The bufio Package

The bufio package buffers I/O and supports convenient line-oriented reading with scanners.

bufio.NewScanner reads text in tokens, lines by default. Call Scan() in a loop, read Text(), and check Err() afterwards. For larger input or more control, buffered readers and writers expose additional methods.

Scanner has a configurable token-size limit; it is not appropriate for unlimited-length lines without configuration. This example reads from a string reader so it requires no interactive input or external files.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"bufio"
	"fmt"
	"strings"
)

func main() {
	input := strings.NewReader("red\nblue\ngreen\n")
	scan := bufio.NewScanner(input)
	count := 0
	for scan.Scan() {
		count++
		fmt.Println(count, scan.Text())
	}
	if err := scan.Err(); err != nil {
		fmt.Println("Read error:", err)
	}
}

Output

1 red
2 blue
3 green

The scanner reads three lines in order. Each Scan call updates the current token; after scanning completes, Err is checked for a read failure.

• Always check Scanner.Err after the loop.
• Long tokens may exceed Scanner’s default maximum token size.

Change the sample input to four lines and count how many are nonempty.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced The bufio Package in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.