C - Write program to convert integer to string

Requirements

Define a function to return a string representation of an integer.

The integer is passed as the argument.

For example, if the argument is 25, the function will return "25".

If the argument is -98, the function will return "-98".

Demo

#include <stdio.h>
#include <stdbool.h>

#define STR_LEN 6                         // Length of string to store value string

char* itoa(int n, char str[], size_t size) {
  size_t i = 0;                           // character count
  bool negative = n < 0;                  // Indicates negative integer 
  int length = 0;                         // Length of string
  char temp = '0';                        // Temporary storage

  if (negative)                            // If it's negative...
    n = -n;                               // make it positive

                        // Generate digit characters in reverse order
  do/*from   w  w  w  .  j  a  v  a 2 s .  c  o  m*/
  {
    if (size == i + 1 + (negative ? 1 : 0))
    {
      printf("String not long enough.\n");
      return NULL;
    }
    str[i++] = '0' + n % 10;         // Create a rightmost digit
    n /= 10;                         // Remove the digit
  } while (n > 0);                     // Go again if there's more digits

  if (negative)                        // If it was negative...
    str[i++] = '-';                  // ...append minus

  str[i] = '\0';                       // Append terminator
  length = i;                          // Save the length including null character

                     // Now reverse the string in place by switching first and last,
                     // second and last but one, third and last but two, etc.
  for (i = 0; i < length / 2; ++i)
  {
    temp = str[i];
    str[i] = str[length - i - 1];
    str[length - i - 1] = temp;
  }
  return str;                                    // Return the string
}
int main(void)
{
  char str[STR_LEN];          // Stores string representation of an integer
  long testdata[] = { 3L, -98L, 110L, -1L, 999L, -12345L, 12345L };

  for (int i = 0; i < sizeof testdata / sizeof(long); ++i)
  {
    if (itoa(testdata[i], str, sizeof(str)))
      printf("Integer value is %d, string is %s\n", testdata[i], str);
  }

  return 0;
}

Result

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