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Sep 5th, 2008

Recursive function not fully working

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The recursive function is only reversing the first and last characters of a string. I've been trying for hours but cant find the error. Any help would be awesome. The code:

C++ Syntax (Toggle Plain Text)
#include <iostream>
#include <string>
using namespace std;
 
void str_reverse(char str[], int length)
{
 
    char temp, temp1;
    int starti=0;
    int endi = length-starti;
    temp1 = str[endi-1];
    temp = str[starti];
 
    if(starti>=length-starti)
    {
         return;
    }
    str[starti] = temp1;
    str[endi-1] = temp;
    starti++;
    return (str_reverse(str, length-1));
}
 
int main()
{
    string test= "lighter";
    str_reverse(test, 7);
    cout << "output= " << test;
    return 0;
}
#include <iostream>
#include <string>
using namespace std;

void str_reverse(char str[], int length)
{
	
    char temp, temp1;
    int starti=0;
    int endi = length-starti;
    temp1 = str[endi-1];
    temp = str[starti];
    
    if(starti>=length-starti)
    {
         return;
    }
    str[starti] = temp1;
    str[endi-1] = temp;
    starti++;
    return (str_reverse(str, length-1));
}

int main()
{
    string test= "lighter";
    str_reverse(test, 7);
    cout << "output= " << test;
    return 0;
}

nizbit

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Sep 5th, 2008

Re: Recursive function not fully working

Click to Expand / Collapse Quote originally posted by nizbit ...

The recursive function is only reversing the first and last characters of a string. I've been trying for hours but cant find the error. Any help would be awesome. The code:
C++ Syntax (Toggle Plain Text)
#include <iostream>
#include <string>
using namespace std;
 
void str_reverse(char str[], int length)
{
 
    char temp, temp1;
    int starti=0;
    int endi = length-starti;
    temp1 = str[endi-1];
    temp = str[starti];
 
    if(starti>=length-starti)
    {
         return;
    }
    str[starti] = temp1;
    str[endi-1] = temp;
    starti++;
    return (str_reverse(str, length-1));
}
 
int main()
{
    string test= "lighter";
    str_reverse(test, 7);
    cout << "output= " << test;
    return 0;
}
#include <iostream> #include <string> using namespace std; void str_reverse(char str[], int length) { char temp, temp1; int starti=0; int endi = length-starti; temp1 = str[endi-1]; temp = str[starti]; if(starti>=length-starti) { return; } str[starti] = temp1; str[endi-1] = temp; starti++; return (str_reverse(str, length-1)); } int main() { string test= "lighter"; str_reverse(test, 7); cout << "output= " << test; return 0; }

A few problems. One, look at this call and this function:

(Toggle Plain Text)

    string test= "lighter";
    str_reverse(test, 7);

    void str_reverse(char str[], int length)

It gave me a compile error until I changed test to a character array to match the function.

Two:

(Toggle Plain Text)

void str_reverse(char str[], int length)

return (str_reverse(str, length-1));

Not an error, or at least my compiler let me get away with it, but you are returning something from a void function here. Just call it. As to the rest of your problem, look carefully at the two arguments in this line and make sure they are what you want.:

(Toggle Plain Text)

return (str_reverse(str, length-1));

Last edited by VernonDozier; Sep 5th, 2008 at 11:51 pm.

VernonDozier

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Sep 6th, 2008

Re: Recursive function not fully working

The first part was just a stub. I had the wrong string in my mind. I'm still not seeing whats wrong. Could you explain a little farther.

nizbit

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Re: Recursive function not fully working

Click to Expand / Collapse Quote originally posted by nizbit ...

The first part was just a stub. I had the wrong string in my mind. I'm still not seeing whats wrong. Could you explain a little farther.

Post the exact code that compiles and gives you the problem, along with your results, whether it is working or not. To help you debug, place the following line:

C++ Syntax (Toggle Plain Text)

cout << str << endl;

before the recursive call so you can see each iteration. This may help you see what is occurring.

VernonDozier

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Sep 6th, 2008

Re: Recursive function not fully working

A lot of the code in your function is superfluous. Try simplifying it to what you actually need. What is the point of starti? it's always read as 0. starti is a local non-static variable, so your starti++; at the end of the function does nothing useful.

Also, in the recursive call to reverse, length-1 should be length-2, as you've just swapped the first AND last characters (that's 2 characters). You'll also want to increment the pointer for each call so that you don't keep swapping the first character with every other character.

CPP Syntax (Toggle Plain Text)
void reverse(char *str, int len){
   if(len >= 2){
      swap(str[0], str[len-1]);  // swap first & last chars
 
      reverse(&str[1], len - 2); // swap the rest - (but not the chars we just swapped)
      }
   }
void reverse(char *str, int len){
   if(len >= 2){
      swap(str[0], str[len-1]);  // swap first & last chars
      
      reverse(&str[1], len - 2); // swap the rest - (but not the chars we just swapped)
      }
   }

dougy83

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Sep 6th, 2008

Re: Recursive function not fully working

Thanks for the help everyone. My recursive function is working!!

nizbit

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