Question

Your team will write a function that reverses a C-string, to practice using pointers. You must use pointers and C-strings for this lab, not string objects. Your program should ask for the user's input, as "Enter the original string: " Then, it should print out the reversed string to the console.

Answer 1

Answer:

Here is the C++ program:

#include <iostream> //to use input output functions

#include<string.h> // to manipulate strings

using namespace std; //to identify objects like cin cout

void ReverseString(char* original_str){ // function that reverses a C-string

       char* ptr1 = original_str;  // pointer that points to the start of C-string

       char* ptr2 = original_str; // pointer that points to the start of C-string initially

       for (int i = 0; i < strlen(original_str)- 1; i++) {//iterates through the length of the C-string

           ptr2++;} //moves ptr2 to the end of the C-string

       while(ptr1 < ptr2) {// swaps each character of C-string using pointers

               char temp = *ptr1; //declares a temp variable to hold character that *ptr1 points to

               *ptr1 = *ptr2;

               *ptr2 = temp;

               ptr1++; //increments ptr1 moving it forward

               ptr2--;   }} //decrements ptr2 moving it backwards

int main(){ //start of main() function

       char input_str[]="";  /// declare a C-string

       cout<<"Enter original string: "; //prompts user to enter a string

       cin>>input_str; // reads that C-string from user

       ReverseString(input_str); //calls function to revserse the input string

       cout<<"\nReversed string: "<<input_str;} // displays the reversed string

Explanation:

The program has a function ReverseString that takes a C-style string as parameter and reverses the string using pointers.

The ptr1 and ptr2 are pointers that initially point to the beginning of original string.

for (int i = 0; i < strlen(original_str)- 1; i++) this statement has a for loop that iterates through the original string and moves the ptr2 to point at the end or last character of the C-string. strlen() method is used to get the length of the C-string. At each iteration ptr2 moves one time forward until the last character of the C-string is reached.

while(ptr1 < ptr2) statement has a while loop that works as follows:

checks if ptr1 is less than ptr2. This evaluate to true because the index of first character that ptr1 points to is 0 and last character is 5 so 0<5. Hence the statements inside while loop execute which swaps the character of C-string from beginning and end index using ptr1 and ptr2. a temp variable is declated to hold character that *ptr1 points to.

For example

original_string = "xyz" Then temp = *ptr1; stores the character that *ptr1 points to i.e. 'x'

*ptr1 = *ptr2; This statement assigns the character that *ptr2 points to the *ptr1. Basically these pointers are used as indices to the C-string. So this means that the *ptr1 now stores the last character of the C-string that is pointed to by *ptr2. So *ptr1 = 'z'

*ptr2 = temp; This statement assigns the character that temp stores to the *ptr2. We know that temp contains the first character i.e 'x'. So *ptr2 = 'x'      

ptr1++; The ptr1 is incremented to 1. This means it moves one position ahead and now points to the second character of the C-string.

ptr2--; The ptr2 is decremented to 1. This means it moves one position backwards and now points to the second character of the C-string.

Now the while loop breaks because ptr1 < ptr2 condition evaluates to false.

So the original string is reversed using pointers. Hence the output is:

zyx

screenshot of the program along with its output is attached.