RCU 21BCA1C1P · Lab Program 5

Fibonacci Sequence of n Terms in C

Read n and print the first n terms of the Fibonacci sequence, where each term is the sum of the two before it.

Algorithm

Read n and reject values below 1.
Start with the first two terms a = 0 and b = 1.
Print a, then move on: the next term is a + b, so set a = b and b = next.
Repeat until n terms have been printed.

C program

#include <stdio.h>

int main(void) {
    int n, i, a = 0, b = 1, next;

    printf("Enter the number of terms (at least 1):\n");
    if (scanf("%d", &n) != 1 || n < 1) {
        printf("Error: enter a positive number of terms\n");
        return 1;
    }

    for (i = 1; i <= n; i++) {
        printf("%d", a);
        if (i < n) {
            printf(" ");
        }
        next = a + b;
        a = b;
        b = next;
    }
    printf("\n");
    return 0;
}

Sample input and output

Seven terms

Input

Output

Enter the number of terms (at least 1):
0 1 1 2 3 5 8

Edge case: one term

Input

Output

Enter the number of terms (at least 1):
0

How the program works

Two variables carry the last two terms. Each step prints the current term and then shifts the pair forward, so no array is needed.

The space is printed only between terms, so the output has no trailing space and reads exactly as the sequence.

Run this code

Edit the program and press Run. It is free and needs no sign-in.

Program input (what the program reads with scanf)

Copy a sample and paste it into the STDIN box in the editor before you press Run.

Seven terms

Expected output

Enter the number of terms (at least 1):
0 1 1 2 3 5 8

Edge case: one term

Expected output

Enter the number of terms (at least 1):
0

Viva questions

How is each Fibonacci term defined?

F(n) = F(n - 1) + F(n - 2), with F(0) = 0 and F(1) = 1.

Why is a temporary variable next needed?

The new sum uses a and b before they are overwritten, so it must be stored first.

What is the first term printed when n is 1?

Why does the loop not use an array?

Only the last two terms are needed, so two variables are enough.

When do the Fibonacci numbers overflow an int?

Around the 47th term, so very large n needs a wider type.

Common mistakes

Printing an extra term by updating the variables before printing.
Printing a trailing space after the last term.

Back to all RCU 21BCA1C1P lab programs.

C program

#include <stdio.h> int main(void) { int n, i, a = 0, b = 1, next; printf("Enter the number of terms (at least 1):\n"); if (scanf("%d", &n) != 1 || n < 1) { printf("Error: enter a positive number of terms\n"); return 1; } for (i = 1; i <= n; i++) { printf("%d", a); if (i < n) { printf(" "); } next = a + b; a = b; b = next; } printf("\n"); return 0; }

Viva questions

How is each Fibonacci term defined?

F(n) = F(n - 1) + F(n - 2), with F(0) = 0 and F(1) = 1.

Why is a temporary variable next needed?

The new sum uses a and b before they are overwritten, so it must be stored first.

What is the first term printed when n is 1?

Why does the loop not use an array?

Only the last two terms are needed, so two variables are enough.

When do the Fibonacci numbers overflow an int?

Around the 47th term, so very large n needs a wider type.