10       print our string printf   s n   snum       If you want to output your structure into a file there is no need to convert any value beforehand  You can just use the printf format specification to indicate how to output your values and use any of the operators from printf family to output your data

User · Answer

After having looked at various versions of itoa for gcc  the most flexible version I have found that is capable of handling conversions to binary  decimal and hexadecimal  both positive and negative is the fourth version found at http   www strudel org uk itoa   While sprintf snprintf have advantages  they will not handle negative numbers for anything other than decimal conversion  Since the link above is either off-line or no longer active  I ve included their 4th version below          C   version 0 4 char  style  itoa      Written by Luk  s Chmela    Released under GPLv3      char  itoa int value  char  result  int base           check that the base if valid     if  base  lt  2    base  gt  36     result     0   return result         char  ptr   result   ptr1   result  tmp char      int tmp value       do           tmp value   value          value    base           ptr      zyxwvutsrqponmlkjihgfedcba9876543210123456789abcdefghijklmnopqrstuvwxyz   35    tmp value - value   base          while   value            Apply negative sign     if  tmp value  lt  0   ptr      -        ptr--     0       while ptr1  lt  ptr            tmp char    ptr           ptr--   ptr1           ptr1     tmp char            return result

User · Answer

Function return size of string and convert signed      integer to ascii value and store them in array of      character with NULL at the end of the array            int itoa int value char  ptr                 int count 0 temp          if ptr  NULL              return 0             if value  0                            ptr  0               return 1                     if value lt 0                        value   -1                    ptr    -               count                      for temp value temp gt 0 temp  10 ptr              ptr   0           for temp value temp gt 0 temp  10                         --ptr temp 10  0               count                      return count

[c] How to convert an int to string in C?

Examples related to c

Examples related to string

Examples related to integer