Go Academy · Lesson

Sprintf and Fprintf

Format to strings and writers.

Sprintf and Fprintf is a free Go Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

The Printf Family

The fmt package has three formatting cousins:

Printf writes to standard output.
Sprintf returns a formatted string.
Fprintf writes to any writer.

package main

import "fmt"

func main() {
    fmt.Printf("hello %s\n", "world")
}

Sprintf Returns a String

fmt.Sprintf formats exactly like Printf but, instead of printing, it returns the result as a string you can store or reuse.

package main

import "fmt"

func main() {
    msg := fmt.Sprintf("%s scored %d", "Bob", 95)
    fmt.Println(msg)
}

Building Identifiers

Sprintf is handy for assembling keys, file names, or labels from parts.

package main

import "fmt"

func main() {
    id := fmt.Sprintf("user-%04d", 42)
    fmt.Println(id)
}

Formatting Numbers into Text

Use Sprintf to turn numbers into nicely formatted strings, such as currency.

package main

import "fmt"

func main() {
    label := fmt.Sprintf("Price: $%.2f", 7.5)
    fmt.Println(label)
}

Fprintf Writes to a Writer

fmt.Fprintf takes an io.Writer as its first argument and formats into it. os.Stdout is one such writer.

package main

import (
    "fmt"
    "os"
)

func main() {
    fmt.Fprintf(os.Stdout, "to stdout: %d\n", 1)
}

Writing to Standard Error

Send diagnostics to os.Stderr with Fprintf so they stay separate from normal output.

package main

import (
    "fmt"
    "os"
)

func main() {
    fmt.Fprintf(os.Stderr, "warning: %s\n", "low disk")
}

Fprintf into a Builder

Because strings.Builder is also an io.Writer, you can format directly into it and read the string at the end.

package main

import (
    "fmt"
    "strings"
)

func main() {
    var b strings.Builder
    fmt.Fprintf(&b, "%s=%d;", "x", 10)
    fmt.Fprintf(&b, "%s=%d", "y", 20)
    fmt.Println(b.String())
}

Fprintf into a bytes.Buffer

A bytes.Buffer is another common writer target. Build up content, then convert to a string.

package main

import (
    "bytes"
    "fmt"
)

func main() {
    var buf bytes.Buffer
    fmt.Fprintf(&buf, "row %d\n", 1)
    fmt.Fprintf(&buf, "row %d\n", 2)
    fmt.Print(buf.String())
}

Sprint and Fprint Without f

The f versions take a format string. Drop the f for Sprint/Fprint, which simply concatenate arguments with default formatting.

package main

import "fmt"

func main() {
    s := fmt.Sprint("a", 1, true)
    fmt.Println(s)
}

Sprintln Adds Spaces

fmt.Sprintln joins arguments with spaces and appends a newline, returning the string.

package main

import "fmt"

func main() {
    s := fmt.Sprintln("x", 1, "y", 2)
    fmt.Print(s)
}

Choosing the Right Tool

Pick based on the destination:

Need a string? Use Sprintf.
Writing to stdout? Use Printf.
Writing to a file, buffer, or error stream? Use Fprintf.

package main

import (
    "fmt"
    "os"
)

func main() {
    s := fmt.Sprintf("count=%d", 3)
    fmt.Fprintf(os.Stdout, "%s\n", s)
}

Quick Check

Which function returns the formatted text instead of printing it?

Recap: Sprintf and Fprintf

You learned the formatting family:

Sprintf returns a string.
Fprintf writes to any io.Writer (stdout, stderr, buffers, builders).
The non-f variants use default formatting.

package main

import (
    "fmt"
    "strings"
)

func main() {
    var b strings.Builder
    fmt.Fprintf(&b, "result=%s", fmt.Sprintf("%d%%", 80))
    fmt.Println(b.String())
}

Frequently asked questions

Is the “Sprintf and Fprintf” lesson free?

Yes — the full text of “Sprintf and Fprintf” is free to read here on the web, and the Go Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Go Academy course, upgrade to CoddyKit PRO.

What will I learn in “Sprintf and Fprintf”?

Format to strings and writers. You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Go Academy?

No prior experience is required. Go Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Sprintf and Fprintf” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Go Academy lesson?

Yes. Every Go Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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