Exercise 4.12 - convert integer into string by calling recursive routine

Question

Adapt the ideas of printd to write a recursive version of itoa; that is, convert an integer into a string by calling a recursive routine.

/* recursive version of itoa; that converts an integer string by calling a recursive routine */

#include<stdio.h>
#include<math.h>

#define MAXLEN 100

void itoa(int n,char s[]);


int main(void)
{
    int n;
    char s[MAXLEN];

    n = 1723;

    itoa(n,s);

    printf("%s",s);

    return 0;
}

void itoa(int n,char s[])
{
    static int i;

    if(n/10)
        itoa(n/10,s);
    else
    {
        i = 0;
        if( n < 0)
            s[i++]='-';
    }

    s[i++] = abs(n) % 10 + '0';

    s[i] = '\0';

}
    
Run this

Explanation

The main part of this program is the itoa routine, which takes an integer n and string s and is called recursively.

void itoa(int n,char s[])
{
    static int i;

    if(n/10)
        itoa(n/10,s);
    else
    {
        i = 0;
        if( n < 0)
            s[i++]='-';
    }

    s[i++] = abs(n) % 10 + '0';

    s[i] = '\0';

}

In the first invocation from the main program, this is called with n = 1723 and within the program the number n is divided by 10 (until it is less than 10), and the new number (which is old number / 10) is called with itoa again. When we reach the first digit of the number, the number is converted to a string using abs(n) % 10 + ‘0’ and stored in the s array. The array is closed with 0, in subsequent recurssion, the next values like 7,2,3 will override 0 stored from the previous iteration and in the last call of the recursion, the number the complete number is transformed from integer to string. s will look like [‘1’,‘7’,‘2’,‘8’,‘0’] and this will be printed in the main program.

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