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fmt.Fprintln

Learn to write to a Writer with Fprintln.

fmt.Fprintln

The Fprint family writes output to any io.Writer: a file, an HTTP response, a buffer or os.Stderr.

fmt.Fprintln(os.Stderr, "uyarı:", err)
fmt.Fprintf(w, "<h1>%s</h1>", baslik)  // http.ResponseWriter

Each returns the number of bytes written and an error. When writing to a file or the network this error matters; the disk may fill up or the connection may drop.

Aligned output with tabwriter

text/tabwriter is a Writer that aligns tab-separated text into columns; ideal for command-line tables:

tw := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
fmt.Fprintln(tw, "AD\tFİYAT")
fmt.Fprintln(tw, "Kalem\t12.50")
fmt.Fprintln(tw, "Defter\t7.00")
tw.Flush()

Nothing is written unless Flush is called at the end.

Practice Table output

Write an aligned table to os.Stdout with text/tabwriter: header AD\tADET, rows Kalem\t3 and Defter\t12. Use NewWriter(os.Stdout, 0, 0, 2, ' ', 0) and call Flush.

Starter code

main.go
package main

import (
    "fmt"
    "os"
    "text/tabwriter"
)

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import (
    "fmt"
    "os"
    "text/tabwriter"
)

func main() {
    tw := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
    fmt.Fprintln(tw, "AD\tADET")
    fmt.Fprintln(tw, "Kalem\t3")
    fmt.Fprintln(tw, "Defter\t12")
    tw.Flush()
}

Expected output

AD      ADET
Kalem   3
Defter  12

Questions and answers

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

1 What is the risk of writing user data directly with <code>Fprintf</code> when generating HTML?

  • It's slower
  • It doesn't compile
  • There is no risk
  • XSS: data isn't escaped, scripts can run Correct answer

Explanation: fmt knows nothing about HTML; html/template or html.EscapeString must be used for user data.

2 Write to the error stream

Question 
fmt.____(os.Stderr, "hata:", err)
Answer 
fmt.Fprintln(os.Stderr, "hata:", err)

Explanation: Fprintln writes to the Writer in its first argument; stderr here.

3 Writer target

Term Meaning
os.Stderr Error messages
http.ResponseWriter Web response
tabwriter Aligned table
bytes.Buffer Collecting in memory

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