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';
}

The code does compile fine,the problem is it doesnt show any characters when I print the 2 string. — Lind
– Lind, Commented Mar 10, 2013 at 11:29
@Lind, does the exercise mention any benefit you could gain from using a recursive function to reverse a string in C, whose "strings" cannot be returned easily and therefore do not fit that pattern very well? — Frédéric Hamidi
– Frédéric Hamidi, Commented Mar 10, 2013 at 11:35
It says the same thing as you said.Using a recursive function does not make a faster program,but still it asks for an exercise with a recursive function.I cant move on without solving this exercise. — Lind
– Lind, Commented Mar 10, 2013 at 11:37

wildplasser · Accepted Answer · 2013-03-10 11:56:15Z

5

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);
}

edited Mar 10, 2013 at 11:56

answered Mar 10, 2013 at 11:41

wildplasser

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1 Comment

Lind Over a year ago

I fixed my original solution,and yes I understand it,although I must read pointers again cuz I don't have a clear idea on them.

Sergey Kalinichenko · Accepted Answer · 2013-03-10 11:42:40Z

3

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.

edited Mar 10, 2013 at 11:42

answered Mar 10, 2013 at 11:36

Sergey Kalinichenko

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3 Comments

Lind Over a year ago

Thanks this is the answer I was looking for.I made the condition string1[i]!='\0' in the for loop,but I started counting from the first character,and I think thats the problem here.

Lind Over a year ago

I didnt put n-1 on the function but I declared int n=-1.As for the n>=0 that was necessary thanks.

Sergey Kalinichenko Over a year ago

@Lind Starting n at -1 would work too. By the way, you can drop the declaration of j from main, because it is assigned zero, and is not changed after that, so you can pass 0 as your last parameter of reverse for the same effect.

Michal Rus · Accepted Answer · 2013-03-10 11:42:51Z

2

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);
    }
}

answered Mar 10, 2013 at 11:42

Michal Rus

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Comments

kriya · Accepted Answer · 2013-12-09 12:03:33Z

1

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';
}

answered Dec 9, 2013 at 12:03

kriya

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Comments

Martin Voroňák · Accepted Answer · 2015-06-03 15:47:19Z

0

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

answered Jun 3, 2015 at 15:47

Martin Voroňák

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