How to pad a number with zeros when printing?

How can I print a number or make a string with zero padding to make it fixed width?

For instance, if I have the number 12 and I want to make it 000012.

The fmt package can do this for you:

fmt.Printf("|%06d|%6d|\n", 12, 345)

Output:

|000012|   345|

Notice the 0 in %06d, that will make it a width of 6 and pad it with zeros. The second one will pad with spaces.

Try it for yourself here: http://play.golang.org/p/cinDspMccp

Use the Printf function from the fmt package with a width of 6 and the padding character 0:

import "fmt"
fmt.Printf("%06d", 12) // Prints to stdout '000012'

Setting the width works by putting an integer directly preceding the format specifier ('verb'):

fmt.Printf("%d", 12)   // Uses default width,                          prints '12'
fmt.Printf("%6d", 12)  // Uses a width of 6 and left pads with spaces, prints '    12'

The only padding characters supported by Golang (and most other languages) are spaces and 0:

fmt.Printf("%6d", 12)   // Default padding is spaces, prints '    12'
fmt.Printf("%06d", 12)  // Change to 0 padding,       prints '000012'

It is possible to right-justify the printing by prepending a minus -:

fmt.Printf("%-6d", 12)   // Padding right-justified, prints '12    '

Beware that for floating point numbers the width includes the whole format string:

fmt.Printf("%06.1f", 12.0) // Prints '0012.0' (width is 6, precision is 1 digit)

It is useful to note that the width can also be set programmatically by using * instead of a number and passing the width as an int parameter:

myWidth := 6
fmt.Printf("%0*d", myWidth, 12) // Prints '000012' as before

This might be useful for instance if the largest value you want to print is only known at runtime (called maxVal in the following example):

myWidth := 1 + int(math.Log10(float64(maxVal)))
fmt.Printf("%*d", myWidth, nextVal)

Last, if you don't want to print to stdout but return a String, use Sprintf also from fmt package with the same parameters:

s := fmt.Sprintf("%06d", 12) // returns '000012' as a String

There is one simplest way to achieve this. Use

func padNumberWithZero(value uint32) string {
    return fmt.Sprintf("%02d", value)
}

fmt.Sprintf formats and returns a string without printing it anywhere.
Here %02d says pad zero on left for value who has < 2 number of digits. If given value has 2 or more digits it will not pad. For example:

• If input is 1, output will be 01.
• If input is 12, output will be 12.
• If input is 1992, output will be 1992.

You can use %03d or more for more zeros padding.

Just in case if you want to prefix or suffix to form another word by concatenating you can use below code.

package main

import "fmt"

func main() {
	concatenatedWord:= "COUNTER_"+fmt.Sprintf("%02d", 1)
	// use concatenatedWord 
        fmt.Println("ConcatenatedWordword is", concatenatedWord)
}

output : ConcatenatedWordword is COUNTER_01

link : https://play.golang.org/p/25g3L8TXiPP

The question "List of printing format in Go lang" reminds us that there is also the flag:

> - pad with spaces on the right rather than the left (left-justify the field)

You can see more padding examples with DaddyOh/golang-samples/pad.go, if you want to pad with other string sequences (more complex than '0' or ' '):

• leftPad(s string, padStr string, pLen int)
• rightPad(s string, padStr string, pLen int)
• leftPad2Len(s string, padStr string, overallLen int)
• rightPad2Len(s string, padStr string, overallLen int)

See <kbd>play.golang.org</kbd>:

1234567890

leftPad(str, "*", 3)  ***1234567890
leftPad2Len(str, "*-", 13)  -*-1234567890
leftPad2Len(str, "*-", 14)  *-*-1234567890
leftPad2Len(str, "*", 14)  ****1234567890
leftPad2Len(str, "*-x", 14)  x*-x1234567890
leftPad2Len(str, "ABCDE", 14)  BCDE1234567890
leftPad2Len(str, "ABCDE", 4)  7890
rightPad(str, "*", 3)  1234567890***
rightPad(str, "*!", 3)  1234567890*!*!*!
rightPad2Len(str, "*-", 13)  1234567890*-*
rightPad2Len(str, "*-", 14)  1234567890*-*-
rightPad2Len(str, "*", 14)  1234567890****
rightPad2Len(str, "*-x", 14)  1234567890*-x*
rightPad2Len(str, "ABCDE", 14)  1234567890ABCD
rightPad2Len(str, "ABCDE", 4)  1234

func lpad(s string,pad string, plength int)string{
	for i:=len(s);i<plength;i++{
		s=pad+s
	}
	return s
}

lpad("3","0",2) result: "03"

lpad("12","0",6) result: "000012"

Here's my solution:

func leftZeroPad(number, padWidth int64) string {
	return fmt.Sprintf(fmt.Sprintf("%%0%dd", padWidth), number)
}

Example usage:

fmt.Printf("%v", leftZeroPad(12, 10))

prints:

0000000012

The advantage of this is that you can specify the pad length at run time if needed.

For those that want to right pad, you can do this:

str2pad := "12"
padWith := "0"
amt2pad := 6

//This will make sure there is always 6 characters total, padded on the right side
//Note to check if strings.Repeat returns a negative value
paddedStr := str2pad + strings.Repeat(padWith, amt2pad - len(str2pad))

//Outputs 120000

Another option is the golang.org/x/text/number package:

package main

import (
   "golang.org/x/text/language"
   "golang.org/x/text/message"
   "golang.org/x/text/number"
)

var fmt = message.NewPrinter(language.English)

func main() {
   n := number.Decimal(
      12, number.Pad('0'), number.FormatWidth(6),
   )
   fmt.Println(n) // 000012
}

https://pkg.go.dev/golang.org/x/text/number

fmt.Printf("%012s", "345")

Result: 000000000345