Reversing the elements of a vector using recursion

Below is an example of how to reverse the elements of a vector using recursion. This tutorial expects you to understand the following.

C++ Recursion

Example of reversing the elements of a vector

Below is an example of how one would reverse the elements of a vector by simply intuition.

// Our vector with size 5
[ X ] [ B ] [ A ] [ N ] [ Y ]
[ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ]

//Step 1: Swap entry 0 (first) with the entry n (last)
[ Y ] [ B ] [ A ] [ N ] [ X ]
[ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ]

//Step 2: Swap entry 1 with entry 3 (n -1)
[ Y ] [ N ] [ A ] [ B ] [ X ]
[ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ]

//Step 3: No swap needed for 3rd entry (or middle entry)
[ Y ] [ N ] [ A ] [ B ] [ X ]
[ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ]
 

Notice how once we arrive at the entry in the middle of the vector we don’t continue. Why? The values beyond this entry have already been swapped. Keep this in mind.

Reversing the elements of a vector using iteration

Before we try making a recursive function to reverse the elements of a vector, let’s make one using iteration. By following the method described in the method above, it’s easy to see that we are simply swapping the ith and (nth – ith) variables (Example: the 0th and the 4th (4th – 0th) element).

Reversing the elements of a vector using iteration

The iterative solution for reversing the elements of a vector is below. Make sure you are trying all these functions if you haven’t before on your own.

//We pass the vector by reference
void reverseElements(vector <int> &vec) {

     //Temp variable for swapping
     int temp;

     int n = vec.size()-1;
 
     //Iterate vec.size()/2 times
     for ( int i = 0; i < vec.size()/2; i++ ) {
 
          // The swapping of elements
          temp = vec[i];
          vec[i] = vec[n-i];
          vec[n] = temp;
   
     }

     return;
}

Make sure you understand this completely before moving forward. Try working it out by hand.

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Printing a sequence of numbers in reverse

10 9 8 7 6 5 4 3 2 1

1 2 3 4 5 6 7 8 9 10

Printing numbers using recursion

The code above is explained in Introduction to Recursion in C++.

Example:

 print(3); 

Produces:

3 2 1

Printing Numbers in Reverse