Input/output with fmt

The standard library's fmt package is your main tool for printing to the console, formatting strings and reading input. If you have used printf in C you'll feel right at home, but with some extra refinement thanks to the %v and %T verbs.

Print, Println, Printf: three functions, three behaviors

fmt.Print("ciao ", "mondo")    // "ciao mondo"  (niente newline finale)
fmt.Println("ciao", "mondo")   // "ciao mondo\n" (newline + spazi tra argomenti)
fmt.Printf("eta=%d\n", 36)     // "eta=36\n"    (format string esplicita)

The verbs you'll use all the time

u := struct{ Name string; Age int }{"Ada", 36}
fmt.Printf("%v\n", u)   // {Ada 36}
fmt.Printf("%+v\n", u)  // {Name:Ada Age:36}
fmt.Printf("%T\n", 3.14) // float64

Sprintf: building strings without printing them

Same interface as Printf, but the result is returned to you as a string:

msg := fmt.Sprintf("L'utente %s ha %d anni", "Ada", 36)
fmt.Println(msg) // L'utente Ada ha 36 anni

You'll use it constantly to build messages, logs and errors.

Errorf: strings + error wrapping

fmt.Errorf creates an error with Printf-style formatting. With the special verb %w you can wrap an existing error to preserve its chain (we'll cover this in depth in the Interfaces module):

if _, err := os.Open("missing"); err != nil {
    return fmt.Errorf("apertura config: %w", err)
}

Your turn

fmt.Printf("?\n", 3.14)

Recap

• Print/Println/Printf differ in spaces and newlines.
• %v is the wildcard that works on any type; %+v shows field names.
• %T prints the type (useful for quick debugging).
• Sprintf builds strings; Errorf builds errors with wrapping (%w).
• Printf does NOT add a newline: write it yourself in the format string.