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fmt.Sprintf

Learn to return a formatted string with Sprintf.

fmt.Sprintf

Sprintf does the same formatting as Printf but returns the result as a string instead of writing it to the screen. It is used to build messages, keys and file names:

dosya := fmt.Sprintf("rapor-%04d-%02d.csv", 2026, 3)
// rapor-2026-03.csv

The three family members: Print (stdout), Fprint (a given Writer), Sprint (a string). Each has …f and …ln variants.

Sprint vs Sprintln

Sprint adds a space between two non-string values but not between strings; Sprintln adds a space between every value and a newline at the end:

fmt.Sprint("a", "b", 1, 2)    // "ab1 2"
fmt.Sprintln("a", "b", 1, 2)  // "a b 1 2\n"

For readable, predictable output Sprintf is usually preferred.

Practice Build a file name

Write dosyaAdi(yil, ay int, tur string) string; it builds a name like "rapor-2026-03.csv" (month in two digits). Print dosyaAdi(2026, 3, "csv").

Starter code

main.go
package main

import "fmt"

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import "fmt"

func dosyaAdi(yil, ay int, tur string) string {
    return fmt.Sprintf("rapor-%d-%02d.%s", yil, ay, tur)
}

func main() {
    fmt.Println(dosyaAdi(2026, 3, "csv"))
}

Expected output

rapor-2026-03.csv

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 In a hot loop, which is faster for converting a single number to a string?

  • string(n)
  • strconv.Itoa(n) Correct answer
  • fmt.Sprintf("%d", n)
  • fmt.Sprint(n)

Explanation: Sprintf parses the format string and boxes the value into an interface; strconv converts directly. string(n) turns the number into a rune, not digits.

2 Build text

Question 
anahtar := fmt.____("kullanici:%d", id)
Answer 
anahtar := fmt.Sprintf("kullanici:%d", id)

Explanation: Sprintf returns the formatted text as a string.

3 Function and target

Term Meaning
Printf Standard output
Fprintf A given io.Writer
Sprintf A returned string
Errorf A returned error

All lessons in Intermediate

1 Goroutines

  1. What is a goroutine?
  2. The go keyword
  3. Goroutine and function
  4. The main goroutine
  5. Number of goroutines
  6. Summary - Goroutines

2 Channels

  1. What is a channel?
  2. Channel with make
  3. Sending data
  4. Receiving data
  5. Buffered channel
  6. Summary - Channels

3 Select

  1. What is select?
  2. Multiple channels
  3. default and non-blocking
  4. Select with timeout
  5. Closing a channel
  6. Summary - Select

4 sync package

  1. sync.Mutex
  2. sync.RWMutex
  3. sync.WaitGroup
  4. sync.Once
  5. sync.Pool
  6. Summary - sync package

5 Project: Concurrency

  1. Worker pool skeleton
  2. Job queue
  3. Collecting results
  4. Error handling

6 Context

  1. context.Background
  2. WithCancel
  3. WithTimeout
  4. WithValue
  5. WithDeadline
  6. Summary - Context

7 io package

  1. io.Reader
  2. io.Writer
  3. os.Stdin and os.Stdout
  4. bufio.Scanner
  5. io.Copy
  6. Summary - io package

8 Testing

  1. Test function
  2. t.Error and t.Fatal
  3. Table-driven tests
  4. Test helpers
  5. Example tests
  6. Summary - Testing

9 Benchmark

  1. Benchmark function
  2. b.ResetTimer
  3. Memory benchmark
  4. Sub-benchmark
  5. Comparing benchmarks
  6. Summary - Benchmark

10 fmt package

  1. fmt.Printf
  2. Format verbs
  3. fmt.Sprintf
  4. fmt.Errorf
  5. fmt.Fprintln
  6. Summary - fmt package

11 encoding/json

  1. The encoding/json package
  2. Struct tags
  3. Marshal and Unmarshal
  4. Encoder and Decoder
  5. JSON and map
  6. Summary - encoding/json

12 net/http

  1. Client with net/http
  2. http.Get
  3. http.Handler
  4. http.HandleFunc
  5. JSON API
  6. Summary - net/http

13 flag package

  1. The flag package
  2. flag.String and flag.Int
  3. flag.Parse
  4. Subcommand
  5. os.Args
  6. Summary - flag package

14 Intermediate final

  1. Challenge 1 - Concurrency
  2. Challenge 2 - HTTP service
  3. Challenge 3 - Test and benchmark
  4. Challenge 4 - CLI tool