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.

A review

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