fmt Package

The fmt package in Go provides formatted I/O functions similar to C’s printf family. It is one of the most commonly used standard libraries in Go and supports:

• Printing to the console (Print, Printf, Println)
• Formatting strings (Sprintf)
• Reading input (Scan, Scanf)
• Formatting values with verbs (%d, %s, %v, etc.)

Basic Output Functions

Function	Description
fmt.Print()	Prints values to standard output without a newline
fmt.Println()	Prints values followed by a newline
fmt.Printf()	Prints formatted text using verbs

Examples

fmt.Print("Hello")           // Output: Hello
fmt.Println("Hello", "Go")   // Output: Hello Go\n
fmt.Printf("Age: %d", 25)    // Output: Age: 25

Printf and Formatting Verbs

Printf is used to format strings before printing them. It accepts a format string followed by variables.

name := "Alice"
age := 30
fmt.Printf("Name: %s, Age: %d\n", name, age)

Common Formatting Verbs

General

Verb	Description
%v	Default format
%#v	Go syntax representation
%T	Type of the value
%%	A literal percent sign

Integer

Verb	Description
%d	Decimal
%b	Binary
%o	Octal
%x	Hex (lowercase)
%X	Hex (uppercase)

Float

Verb	Description
%f	Decimal point
%e	Scientific notation (e)
%E	Scientific notation (E)
%.2f	Precision: 2 decimals

String and Byte

Verb	Description
%s	Plain string
%q	Double-quoted string (escaped)
%x	Hex dump of bytes

Pointer

Verb	Description
%p	Pointer address in hex

Sprintf – Format Without Printing

Sprintf works like Printf but returns the formatted string instead of printing it.

Examples

greeting := fmt.Sprintf("Hello, %s!", "Bob")
fmt.Println(greeting) // Output: Hello, Bob!

Useful for constructing strings dynamically or logging.

Formatting Width and Alignment

We can control width, padding, and alignment with formatting directives.

Examples

fmt.Printf("|%10s|\n", "Go")     // Right-aligned
fmt.Printf("|%-10s|\n", "Go")    // Left-aligned
fmt.Printf("|%6d|\n", 42)        // Right-aligned int
fmt.Printf("|%06d|\n", 42)       // Zero-padded

Output:

|        Go|
|Go        |
|    42|
|000042|

Formatting Numbers with Precision

f := 3.141592
fmt.Printf("%.2f\n", f)  // Output: 3.14
fmt.Printf("%6.2f\n", f) // Output: "  3.14" (width 6, precision 2)

Using %#v and %T for Debugging

type Person struct {
    Name string
    Age  int
}

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

Multiline Formatting with Sprintf

msg := fmt.Sprintf(`
Name: %s
Age: %d
`, "Alice", 30)
fmt.Println(msg)