C Program to Print Fibonacci Series

A C program to print the Fibonacci series uses a loop to generate numbers where each number is the sum of the two previous ones, starting with 0 and 1. For example: int a=0, b=1, next; for(int i=0; i prints the first n Fibonacci numbers.

Examples

Input5

Output0 1 1 2 3

Input1

Output0

Input10

Output0 1 1 2 3 5 8 13 21 34

How to Think About It

To print the Fibonacci series, start with the first two numbers 0 and 1. Then, repeatedly add the last two numbers to get the next one. Keep printing each number until you reach the desired count.

Algorithm

1

Get the number of terms to print (n).

2

Initialize two variables with 0 and 1 as the first two Fibonacci numbers.

3

Repeat n times: print the current number, calculate the next by adding the last two, then update the last two numbers.

4

Stop when n numbers are printed.

Code

c

#include <stdio.h>
int main() {
    int n = 10, a = 0, b = 1, next;
    for (int i = 0; i < n; i++) {
        printf("%d ", a);
        next = a + b;
        a = b;
        b = next;
    }
    return 0;
}

Output

0 1 1 2 3 5 8 13 21 34 

Dry Run

Let's trace printing the first 5 Fibonacci numbers through the code.

1

Initialize variables

n=5, a=0, b=1

2

Iteration 1

Print a=0; next=0+1=1; a=1; b=1

3

Iteration 2

Print a=1; next=1+1=2; a=1; b=2

4

Iteration 3

Print a=1; next=1+2=3; a=2; b=3

5

Iteration 4

Print a=2; next=2+3=5; a=3; b=5

6

Iteration 5

Print a=3; next=3+5=8; a=5; b=8

Iteration	a (printed)	b	next
1	0	1	1
2	1	1	2
3	1	2	3
4	2	3	5
5	3	5	8

Why This Works

Step 1: Start with first two numbers

The Fibonacci series begins with 0 and 1, so we set a=0 and b=1.

Step 2: Print current number

We print the current number a in each loop iteration to show the series.

Step 3: Calculate next number

The next Fibonacci number is the sum of the last two, so we compute next = a + b.

Step 4: Update variables

We then update a to b and b to next to move forward in the series.

Alternative Approaches

Recursive approach

c

#include <stdio.h>
int fib(int n) {
    if (n <= 1) return n;
    return fib(n-1) + fib(n-2);
}
int main() {
    int n = 10;
    for (int i = 0; i < n; i++) {
        printf("%d ", fib(i));
    }
    return 0;
}

This method is simple but inefficient for large n due to repeated calculations.

Using array to store series

c

#include <stdio.h>
int main() {
    int n = 10, fib[10];
    fib[0] = 0; fib[1] = 1;
    for (int i = 2; i < n; i++) {
        fib[i] = fib[i-1] + fib[i-2];
    }
    for (int i = 0; i < n; i++) {
        printf("%d ", fib[i]);
    }
    return 0;
}

This stores all numbers but uses more memory; useful if you need to access series later.

Complexity: O(n) time, O(1) space

Time Complexity

The loop runs n times, so the time complexity is O(n). Each iteration does constant work.

Space Complexity

Only a few variables are used to store numbers, so space complexity is O(1).

Which Approach is Fastest?

The iterative loop approach is fastest and most memory efficient compared to recursion or array storage.

Approach	Time	Space	Best For
Iterative loop	O(n)	O(1)	Fast and memory efficient
Recursive	O(2^n)	O(n)	Simple code but slow for large n
Array storage	O(n)	O(n)	When series needs to be reused