Converting Integer to string in C Without sprintf

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Today while solving a problem, I had to convert integers to string in C to use its indexing capabilities. One simple solution was to use sprintf, which can be done in 2-3 lines, but i was temped to solve this problem without any string manipulation functions.

The Code


Code: c
// Change a positive integer to a string in C 
  
 #include <stdio.h> 
 #include <stdlib.h>  // for malloc
 #include <math.h>  // for pow
  
 int get_number_of_digits(int n) 
 { 
     int i, count; 
     for (i = 10, count = 0; ; i *= 10, count++) { 
         if ((int)n / i == 0) { 
             break; 
         } 
     } 
     return count + 1; 
 } 
  
 int main() 
 { 
     int number, no_of_digits, prev = 0, i, j; 
     char *number_string; 
  
     scanf("%d", &number); 
     no_of_digits = get_number_of_digits(number); 
  
     number_string = (char *)malloc(sizeof(char) * (no_of_digits + 1)); // +1 for NULL 
     for (i = 0, j = pow(10, no_of_digits - 1); i < no_of_digits; i++, j /= 10) { 
         number_string[i] = (number / j - (prev * 10)) + 48;  // int to ascii
         prev             = number / j; 
     } 
     number_string[i] = '\0'; 
     printf("%s\n", number_string); 
 }
In the above program we used malloc (for dynamic memory allocation) and pow. I had to look at an ASCII table for reference on how to convert from int to ASCII and rest all is pure logic.

Compiling :

Code:
 gcc -lm int_to_str.c -o int_to_str
we used the -lm flag for math libraries

Output:

Code:
 lionaneesh@lionaneesh:~/Challenges/$ ./int_to_str  
 123 
 123
John Hoder
20Mar2012,00:48   #2
Scripting's Avatar
Awesome example how flexible C is Very good tut, althouhg I miss some more explanation about the code...
skl
Newbie Member
20May2012,21:56   #3
skl's Avatar
A faster and more elegant implementation:

Code:
inline int inttostr( char *s, int n )
{
        unsigned int i = 1000000000;
                /* largest base 10 'place' < 2^32 */
                /* for a 64 bit int - 10000000000000000000 */
                /* for a 16 bit int - 10000 */
                /* etc */

        /* if we are signed, put the sign in the string;
           afterwards, it doesn't matter */

        if( ((signed)n) < 0 ) {
                *s++ = '-';
                n = 0 - n;
        }

        do
                *s++ = '0' + (((n-n%i)/i)%10);
        while( i /= 10 );

        /* terminating null */

        *s = 0; 

        /* return value should always be zero, unless
           there was some unpredicted badness */

        return n;       
}
comparison:

one million reps of this:

Code:
real	0m0.198s
user	0m0.188s
sys	0m0.000s
vs one million reps of above

Code:
real	0m0.228s
user	0m0.224s
sys	0m0.000s
skl
Newbie Member
20May2012,22:19   #4
skl's Avatar
oops ... I did that too quickly without thinking.

above inserts leading zeroes!

should insert the following before the main loop:

Code:
     while( i > n ) i /= 10;
i.e.

Code:
 int inttostr( char *s, int n )
{
        unsigned int i = 1000000000;

        if( ((signed)n) < 0 ) {
                *s++ = '-';
                n = -n;
        }

        while( i > n ) i /= 10;

        do {
                *s++ = '0' + (n-n%i)/i%10;
        } while( i /= 10 );

        *s = 0;

        return n;
}
this is actually considerably faster

Code:
real	0m0.112s
user	0m0.104s
sys	0m0.004s
Light Poster
2Jun2012,06:29   #5
Mark Brown's Avatar
educative. I need such help to develop my skill.