Get lines, file

For file processing, we often need to get each line in a file. We can store the lines in a string slice, or process each one as we encounter it.

In Go, the Scanner type (returned by bufio.NewScanner) is helpful here. We can use append() to build up our string slice of file lines. The code can be placed in a separate method.

Example

To begin, please notice how we have imported the "bufio" and "os" packages. These are used in LinesInFile, our most important method in the example.

package main

import (
    "bufio"
    "fmt"
    "os"
)

func LinesInFile(fileName string) []string {
    f, _ := os.Open(fileName)
    // Create new Scanner.
    scanner := bufio.NewScanner(f)
    result := []string{}
    // Use Scan.
    for scanner.Scan() {
        line := scanner.Text()
        // Append line to result.
        result = append(result, line)
    }
    return result
}

func main() {
    // Loop over lines in file.
    for index, line := range LinesInFile(`C:\programs\file.txt`) {
        fmt.Printf("Index = %v, line = %v\n", index , line)
    }

    // Get count of lines.
    lines := LinesInFile(`C:\programs\file.txt`)
    fmt.Println(len(lines))
}
Index = 0, line = Thank you
Index = 1, line = Friend
2

For-range loop

Look carefully at the for-loop in the main method. We receive 2 values for each iteration—the index and the string itself. These can be used in the loop body.

Note, loop

The LinesInFile func is invoked only once for the entire loop. So the file is only opened once. We do not need to worry about excess IO or allocations.

Review

The LinesInFile method is useful for iterating over the lines in a text file in Go in an efficient way. It handles common forms of newlines well.