⚡ TL;DR
Display the Fibonacci series of the first n terms in C using a for loop. Complete source code with output, step-by-step explanation, and complexity analysis.
In this tutorial, you’ll learn how to display the Fibonacci sequence of the first n numbers (entered by the user) in the C programming language. The Fibonacci series starts with 0 and 1, and every subsequent term is the sum of the two terms before it: 0, 1, 1, 2, 3, 5, 8, 13, ...
Expected Output
For an input of 8 terms, the program should print:
Enter a positive number:
8
First 8 terms of Fibonacci series:
0
1
1
2
3
5
8
13Implementation
Fibonacci Series in C using a for loop:
#include<stdio.h>
int main()
{
int count, start = 0, second = 1, next, i;
//Ask user to input number of terms
printf("Enter a positive number:\n");
scanf("%d",&count);
printf("First %d terms of Fibonacci series:\n",count);
for ( i = 0 ; i < count ; i++ )
{
if ( i <= 1 )
next = i;
else
{
next = start + second;
start = second;
second = next;
}
printf("%d\n",next);
}
return 0;
}Output
Enter a positive number:
8
First 8 terms of Fibonacci series:
0
1
1
2
3
5
8
13How It Works
The program keeps track of only the two most recent terms at any time:
- Seed values:
start = 0andsecond = 1are the first two Fibonacci numbers. - First two iterations: when
i <= 1, the term is simplyiitself (0, then 1) — no addition needed yet. - Every later iteration:
next = start + secondcomputes the new term, then the window slides forward —starttakes the oldsecond, andsecondtakes the freshly computednext. - Printing: each term is printed on its own line with
printf("%d\n", next).
Because the loop only ever stores three integers (start, second, next), the memory usage stays constant no matter how many terms you print.
Complexity
- Time: O(n) — one pass over the
nterms - Space: O(1) — only a fixed number of integer variables
Practice Variations
- Print the series up to a maximum value instead of a term count (e.g., all Fibonacci numbers below 100)
- Print the series on a single line separated by commas
- Compute the nth Fibonacci term using recursion instead of a loop
- Store the series in an array and print it in reverse
Happy coding!
Related Posts
- Star Pattern Series 1 - Triangle pyramid pattern in C programming language using stars
- Star Pattern Series 2 - Triangle pyramid pattern in C programming language using stars
References
- C language reference — cppreference
printfin C — cppreferencescanfin C — cppreference- GCC online documentation — GNU
FAQ
How do I write a C program to print the Fibonacci series?
Initialize two variables to the seed values 0 and 1, then run a for loop for the requested number of terms. The first two terms print directly, and each later term is the sum of the two previous terms, after which the pair of stored values slides forward. The program prints one term per line.
What is the Fibonacci series?
The Fibonacci series is a sequence that starts with 0 and 1, where every subsequent number is the sum of the two before it: 0, 1, 1, 2, 3, 5, 8, 13, and so on. It appears throughout math and nature, and printing its first n terms is a classic beginner exercise for practicing loops.
What is the time complexity of the loop-based Fibonacci program in C?
The loop-based version runs in O(n) time because it computes each of the n terms exactly once in a single pass, and O(1) space because it stores only three integers — the two previous terms and the next one. Naive recursive versions, by contrast, take exponential time unless you add memoization.
Why does the Fibonacci program store only the last two terms?
Because each Fibonacci term depends only on the two before it, the program never needs the full history. It keeps two variables as a sliding window: compute the next term as their sum, then shift the older value out and the new one in. This constant memory is why the loop approach beats storing the series in an array.
Written by Mihir Pipermitwala
Senior Full Stack Consultant & Mobile Engineering Specialist building user-focused applications in healthcare with Flutter, Dart and React Native. I write about what I actually build and test — every tutorial here comes from real project work.
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