FizzBuzz

%, while, if, else, function composition

Do not paste the complete program immediately. Create the file, type the first step, run vpp check, and then run the program when a complete entry point exists. Read the explanation before looking at the snippet. After each step, predict what the new code should do. This turns the page into a lesson rather than a code dump.

The order of checks matters. A number divisible by both 3 and 5 must be recognized as FizzBuzz before the individual Fizz or Buzz cases. The modulo operator returns the remainder, so a remainder of zero means exact divisibility.

After typing the snippet, identify the new names introduced by the step. Ask what each name represents, what type the compiler should infer or check, and what value should exist after the code runs.

fn fizzbuzz(n: int) -> string {
    if n % 15 == 0 {
        return "FizzBuzz"
    }
    if n % 3 == 0 {
        return "Fizz"
    }
    if n % 5 == 0 {
        return "Buzz"
    }
    return "num"
}

The loop owns the sequence of inputs while fizzbuzz owns the decision logic. Separating those responsibilities keeps the function easy to test and the loop easy to read.

Before moving forward, explain how this step connects to the previous one. In particular, identify which values cross a function boundary, which values change, and which values are guaranteed by the type system.

fn main() -> int {
    let mut i = 1
    while i <= 20 {
        let label = fizzbuzz(i)
        if label == "num" {
            print(i)
        } else {
            print(label)
        }
        i = i + 1
    }
    return 0
}

The complete program should produce the following output when run with the command shown below. Exact formatting should follow the current V++ runtime.

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz

Good small programs separate decision logic from iteration. The fizzbuzz function owns the rule while main owns the sequence.

1. Checking 3 and 5 before 15
2. Forgetting to increment the loop counter
3. Returning a string that the caller does not interpret correctly

Extend the loop to 50. Then change the labels and add another divisibility rule.

A strong learner should be able to explain the program without looking at the code, rebuild the core idea from memory, and make the practice change without copying a solution.