Lab 1, FizzBuzz


The purpose of this assignment is to write a C program that demonstrates a basic understanding of programming using C. The core concepts demonstrated by this assignment are the handling of command line arguments, looping, conditionals, and output. You are expected to have already learned, each of these concepts save for command line arugments. I encourage you to attempt to complete this assignment without consulting any outside resources. However, should you need to look something up, please make a mental note of the topic in order to track your progress toward mastery of the topic.


This assignment requires you to write a program, fizzbuzz.c, that produces a single-line output for each number, i, from 1 up to and including the given number NUM. Each line of output is one of the following:

  • i when i is not divisible by 3 and not divisible by 5
  • fizz when i is divisible by 3
  • buzz when i is divisible by 5
  • fizzbuzz when i is divisible by both 3 and 5

The number, NUM, should be read in as the first command line argument. Your program should check for the following error cases and produce the associated error output message. Note that each error message should be followed by a newline:

  • The number of command line arguments is not 2: Usage: fizzbuzz NUM
  • The number, NUM is too small to produce any output: NUM is too small

Compiling and running your program

In this class we will use the clang/clang++ compiler for all assignments. The clang compiler has a number of benefits over the gcc/g++ compiler, most notably with respect to the usefulness of its error and warning messages.

To compile the project:

clang fizzbuzz.c

To run the project:

./a.out NUM

Example input and corresponding output

No command line arguments:

$ ./a.out
Usage: fizzbuzz NUM

Negative value command line argument:

$ ./a.out -1024
NUM is too small

A valid input:

$ ./a.out 15

Submitting the project

Note that you may resubmit this project as many times as necessary up until the deadline. Please review the submission instructions as needed.

Appendix A: Reading command line arguments

Command line arguments can be read by declaring your main() function as follows:

int main(int argc, char *argv[]) {

argc is the number of arguments that the program was run with, while argv is an array with each of the command line arguments listed in order. Therefore, argc will always tell you the number of elements in argv.

Keep in mind that the program name itself (./a.out in this lab) counts as one argument, and will occupy argv[0]. Any additional arguments can be found in argv[1], argv[2], etc.

In the following example, our program prints out all of its command line arguments in the order they were specified in:


int main(int argc, char *argv[]) {
    for(int i = 1; i < argc; i++) {
        printf("%s\n", argv[i]);

    return 0;

Compiling and running yields the following behavior:

One argument:

$ ./a.out hello!

Multiple arguments:

$ ./a.out a b c d e

Appendix B: Converting char* to int

During the course of this lab, you may need to convert char* values to ints. This can be done using the atoi function, which is included in the stdlib.h library. atoi can be used as follows:


int main() {
    char* aStr = "5";
    int b = atoi(aStr) + 1; //not possible without atoi
    printf("%d\n", b);
    return 0;

Compiling and running yields the following behavior:

$ ./a.out

See also


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