String problem.

can anyone help me with a problem I'm kinda new to C++. Here is my question:

Write a function of the form string mirrorEnds(string), that when given a string, looks for a mirror image (backwards) string at both the beginning and end of the given string. In other words, zero or more characters at the very beginning of the given string, and at the very end of the string in reverse order (possibly overlapping). For example, the string "abXYZba" has the mirror end "ab".

mirrorEnds("abXYZba") → "ab"
mirrorEnds("abca") → "a"
mirrorEnds("navan") → "navan"

I think I can do it if I can convert the string into an array of chars can this be done?

>I can convert the string into an array of chars can this be done
Well... std::strings are internally array of characters.. and you can still get individual chars using the [] operator...just like you'd do in a normal array.
Try doing something like the following:

Code:

---

Iterate over the string....(Let i be the index of the current position)
    if the character in the i'th position == char at (size - i)'th position
        append the the i'th char on the result string
    else break out from the loop

---

that's great that helps a lot thanks very much.

ok here's my code I'm getting an error with the strcat function

Code:

---

#include <cstdlib>
#include <iostream>
#include <string.h>

using namespace std;

string mirrorends(string s)
{
      int size = s.size();
      string mirror = " ";
     

      for (int i = 0; i <=size; i++)
      {
          if (s[i]==s[size -i])
          {
              strcat(mirror,s[i]);
          }
      }
     
      cout << mirror << endl;
      return 0;
}



int main(int argc, char *argv[])
{
    mirrorends("racecar");
   
    system("PAUSE");
    return EXIT_SUCCESS;
}

---

That is because you're using a cstring function for C++ strings.
Just use the + operator to append.
Also..you seem to misunderstand what returning values from functions entail....Read up on that topic too.

>> #include <string.h>
That should be #include <string>. The one with the .h is for C style string functions like strcat, but you don't need or want those functions here since the instructions say to use the C++ string class in <string> and that class has everything you need.

>> string mirror = " ";
string mirror; is fine since it defaults to empty. Your version adds a space character, which will make the result wrong.

Otherwise understand and implement manasij7479's advice and you should be good.

ok changed my code again and it compiles and runs but displays nothing.

here's my new (and hopefully improved code):

Code:

---

#include <cstdlib>
#include <iostream>
#include <string>

using namespace std;

string mirrorends(string s)
{
      int size = s.size();
      string mirror;
     

      for (int i = 0; i <=size; i++)
      {
          if (s[i]==s[size -i])
          {
            mirror = mirror + s[i];
          }
      }
     
      return mirror;
}



int main(int argc, char *argv[])
{
    string a = mirrorends("racecar");
    cout << a << endl;
   
    system("PAUSE");
    return EXIT_SUCCESS;
}

---

Now you just have to debug your program. This is a great example of how to debug since you know the word you're inputting into the function and so you can follow the flow yourself to see what is happening. If you don't have or don't know how to use a debugger, just do it by hand.

To start, you call mirrorends with the parameter "racecar", so inside that function the variable s is "racecar". So go to the first line. What is size? Write that down. Then the next line defines mirror as an empty string, so write down mirror and its current value.

Then move on to the for loop. First write down the new variable i and its initial value. Then check to see if i is less than or equal to size. Then enter the loop. Now, what character is at s[i]? Also, what is size-i and what character is at s[size-i]?

I'll leave you with that for now. Follow that type of logic and see if you get the results you expect. You should quickly see where the problem is, and then you can think about how to solve it.

By the way, make sure to remember that C++ strings are different than C style strings if you're used to those. There is no null-terminator or anything like that. The string is just the characters you see.

yeah I followed all that but still can't find my problem I know it's in the for loop when I try append the characters.
so s[i] is my first character of the string and I want to compare it to the last character, [size-i].
so when loop starts i =o, so s[i] gives me s[0] which is the first character,
then size = 7, so i get the last character s[7-0].

I fear this problem is so simple and just stearing me in the face ha. What am I doing wrong?

Write down the value of the string at those positions.
Don't just assume. If you note you want to access s[7], then count 8 characters from the beginning and write down what that character is.
Remember that indexing starts at 0, not 1!

| r | a | c | e | c | a | r |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 |

oh just sank in there ha thnk you Elysia, Daved and Manasij7479. Code now works perfect :D

Can you shed some light on how your final code works?

when I had s[size - i] which is [7-0] I was comparing my first character to the character at s[7] which is not the last character as Daved and Elysia pointed out. So my first character did not = to the character at s[7] so the code never entered the loop and just displayed the blank string.

Code:

---

#include <cstdlib>
#include <iostream>
#include <string>
 
using namespace std;
 
string mirrorends(string s)
{
      int size = s.size();
      string mirror;
       
 
      for (int i = 0; i <=size; i++)
      {
          if (s[i]==s[size -(i+1)])
          {
            mirror = mirror + s[i];
          }
      }
       
      return mirror;
}
 
 
 
int main(int argc, char *argv[])
{
    string a = mirrorends("racecar");
    cout << a << endl;
   
    system("PAUSE");
    return EXIT_SUCCESS;
}

---

By the way... there is still a bug in your code.

Continue walking through the loop to find it. Also make sure to test other words besides racecar.

Acutally, I don't understand how it works since you approaches a memory which is not yours to access.
Like people told you before size is not the element indicator because we start from 0.

I changed it completely I put a while loop in instead I think the code is ok now cheers.

sorry walla I don't understand what you mean.

What is your current code?

Ok it's probably still wrong ha.. works fine though.

Code:

---

using namespace std;

string mirrorends(string s)
{
      int size = s.size();
      string mirror;
     

      int i = 0;
     
      while (s[i]==s[size -(i+1)])
          {
            mirror = mirror + s[i];
            i++;
          }
     
      return mirror;
}



int main(int argc, char *argv[])
{
    string a = mirrorends("racecar");
    cout << a << endl;
   
    system("PAUSE");
    return EXIT_SUCCESS;
}

---

why not use iterators? std::string provides forward and reverse iterators, accessible with begin/end and rbegin/rend. you could start a for loop, initializing two iterators with begin and rbegin, and increment both each time through the loop. as long as the values referred to by the iterators are the same, you continue the loop and add the current value of the iterator you received from begin to the end of your string.

>> Ok it's probably still wrong ha.. works fine though.

Works fine for which inputs? I liked your for loop better, honestly. The bug you had was really simple. Did you figure out what it was or did you just want to try the while loop? Note that your while loop actually has the same type of problem as your for loop did (you have to know when to stop).


>> why not use iterators?

For something this simple that is a learning exercise it's probably not necessary, especially since it's unlikely that caffrea4 has been introduced to them. Also note that he or she would have to solve the same problem for your pseudocode as in the for and while loops, so it wouldn't be any more correct than what is there already.

Quote:

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Originally Posted by caffrea4

sorry walla I don't understand what you mean.

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<=size wasn't good, it should be <size since the count begins from 0.