How to add to String Array from inside recursive function in C

Question

I am trying to write a C function that will give me the binary representation of a number n . The function I have prints the number correctly; however I would like the string array word to be updated with the same data as is being printed:

#include <stdio.h>
#include <stdlib.h>

#define MAXBIN 100

void printbitsrec(unsigned n, int n_bits, char *w)
{
    if (n_bits-- > 0) {
        printbitsrec(n >> 1, n_bits, w);
        if (n & 1) {
            printf("1");
            *w++ = '1';
        }
        else {
            printf("0");
            *w++ = '0';
        }

    }
}

void printBits(unsigned n, int n_bits, int ret)
{
    char word[MAXBIN];
    printbitsrec(n, n_bits, &word[0]);
    word[n_bits + 1] = '\0';
    if (ret)
        printf("\n");
    printf("word = %s\n", word);
}

int main() {
    printBits(2, 4, 1);
}

Is there a more elegant way to do this? What am I doing wrong in my code?

Answer 1

Here's my attempt:

#include <stdio.h>
#include <stdlib.h>

#define MAXBIN 100

char * printbitsrec(unsigned n, int n_bits, char *w)
{
    if (n_bits-- > 0) {
        w = printbitsrec(n >> 1, n_bits, w);
        const char c = (n & 1) ? '1' : '0';
        putchar(c);
        *w++ = c;
    }
    return w;
}

void printBits(unsigned n, int n_bits, int ret)
{
    char word[MAXBIN];
    char * w = printbitsrec(n, n_bits, &word[0]);
    *w++ = '\0';
    if (ret)
        printf("\n");
    printf("word = %s\n", word);
}

int main(void) {
    printBits(2, 4, 1);
    return 0;
}

Notable changes:

• Use a returned value to keep track of the end of the string
• Only compute the new character once
• Use putchar rather than printf() , just a basic performance consideration ( printf() is overkill when printing single characters)

Note that no attempt to prevent buffer overflow is done, that could of course be added.

Answer 2

There is issue in recursive function call, it should be below after w++ . Check below code

  1 #include <stdio.h>
  2 #include <stdlib.h>
  3
  4 #define MAXBIN 100
  5
  6 void printbitsrec(unsigned n, int n_bits, char *w)
  7 {
  8     if (n_bits-- > 0) {
  9         if (n & 1) {
 10             printf("1");
 11             *w = '1';
 12         }
 13         else {
 14             printf("0");
 15             *w = '0';
 16         }
 17         w++;
 18         printbitsrec(n >> 1, n_bits, w);
 19     }
 20 }
 21
 22 void printBits(unsigned n, int n_bits, int ret)
 23 {
 24     char word[MAXBIN];
 25     memset(word, 0x00, sizeof(word));
 26     printbitsrec(n, n_bits, &word[0]);
 27     //word[n_bits] = '\0';
 28     if (ret)
 29         printf("\n");
 30     printf("word = %s\n", word);
 31 }
 32
 33 int main() {
 34     printBits(2, 4, 1);
 35 }

Answer 3

Take Any global variable to preserve the array index inside recursive function which will solve your problem.

#include <stdio.h>
#include <stdlib.h>

#define MAXBIN 100

static int i = 0;
void printbitsrec(unsigned n, int n_bits, char *w)
{
    if (n_bits-- > 0) {
        printbitsrec(n >> 1, n_bits, w);
        if (n & 1) {
            printf("1");
            w[i] = '1';
        }
        else {
            printf("0");
            w[i] = '0';
        }
        i++;
    }
}

void printBits(unsigned n, int n_bits, int ret)
{

    char word[MAXBIN];
    printbitsrec(n, n_bits, word);
    word[i] = '\0';
    if (ret)
        printf("\n");
    printf("word = %s\n", word);
}

int main() {
    printBits(2, 4, 1);
}

Answer 4

As you increment *w after the recursive call of printbitsrec() w always points to the same memory Location. If you do not like global variables this works but looks not very nice:

void printBits(unsigned n, int n_bits, int ret)
{
    char word[MAXBIN];

    memset(word,0,MAXBIN);

    printbitsrec(n, n_bits, &word[0]);
//    *w++ = '\0';
    if (ret)
        printf("\n");
    printf("word = %s\n", word);
}

void printbitsrec(unsigned n, int n_bits, char *w)
{
    if (n_bits-- > 0)
    {
        w++;
        printbitsrec(n >> 1, n_bits, w);
        if (n & 1)
        {
            printf("1");
            *(w - 1)= '1';
        }
        else
        {
            printf("0");
            *(w - 1) = '0';
        }
    }
}