String Concatenation

Concatenation combines strings into a new string. It is useful for small messages, and Go offers efficient tools when many pieces need to be assembled.

Use + for a few strings, such as first + " " + last. For many pieces in a loop, strings.Builder avoids repeated intermediate strings and lets you write chunks using WriteString.

The builder’s String() method returns the assembled value. In simple code, choose clarity before performance optimization; for repeated concatenation of many fragments, a builder can make the intended accumulation explicit.

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() {
	greeting := "Hello" + ", " + "Go"
	var builder strings.Builder
	for _, word := range []string{"Learn", "Go", "today"} {
		if builder.Len() > 0 {
			builder.WriteString(" ")
		}
		builder.WriteString(word)
	}
	fmt.Println(greeting)
	fmt.Println(builder.String())
}

Output

Hello, Go
Learn Go today

The first expression combines three pieces into Hello, Go. The loop appends words into a builder, adding a space before every word after the first. The final string is Learn Go today.

• A strings.Builder must not be copied after its first use.
• Repeated += in very large loops can perform unnecessary allocations; use a builder when appropriate.

Add another word to the builder input and verify exactly one space separates adjacent words.

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

You have practiced String Concatenation 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.