C++ Recursion – Printing Elements of a Vector

Diego — Sun, 21 Feb 2010 08:05:33 +0000

Printing elements of a Vector using Recursion

Below is an example of how to print the elements of a vector using recursion. Later, we’ll print the elements in reverse. Why is this useful? You can apply this principle later to more complex data structures such as Binary Search Trees, or Linked Lists.

This tutorial assumes you understand the following:

Iterative version for printing elements of a vector

To understand how to print elements of a vector using recursion, let’s write a simple algorithm that will do it for us using iteration.

void printVector(vector  vec) { 

     for ( int i = 0; i < vec.size(); i++ ) { 

          //Printing ith element  followed by a space
          cout << vec[i] << " ";

     }

     //Printing end line
     cout << endl;
}

Recursive function for printing elements of a vector

So, how do we go from iteration to recursion? Well if you remember how to Print a Sequence of Numbers using Recursion without a vector, you know at every recursive call we print an number larger than the previous. In this case we want to print out an element in the vector in increasing order. So what would be our function header?

void printVector(vector  vec) {

Would this work? Well if you think about it, which element would you print? Using iteration, we used the i variable to print the ith element in the vector. Using recursion, we would simply pass this variable, so our function header would be the following.

void printVector(vector  vec, int i) {

Now, using the knowledge of Printing a Sequence of Numbers using Recursion we could apply this to printing a sequence of elements in a vector.

void printVector(vector vec, int i) { 

     //Returning when i is larger or equal to the size
    //of the vector
     if ( i >= vec.size() ) {
          return;
     }

     //Printing the ith element of the vector
     cout << vec[i] << " ";

     //Calling the function with a larger i
     printVector(vec, i+1);
}

Using this function would be done using the following.

vector  vec; 

//Populate vector...

//Calling the printVector function
printVector(vec, 0);

Have any questions? Feel free to ask below or request any other examples.

Going further

1. How would you make this function's base case take care of another case where your program might crash? (Hint: What is your function was called with a different i value?)
2. How would you make this function be called by simply passing in the vector and not the element to be printed first (in our example, 0). (Hint: Use Helper functions)
3. How would you print the elements of the vector in reverse? (Hint: Printing a sequence of numbers in reverse)

Introduction to Recursion in C++

Diego — Sat, 21 Feb 2009 23:18:23 +0000

Recursion in C++ is simply a function that calls itself that terminates when a base case is met. It’s very useful to learn how recursion for a couple of reasons.

It’s easier to solve certain problems with recursion as the resulting code is usually shorter
Sometimes its the only way

Displaying numbers recursively

So what’s an example of a recursive function? Let’s make one. Let’s start by creating a function that displays a number (it isn’t the best example, but it’s good enough to introduce you to recursion).

Display Function

void display(int n) 
{
     cout << n << " ";
     return;
}

All this function does is simply print the number passed in.

Recursive Display Function

Now let’s have this function call itself but lets decrement the value being displayed each time.

void display(int n)
{
     cout << n << " ";
     display(n-1); //Will call display but with (n-1)
     return;
}

So let’s say we executed display(10)

What would this display? It would display 10 9 8 7 6 5 4…and would never stop. Why? We never told the function when to terminate. This will either cause your program to run until you kill it, or may cause it to segmentation fault. In order to fix this, this calls for the addition of base case.

Base Cases in Recursion

In order for your recursive function to terminate you always need a base case. You may have multiple base cases.

Recursive Display Function with a Base Case

Let’s make the function terminate when n is 0. Or better yet, let’s make it terminate when n is less than or equal to 0. (Why do you think this is better?)

void display(int n)
{
     if ( n <= 0 ) return; 
     cout << n << " ";
     display(n-1); //Will call display but with (n-1)
     return;
}

So now, if we execute display(10), what would it display?

10 9 8 7 6 5 4 3 2 1

How Recursion works in C++

So how does recursion work in C++? Each function creates an activation record on the stack. As a function gets called, it gets added to the top of the stack. Until the function is terminated, is then taken off the stack. A visual example for our display function is below.

(Click to Enlarge)

The first function called was display(10) which is the activation record seen at the bottom of our stack. If you notice, the following function keep getting called as long as the base case isn’t met. Up until n = -1, then the activation record of the functions are popped off.

Comments on Recursion

What is shown previously is a very basic recursive algorithm. More recursion techniques will be discussed later. So even if you don’t understand the purpose of recursion just yet, don’t hesitate. You’ll understand why it’s important when you understand recursion is the only way to do certain problems.

Talk Binary » base case