Pattern Programs

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

Pattern printing uses nested loops to convert row and column positions into text. Practice by deciding what each row should contain before writing code.

This program builds each line in a strings.Builder and prints a right triangle made from asterisks.

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"
	"strings"
)

func main() {
	for row := 1; row <= 4; row++ {
		var line strings.Builder
		for col := 0; col < row; col++ {
			line.WriteByte('*')
		}
		fmt.Println(line.String())
	}
}

Output

*
**
***
****

The outer loop advances through four rows. The inner loop writes as many stars as the current row number, producing one, two, three and four stars.

• Use separate row and column variables for clarity.
• Patterns are about loop reasoning; avoid complex string slicing when a builder or Repeat suffices.

Change the program to print five rows and then an inverted triangle.

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

You have practiced Pattern 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.