Newbie question

[srinivas01]

been a week since i started learning c++, im trying to write a program for Fibonacci series using loops but i was not able to think of some logic so i took help of net. the if loop in this code, im not able to figure out hows it working. can someone plz explain it thx.

#include <iostream>

using namespace std;

int main()
{
int n, first = 0, second = 1 , next, c;
cout<<"enter no of terms";
cin>> n;
cout<<"first" << n << "terms of febonacci series are:" << "\n";
for ( c = 0 ; c < n ; c++ )
{
if ( c <= 1 )
next = c; // n = c
else
{
next = first + second;
first = second;
second = next;
}
cout<< next << "\n";

}

return 0;
}

[Gaminic]

It's actually pretty stupid to use that.

The point is: since the Fibonacci sequence is defined recursively (so value t depends on the values of t-1 and t-2), it is "undefined" without having two starting values (0 and 1, or 1 and 1, depending on preference).

Either you can skip the first two (start at c = 2), or you can use the fix they use here. If you start the series as "0, 1", then you can say that Fib(i) = i for i [0, 1].

Frankly, it's much easier (and it makes more sense) to hardcode the first two steps.

[srinivas01]

well i started c++ just a week ago and i didn't really get what is Fib(i) = i for i [0, 1] lol . i finished till here http://www.cplusplus.com/doc/tutorial/control/ ( till for loop). im taking a day break just to practice loops and other basic concepts.

[ahoysailor]

I don't agree that it's stupid to start there, as fibonacci has two seeded values of 0 and 1 (F0 and F1 consecutively), and this has been compensated for in the code and the user interface quite obviously states that we are being shown the first n terms in the fibonacci sequence (0 being the first).

To show a run through of the code:

the for loop makes int c increment each time it goes through.
if c is less than or equal to 1 then int next is assigned the value of c.
otherwise next =1st + 2nd (0+1 the first time round), then first is given a new value equal to the current value of 2nd, and 2nd is then given the value currently held in next.

so 1st time through you'd get:
c = 0
next = 0

output = 0

2nd time:
c = 1
next = 1

output = 1

3rd:
c = 2
next = 0+1 = 1
first = 1
second = 1

output = 1

4th:
c=3
next = 1 + 1 = 2
first = 1
second = 2

out = 2

5th:
c=4
next=1+2=3
first=2
second=3

out=3

6th:
c=5
next=2+3=5
first=3
second=5

out=5

and so on.

[srinivas01]

got it thx for the reply and do you think i should move to later topics instead of taking a break and practicing loops ?

[ahoysailor]

That all depends on how confident you feel with loops now.
If you haven't already, I'd suggest going through a few more examples of loops just to make sure you understand what's going on and perhaps try creating your own simple loop. Otherwise move on ahead.

[wolfgang]

I don't like the code you found OP so I wrote my own which is easier to explain and follow.

#include <iostream>
int main ()
{
        unsigned prev = 0, curr = 1, next, num_iterations;
        // prev, and curr are the seeds. The names are not important besides we automatically know them at the start of the loop.

        std::cin >> num_iterations;
        // Here I ask for the location in the sequence they want to print. 0 will produce no output. 1 will print one number of the sequence. 2 prints 2 numbers. So on and so forth.
       
        if (num_iterations > 0) // An else if control statement is not useful here as I want both to happen if the num_iterations > 1. There are obviously more efficient ways than this.
                std::cout << prev << " ";
        if (num_iterations > 1)
                std::cout << curr << " ";

        for (int i = 2; i < num_iterations; ++i) // Loop starts at two. If num_iterations < 2 then there is no reason to print the other numbers of the sequence besides the seeds 0 and 1.
        {
                next = prev + curr; // Get the next number in the sequence by adding the previous number and the current number of the sequence.
                std::cout << next << " "; // Print to screen.
                prev = curr; // Set previous number to the old current.
                curr = next; // The current number is the one we just printed.
        }

        return 0; // Program ends.

}

[srinivas01]

thx wolfgang. why did you use unsigned instead of signed ? and whats the difference between std::cin and cin>>?

[wolfgang]

I used unsigned because the Fibonacci sequence does not include negative numbers, so its safe to allow a larger range of positive numbers.

And std::cin is the same as cin>>; std:: is the namespace. If I put using namespace std; at the top of the program I would not need to put std:: infront of cout or cin.