The fmt Package

The fmt package formats values for humans and writes or reads text using verbs such as %s, %d and %v.

fmt.Println writes arguments with appropriate spacing and a newline. fmt.Printf uses a format string; %d is an integer, %s a string, %.2f a floating-point value with two decimal places and %T a value’s type. fmt.Sprintf returns formatted text instead of printing it.

The Scan family reads input, but use fixed variables in this introductory example so it runs deterministically in online Go compilers.

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 main() {
	name, amount := "Mira", 12.5
	fmt.Printf("Customer: %s, total: %.2f\n", name, amount)
	label := fmt.Sprintf("Quantity: %d", 3)
	fmt.Println(label)
}

Output

Customer: Mira, total: 12.50
Quantity: 3

Printf substitutes the name and two-decimal amount. Sprintf creates a string without writing to output, and the final Println prints that string.

• Printf does not automatically add a trailing newline unless the format contains one.
• A mismatched formatting verb may produce diagnostics rather than the intended value.

Add a boolean with %t and print the type of amount with %T.

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

You have practiced The fmt Package 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.