Reverse a string using a recursive function

Question

I am currently studying C and I can't get past this exercise. I must create a recursive function to reverse string1 into string2 . Here is my code. I would gladly appreciate your help.

#include <stdio.h>
#define MAX 100

void reverse(char s1[],char s2[],int n,int j);

int main()
{
    char string1[MAX]="How Are You Mate";
    char string2[MAX]="";
    int n=0;
    int i=0;
    int j=0;

    for(i=0;string1[i]!='\0';i++)
        n++;
    reverse(string1,string2,n,j);
    printf("String-a normal:\n%s\n",string1);
    printf("String-a reverse:\n%s\n",string2);
    return 0;
}

void reverse(char s1[],char s2[],int n,int j)
{
     if(n>0)
     {
            s2[j]=s1[n];
            reverse(s1,s2,n-1,j+1);
     }
     else
            s2[j]='\0';
}

Answer 1

in-place (the caller could make a copy of the string before calling this function) string reverse with tail-recursion

void reverse (char *str, size_t len)
{
  char tmp;
  if (len-- < 2) return;

  tmp = *str;
  *str = str[len];
  str[len] = tmp;

  reverse (str+1, len -1);
}

O, if you don't want pointers:

void reverse (char str[], size_t len)
{
  char tmp;
  if (len-- < 2) return;

  tmp = str[0];
  str[0] = str[len];
  str[len] = tmp;

  reverse (str+1, len -1);
}

Answer 2

The reversing starts by copying the n -th character of string1 array into string2 . The n -th character happens to be the null terminator. It becomes the first character of your new string, so the string looks empty to all standard C routines, including printf .

Calling

reverse(string1,string2,n-1,j);

from the main should fix the problem. The condition in the reverse should be changed from if(n>0) to if(n>=0) as well.

Answer 3

Although it does not save the resulting string anywhere, you get the idea.

#include <stdio.h>

void rev (const char* str);

int main () {
    const char str[] = "!dlrow ,olleH";

    printf("%s\n", str);

    rev(str);
    printf("\n");

    return 0;
}

void rev (const char* str) {
    char c = *str;
    if (c != '\0') {
            rev(str + 1);
        printf("%c", c);
    }
}

Answer 4

I have corrected the program. Please find the changes below

void reverse(char s1[],char s2[],int n,int j)
{
 if(n>0)
 {
        s2[j]=s1[n-1];
        reverse(s1,s2,--n,++j);
 }
 else
        s2[j]='\0';
}

Answer 5

i recommend using library , size=strlen(array) in stead of

for(i=0;string1[i]!='\0';i++)
n++;

to count how many characters in arra