Formatted Output with fmt

The fmt package has three families of printing functions:

• fmt.Print / fmt.Println — print values as they are (Println adds spaces between arguments and a newline at the end).
• fmt.Printf — print using a format string with verbs like %d. It does not add a newline — write \n yourself.
• fmt.Sprintf — same as Printf, but returns the string instead of printing it.

Syntax

The verbs you'll use most:

• %v — the value in a default format (works for anything)
• %+v — like %v, but includes field names for structs
• %T — the value's type
• %d — an integer
• %s — a string
• %q — a string in double quotes, with special characters escaped
• %.2f — a float with 2 decimal places
• %5d / %-5s — pad to a width of 5 (right / left aligned)
• %% — a literal percent sign

Example

package main

import "fmt"

// Point is a struct — you'll learn structs in a later lesson.
type Point struct {
    X, Y int
}

func main() {
    name := "Ada"
    age := 36
    price := 19.5
    p := Point{X: 1, Y: 2}

    fmt.Printf("%v %v %v\n", name, age, price)
    fmt.Printf("%v | %+v\n", p, p)
    fmt.Printf("%T %T %T %T\n", name, age, price, p)
    fmt.Printf("%d years\n", age)
    fmt.Printf("Hello, %s!\n", name)
    fmt.Printf("%q\n", name)
    fmt.Printf("%.2f\n", price)
    fmt.Printf("[%5d] [%-5s]\n", 42, "go")
    fmt.Printf("%d%%\n", 50)

    msg := fmt.Sprintf("%s is %d years old", name, age)
    fmt.Println(msg)
}

Expected output

Ada 36 19.5
{1 2} | {X:1 Y:2}
string int float64 main.Point
36 years
Hello, Ada!
"Ada"
19.50
[   42] [go   ]
50%
Ada is 36 years old

Common mistakes

• Forgetting \n at the end of a Printf format string, so the next output runs onto the same line.
• Using the wrong verb for a value. The program still compiles, but prints something like %!d(string=Ada). Run go vet — it catches these mismatches:

./main.go:8:5: fmt.Printf format %d has arg name of wrong type string

• Passing a format string to Println — it prints the % verbs literally instead of substituting values.

Practice

With item := "Coffee" and price := 3.5, print exactly Coffee costs $3.50 using Printf. Then build the same text with Sprintf and print its length.

package main

import "fmt"

func main() {
    item := "Coffee"
    price := 3.5

    fmt.Printf("%s costs $%.2f\n", item, price)

    text := fmt.Sprintf("%s costs $%.2f", item, price)
    fmt.Println(len(text)) // 18
}