C++ problem (please help)

[allaboutdrew]

Hi all

Could someone please explain this program to me, especially lines 25 and 30. I understand that line 30 is a for loop and I understand how it works. However, I am unclear why it is minusing 3 from n and is n the variable passed from line 14? Line 25, I believe, consists of variables and I understand that they are defined by line 7. However, I am unsure how their values change during the running of the program. I would apprciate any help. Thank you.

Listing 7.15. Solving the nth Fibonacci numberusing iteration.

// Listing 7.15
  // Demonstrates solving the nth
  // Fibonacci number using iteration

  #include <iostream.h>

  typedef unsigned long int ULONG;

  ULONG fib(ULONG position);
  int main()
  {
      ULONG answer, position;
      cout << "Which position? ";
      cin >> position;
      cout << "\n";

      answer = fib(position);
      cout << answer << " is the ";
      cout << position << "th Fibonacci number.\n";
     return 0;
  }

  ULONG fib(ULONG n)
  {
      ULONG minusTwo=1, minusOne=1, answer=2;

      if (n < 3)
          return 1;

      for (n -= 3; n; n--)
      {
          minusTwo = minusOne;
          minusOne = answer;
          answer = minusOne + minusTwo;
      }

      return answer;
}

[strmstn]

ULONG fib(ULONG n) is a function and what is passed to that function is in main known as "position".

The "\n" on line 15 is simply a newline character, try removing the whole cout statement to see the difference.

The typedef on line 7 is just something telling the compiler that anything named ULONG should be the exact same thing as the standard unsigned long, or unsigned long int. Which is a 64 bit unsigned ( >= 0 ) variable.

The ULONG variables that are declared on line 25 are just local variables and they will be gone when the function reaches its final bracket.

[Sumyungi]

On line 25, the three variables are set to 1, 1, and 2 because these are the first three numbers in a fibonacci sequence (not including zero).

Roughly translated, line 30 would read: "initially set n to n - 3, then decrement n by one and execute some code so long as n is greater than zero."

n is initialized to n - 3 because answer is initialized to the 3rd element in the sequence (the value 2 is element 3). From that point on n is only used as a counter so the for loop knows where to stop, so n needs to be offset by 3 initially to return the nth element.

Let F represent the fibonacci sequence. Each F[n] (excluding elements 0 and 1) is equal to F[n-2] + F[n-1]. So starting with the initial values for minusTwo, minusOne, and answer, respectively, they will each take on the following values after each iteration of the for loop:

1,2,3
2,3,5
3,5,8

and so on.

Once n = 0, the nth element of the sequence will be reached, and the algorithm spits out the answer.

[allaboutdrew]

On line 25, the three variables are set to 1, 1, and 2 because these are the first three numbers in a fibonacci sequence (not including zero).

Roughly translated, line 30 would read: "initially set n to n - 3, then decrement n by one and execute some code so long as n is greater than zero."

n is initialized to n - 3 because answer is initialized to the 3rd element in the sequence (the value 2 is element 3). From that point on n is only used as a counter so the for loop knows where to stop, so n needs to be offset by 3 initially to return the nth element.

Let F represent the fibonacci sequence. Each F[n] (excluding elements 0 and 1) is equal to F[n-2] + F[n-1]. So starting with the initial values for minusTwo, minusOne, and answer, respectively, they will each take on the following values after each iteration of the for loop:

1,2,3
2,3,5
3,5,8

and so on.

Once n = 0, the nth element of the sequence will be reached, and the algorithm spits out the answer.

THANK YOU, THANK YOU, THANK YOU, THANK YOU xx