How to use a recursive char * function to reverse a string in C++

Question

Just for full disclosure, this is a homework assignment.

I need to reverse a char string using a recursive function that accepts only a "char *string" input

I've done a lot of googling and whatnot, but just can't seem to figure it out. If I could use a void function, this would be done hours ago, but alas, that's not allowed.

The following code returns nothing, we're not allowed to change the function signature either

char *reverseString(char *string)
{

    if (*string =='\0') return string;

    else
    {
        return reverseString(string + 1) + *string;
    }

}

this is my multiple attempts at calling this function from the main to get output from it, and though the code runs, I get no output in the console

char Tstring[] = "TestString";
cout << reverseString(Tstring);
cout << *reverseString(Tstring);
char *answer2 = reverseString(Tstring);
cout << *answer2;

I'm not asking for a direct solution obviously, but I'm having trouble wrapping my head around how to do this using a pointer as an input and whatnot. Any tips or nudges in the right direction are appreciated

Answer 1

You cannot use the plus operator to concatenate char* pointers.

If you're allowed to, I'd rather take an inplace approach. Don't want to say too much here, but you can reverse a string without accessing any memory outside of the string. Your recursion could then pass pointers around.

You'll probably need a helper function

void reverse(char *start, char *end)

you're calling from your main algorithm.

edit: It would be a recursive solution, but the recursive call would be to the helper function rather than to the main function. Don't know whether that's allowed in your assignment. There might also be a way of making it recurse on the main function, but that'd be more complicated.

Answer 2

You need to remember the last character and then reverse the string without the shorter character.

So

F("yourstring") = "g" + F("yourstrin") 

etc etc

Which fortunately can be done in place with careful use of memcpy

Answer 3

I think this code will work. Sorry for writing such a basic one. I am just learning C++.

#include<iostream.h>
#include<string.h>
#include<conio.h>
char *rev(char *s,int n,int i)
{
char t;
if(i<n/2)
{
t=s[i];
s[i]=s[n-i-1];
s[n-i-1]=t;
i++;
rev(s,n,i);
}
return s;
}
void main()
{
char *s;
cout<<"\n\tEnter a String ";
cin>>s;
int n=strlen(s);
rev(s,n,0);
cout<<"\n\tReversed String :"<<s;
getch();
}

Answer 4

If it's recursive function, it should handle input like this:

1. F( "yourstring" )
2. F( "ourstring" )
3. F( "urstring" )

It prompts to handle one char each time. pseudo-code should like

char* F( char* input)
{
    move last char to the first;
    F(input+1);
    return input;
}

Working code:

char* reverse( char* input )
{
    int len = strlen( input );
    char tmp = input[ len - 1 ];
    if( len > 1 )
    {
        for( int i = len - 1; i > 0; i-- )
        {
            input[ i ] = input[ i - 1 ];
        }
        input[ 0 ] = tmp;
        reverse( input + 1 );
    }
    return input;
}

Answer 5

char* reverse(char *str)
{
    static int i=0;
    static char revstr[100] = "";
   if (*str)
   {
       reverse(str+1);
       revstr[i++]=*str;
   } 
   return revstr;
}