How does this c++ function work?

Question

#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.

A debugger, some inline print statements or just a plain trace on paper of the function run can tell you — StoryTeller - Unslander Monica, Feb 06 '18 at 22:57
@RSahu: I guess you can terminate once you figure it out. 8v) — Fred Larson, Feb 06 '18 at 23:03
Maybe it would help to see the problem in F#: https://stackoverflow.com/questions/2845744/generating-fibonacci-series-in-f — Eljay, Feb 07 '18 at 01:30

Arnav Borborah · Answer 1 · 2018-02-06T23:13:05.747

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.

How does this c++ function work?

1 Answers1