Number Programs

Learn number programs through an executable example, then connect the small demonstration to production code.

Number exercises strengthen comparisons, remainder, loops and edge-case handling. Start with even/odd checks and move toward digit counting and simple divisor tests.

The example checks primality without relying on a math-library square root. Values below two are not prime.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func isPrime(n int) bool {
	if n < 2 {
		return false
	}
	for d := 2; d <= n/d; d++ {
		if n%d == 0 {
			return false
		}
	}
	return true
}
func main() {
	for _, n := range []int{1, 2, 9, 17} {
		fmt.Printf("%d prime: %t\n", n, isPrime(n))
	}
}

Output

1 prime: false
2 prime: true
9 prime: false
17 prime: true

1 is rejected by the n < 2 rule; 2 has no possible divisors to test; 9 is divisible by 3; 17 passes each tested divisor.

• Always handle zero, one and negative inputs in prime checks.
• Testing all divisors through n is correct but does unnecessary work.

Print all prime numbers from 1 to 30 using isPrime.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Number Programs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.