# Optimisation

• 10-29-2001
EvenFlow
Optimisation
I was fooling around with a fibonacci program whereby a recursive call is used.

Code:

```#include <stdio.h> long fibonacci(long); int main (void) {         long result, number;         printf("Fibonacci Program\n\n");         printf("Enter an integer please: ");         scanf("%d", &number);         result = fibonacci(number);         printf("Fibonacci %ld = %ld\n", number, result);         return 0; } long fibonacci(long n) {         if (n == 0 || n == 1)                 return n;         else                 return fibonacci(n - 1) + fibonacci(n - 2); }```
This program does its job for any integer from 1 - 26, without slowing down. The number of recursive calls that get executed to calculate the nth fibonacci number is on the order of 2n. Say you entered 20 - this would require 220 calls (about a million or so).

Interestingly enough, I ran this on one of the computers at university (1GHz Pentiums). Entering in 40 it still took quite some time to process (at least 10-12 seconds by my count).

I guess it is easy to see that even though most modern CPUs are incredibly fast, it is interesting to see that no matter how fast they are, a program such as this (or any that isn't optimised) still runs slowly. Hence my point - optimising code is still important and shouldn't be neglected.
• 10-29-2001
zen
Quote:

Hence my point - optimising code is still important and shouldn't be neglected.
Neither should planning, if you wanted to do it quickly for a high number you wouldn't use recursion :).
• 10-29-2001
EvenFlow
I was deliberately using recursion to demonstrate that fact - i.e. a better method could have been used, and that is what is slowing down the program.
• 10-29-2001
SilentStrike
There is a difference between optimizing and starting with horribly inefficient algorithms. Using O(2^N) algorithms does not mandate the need for optimization. One can write fairly efficient fibonacci code without breaking out a profiler and assembler.
• 10-29-2001
EvenFlow
Guess I could've used a better example couldn't I? But thanks for pointing that out ss.
• 10-29-2001
Nick
There's nothing wrong recursion it's that
you are recalculating every value so many times.

Code:

```int f(int n, int a, int b, int num) {       if (n == num)                 return b;             return f(n+1, b, a+b, num); } int fib(int num) {         if (num <= 2) return 1;         return f(0, 1, 2, num-3); }```
I would expect to be much faster than the other recursive routine.
A really good optmizing c compiler would be able to optimize the function calls out and do jumps.