Padding, Printf

Often in Go we want to output columns of data from our important computations. With the fmt.Printf method, we can specify a couple characters to apply padding.

With a minus sign, we add spaces to the right. With a positive number (no minus sign) we add spaces to the left, which pushes the text to the right.

Example program

An example of right-align and left-align is more helpful. Here we create columns of 10 characters wide. If a string is not long enough, it is padded with spaces.

package main

import "fmt"

func main() {
    values := []string{"bird", "5", "animal"}

    // Pad all values to 10 characters.
    // ... This right-justifies the strings.
    //     Three periods just for decoration.
    for i := range(values) {
        fmt.Printf("%10v...\n", values[i])
    }

    // Pad all values to 10 characters.
    // ... This left-justifies the strings.
    //     Vertical bars just for decoration.
    for i := range(values) {
        fmt.Printf("|%-10v|\n", values[i])
    }
}
      bird...
         5...
    animal...
|bird      |
|5         |
|animal    |

Sprintf, padding

Sometimes we want to get a string with padding—not directly print it to the console. Here fmt.Sprintf is useful. It returns a padded string.

package main

import "fmt"

func main() {
    input := "pad"
    // Pad the string and store it in a new string.
    padded := fmt.Sprintf("%12v", input)
    fmt.Println("Len:", len(padded))
    fmt.Println("[" + padded + "]")
}
Len: 12
[         pad]

Notes, padding

We use a positive or negative number in front of the format code (like "%v"). The "v" stands for "value" and can handle many types.

Padding can make an unreadable list of values readable. With Printf we can write multiple strings onto the same line, creating columnar layouts.