Thread: Reversing a string with RULES!

My goal is to take a simple string like "My name is Don Knuth!", print it, then print it in reverse order. The rules are, I need to use this class

Code:

class ch_stack{
           public:
                  void reset() {top = EMPTY; }
                  void push(char c) {top++; s[top] = c; }
                  char pop() {return s[top--]; }
                  char top_of() const {return s[top]; }
                  bool empty() const { return (top == EMPTY); }
                  bool full() const { return (top == FULL); }
           private:
                   enum {max_len = 100, EMPTY = -1, FULL = max_len-1 };
                   char s[max_len];
                   int top;
     };

And the function it comes out of can't actually print anything...it has to be a void function. So I have to send a string (let's call it s1) to this function, the function reverses the string into a new string (let's call it s2) and then int main simply prints s2 and there we should have it.

This is what I have

Code:

#include<iostream>
using namespace std;

//Function to modify a string and return its value in reverse order
void reverse(const char s1[], char s2[])
{
     class ch_stack{
           public:
                  void reset() {top = EMPTY; }
                  void push(char c) {top++; s[top] = c; }
                  char pop() {return s[top--]; }
                  char top_of() const {return s[top]; }
                  bool empty() const { return (top == EMPTY); }
                  bool full() const { return (top == FULL); }
           private:
                   enum {max_len = 100, EMPTY = -1, FULL = max_len-1 };
                   char s[max_len];
                   int top;
     };
     
     ch_stack stack;
     int i = 0;    
     while (s1[i] && !stack.full())
          stack.push(s1[i++]);
     while (!stack.empty())
          s2[i] = stack.pop();
}

int main()
{
    void reverse(const char s1[], char s2[]);
    
    char s1[30] =  { "My name is Don Knuth!" };
    char s2[30];                      //String to contain reverse values
    
    cout << s1 << endl;               //Show string's original value
    reverse(s1, s2);
    cout << s2 << endl;               //Show string's final value
    
    system("pause");
}

The two things I don't understand are how exactly to make a while loop, implementing that class, to reverse the string, and to have it all in a void function. How does int main know what s2 is if it's not part of its file scope? Does it have something to do with static or something like it? Any help would be great. I am a beginner, so please treat me like one.

>> The two things I don't understand are how exactly to make a while loop, implementing that class, to reverse the string
Doesn't the posted code already do that?

>> How does int main know what s2 is if it's not part of its file scope?
What do you mean by this? There are two s2 variables in this code. One is declared in main and the other is declared as a parameter of the reverse function. Each function is using the one it declared. The data ends up being the same for both because when the reverse function is called, the s2 from main is passed to reverse so the s2 in reverse refers to the same array.

Sweet, thanks for the advice. I'm making progress. This is what I have now.

The int main() function is the same...the reverse function has been changed to

Code:

void reverse(const char s1[], char s2[])
{
     class ch_stack{
           public:
                  void reset() {top = EMPTY; }
                  void push(char c) {top++; s[top] = c; }
                  char pop() {return s[top--]; }
                  char top_of() const {return s[top]; }
                  bool empty() const { return (top == EMPTY); }
                  bool full() const { return (top == FULL); }
           private:
                   enum {max_len = 100, EMPTY = -1, FULL = max_len-1 };
                   char s[max_len];
                   int top;
     };
     
     ch_stack stack;
     int i = 0;
     stack.reset();    
     while (s1[i] && !stack.full())
          stack.push(s1[i++]);
     int w = 0;
     while (!stack.empty())
          s2[w++] = stack.pop();
}

It successfully reverse the string, but now the out put is...

"My name is Luis Camacho III!
!III ohcamaC siuL si eman yMπ┬┬wMy name is Luis Camacho III!
Press any key to continue . . ."

So it's succesfully reversing the string and returning it, but I'm getting garbage and the original string is still there...Any advice?

Don't forget to add the null terminator to the second string. A C style string must have a null character to indicate the end of the characters in the string.

Another option is to initialize s2 to be all null characters (initialize it to 0) so that wherever the reversal stops the rest of the characters will be null already.

This may sound stupid, but how exactly to I insert the null terminator into the second string? I'm drawing a blank. I know what it is and how it works, but I don't know the syntax for hard coding it in there.