CODEWITHSUNDEEP
c++
Dimitris Siomis
Dimitris Siomis

Reputation: 11

How does this c++ function work?

#include <cstdlib>

#include <iostream>

using namespace std;

int myFunction(int n)
{
    int x;
    if (n==1 || n==2)
       x = 1;
    else
        x = myFunction(n-2) + myFunction(n-1);
    return x;
}

int main(int argc, char *argv[])
{
    int n,a;
    n = 7;
    a = myFunction(n);
    cout << "x is: " << a; 

    system("PAUSE");
    return EXIT_SUCCESS;
}

The output to this is "x is: 13". How do I get x = 13 when I do n = 7? It seems that the function repeats some number of times until x=1.

Upvotes: 0

Views: 90

Answers (1)

Arnav Borborah
Arnav Borborah

Reputation: 11769

I will walk you through it for a simpler example where the function is 4, but the overall concept is the same for 7 as well.

First call: myFunction(4) and inside the function n = 4.

Since n does not equal 1 or 2, we skip to the else:

x = myFunction(3) + myFunction(2);
x = myFunction(1) + myFunction(2) + 1; // If there is a 1, that means 
                                       // that we went to the if 
                                       // clause, not the else, since
                                       // n was either 1 or 2
x = 1 + 1 + 1;
x = 3;

This concept is called recursion and can be very useful at times. Your function currently calculates the nth fibonacci number.

On a side note, you should read why using system("PAUSE"); is a bad idea.

Upvotes: 1

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