Format Verbs

Go's fmt package uses format verbs to control how values are printed. %v is the default format that works with any type, and there are also type-specific verbs available.

Syntax

// Basic usage (used inside fmt.Printf, fmt.Sprintf)
fmt.Printf("%v\n", value)       // Default format
fmt.Printf("%d\n", integer)     // Decimal integer
fmt.Printf("%s\n", str)         // String
fmt.Printf("%f\n", floatNum)    // Floating-point number

// Specifying width and precision
fmt.Printf("%10d\n", 42)        // Width 10, right-aligned
fmt.Printf("%-10d\n", 42)       // Width 10, left-aligned
fmt.Printf("%010d\n", 42)       // Width 10, zero-padded
fmt.Printf("%.2f\n", 3.14159)   // 2 decimal places
fmt.Printf("%8.2f\n", 3.14)     // Width 8, 2 decimal places

Format Verb Reference

Verb	Description
%v	Default format. Prints values in a natural representation for their type.
%+v	Like %v, but includes field names when printing structs.
%#v	Prints the value in Go syntax (with package name and type name).
%T	Prints the type name of the value.
%d	Prints an integer in decimal (base 10).
%b	Prints an integer in binary (base 2).
%o	Prints an integer in octal (base 8).
%x / %X	Prints an integer in hexadecimal (lowercase / uppercase).
%e / %E	Prints a floating-point number in scientific notation.
%f	Prints a floating-point number in decimal notation (default precision: 6 digits).
%g / %G	Uses %e or %f, whichever is more compact for the value.
%s	Prints a string or byte slice.
%q	Prints a string as a double-quoted Go string literal.
%c	Prints the Unicode character corresponding to the integer value.
%t	Prints a boolean as `true` or `false`.
%p	Prints a pointer as a hexadecimal address.
%%	Prints a literal percent sign `%`.

Sample Code

package main

import "fmt"

type Person struct {
    Name string
    Age  int
}

func main() {
    // General-purpose verbs
    p := Person{"Alice", 30}
    fmt.Printf("%v\n", p)    // {Alice 30}
    fmt.Printf("%+v\n", p)   // {Name:Alice Age:30}
    fmt.Printf("%#v\n", p)   // main.Person{Name:"Alice", Age:30}
    fmt.Printf("%T\n", p)    // main.Person

    fmt.Println()

    // Integer in different bases
    n := 255
    fmt.Printf("Decimal: %d\n", n)
    fmt.Printf("Binary: %b\n", n)
    fmt.Printf("Octal: %o\n", n)
    fmt.Printf("Hex (lower): %x\n", n)
    fmt.Printf("Hex (upper): %X\n", n)

    fmt.Println()

    // Floating-point formatting
    f := 3.14159265
    fmt.Printf("Default: %f\n", f)
    fmt.Printf("Precision: %.2f\n", f)
    fmt.Printf("Scientific: %e\n", f)
    fmt.Printf("Compact: %g\n", f)

    fmt.Println()

    // Width and alignment
    fmt.Printf("|%10d|\n", 42)    // Right-aligned
    fmt.Printf("|%-10d|\n", 42)   // Left-aligned
    fmt.Printf("|%010d|\n", 42)   // Zero-padded

    // Special verbs
    fmt.Printf("%q\n", "Hello, 世界") // "Hello, 世界" (escaped string literal)
    fmt.Printf("%c\n", 65)           // A (character code 65)
    fmt.Printf("100%%\n")            // 100% (%% prints a literal percent sign)
}

Notes

Format verbs are used everywhere from debug output to production logging. Use %v or %#v to inspect values of unknown type. For numeric formatting, combining %d or %f with width and precision flags is very powerful. To customize formatting for your own types, implement the String() string method (the Stringer interface) — it will be called automatically by %v and %s.

If you use a verb that does not match the value's type — for example, %s with an integer or %d with a string — Go prints a warning string like %!s(int=42). Run go vet to catch these mismatches before compiling.

For details on fmt.Printf(), see fmt.Println() / Printf(). For string formatting, see fmt.Sprintf() / Fprintf().

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Format Verbs

Syntax

Format Verb Reference

Sample Code

Notes