How do I read text files in Golang?

Reading Text Files in Golang

When working with text files, reading their contents into variables becomes necessary. Golang provides several ways to achieve this, as demonstrated below:

Direct Output

To print the entire text file's contents, use fmt.Print(file). However, this will output the file descriptor's pointer value, not the file's contents.

ioutil.ReadAll

This function reads all file contents into memory as bytes:

b, err := io.ReadAll(file)
fmt.Print(b)

io.Reader.Read

Reading in smaller chunks can be more memory-efficient for large files:

buf := make([]byte, 32*1024) // Define buffer size

for {
    n, err := file.Read(buf)
    if n > 0 {
        fmt.Print(buf[:n])
    }

    if err == io.EOF {
        break
    }
    if err != nil {
        log.Printf("read %d bytes: %v", n, err)
        break
    }
}

bufio.Scanner

Using a Scanner tokenizes the file based on separators, with the default being newlines:

scanner := bufio.NewScanner(file)

for scanner.Scan() {
    fmt.Println(scanner.Text()) // Token as unicode characters
    fmt.Println(scanner.Bytes()) // Token as bytes
}

For additional information and examples, refer to the Golang file cheatsheet for comprehensive file handling techniques.

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