Sorting Data in Go

July 22, 2020

This is the tenth entry of my weekly series Learning Go. Last week I talked about Pointers, Marshalling, and Unmarshalling Data in Go. This week I will be talking about how to sort your data in a Go Program. Let’s get to it.

I will be showing you multiple examples of how to sort your data, these examples can be grouped into two categories:

  • using the sort package
  • writing custom sorting functions

Using the sort Package

  • Can be used with built-in types (string, int, etc), as well as user-defined types
  • Sorting happens “in-place”. Changes to a slice, for example, would not return a new slice
  • Much like the array method sort in JavaScript, the sort package in Go compares the ASCII values for string sorting

Let’s try sorting a string:

package main

import (
	"fmt"
	"sort"
)

func main() {
	str := []string{"c", "a", "b"}

	sort.Strings(str)

	fmt.Println(str)
	// [a, b, c]
}

Inside of our func main we create a new variable with the identifier str

The value of str is a slice of values that are of type string. We use a composite literal to assign those values

str := []string{"c", "a", "b"}

We then use the sort package to invoke the Strings method and pass the str variable as the only argument

sort.Strings(str)

Note: the sort methods are specific to their built-in type i.e. string -> Strings, int -> Ints, etc.

We use the fmt package to print the result, and we can see that the value of str has now been properly sorted

fmt.Println(str)
// [a, b, c]

Let me show you an example of sorting values of type int

package main

import (
	"fmt"
	"sort"
)

func main() {
	ints := []int{4, 1, 9, 3, 8}

	sort.Ints(ints)

	fmt.Println(ints)
	// [1, 3, 4, 8, 9]
}

Inside of our func main we create a new variable with the identifier ints

The value of ints is a slice of values that are of type int. We use a composite literal to assign those values

ints := []int{4, 1, 9, 3, 8}

We then use the sort package to invoke the Ints method and pass the ints variable as the only argument

sort.Ints(ints)

We use the fmt package to print the result, and we can see that the value of ints has now been properly sorted

fmt.Println(ints)
// [1, 3, 4, 8, 9]

But what if you need something a little more in-depth, or customized? Well, you can certainly do that with ease in Go. Let me show you some examples of some custom sorting functions that can add a lot of value to your programs.

Custom Sort Functions

Let’s jump right into an example

package main

import (
	"fmt"
	"sort"
)

type Person struct {
	First string
	Age int
}

type ByAge []Person

func (a ByAge) Len() int {
	return len(a)
}

func (a ByAge) Swap(i, j int) {
	a[i], a[j] = a[j], a[i]
}

func (a ByAge) Less(i, j int) bool {
	return a[i].Age < a[j].Age
}

func main() {
	me := Person{"martin", 29}
	brother := Person{"noah", 20}
	sisterOne := Person{"miranda", 26}
	sisterTwo := Person{"alexis", 23}

	family := []Person{me, brother, sisterOne, sisterTwo}

	fmt.Println(family)

	sort.Sort(ByAge(family))

	fmt.Println(family)
}

To make any sorting happen, we have to make sure that we import the sort package

Next, we create a custom type with the identifier Person which is of type struct

The fields for our Person type are: First of type string and Age of type int

type Person struct {
	First string
	Age int
}

We create another custom type with an identifier ByAge, it is a slice of our other custom type Person

type ByAge []Person

Next, we create three functions that all take a receiver type with an identifier of a of type ByAge.

Note: we have to create these three functions because the Sort function makes one call to a Len function to determine length, and then makes calls to Less and Swap. You can read more about that in the official Go docs

Let’s walk through each of these functions I have created starting with Len

func (a ByAge) Len() int {
	return len(a)
}

As mentioned earlier, this function has a receiver type with an identifier of a and of type ByAge

The identifier of this function is Len

We pass the value of a, supplied from our receiver, as the only argument for the len function that we return from Len

Next, we create the Swap function

func (a ByAge) Swap(i, j int) {
	a[i], a[j] = a[j], a[i]
}

Much like the Len function, Swap has a receiver type with an identifier of a and of type ByAge

The Swap function has two parameters, i and j, both of type int

Inside of Swap we see that we are swapping the order of the values each time Swap is called

The value a[i] will be replaced with the value a[j], and the value a[j] will be replaced with the value a[i]

The last function that Sort needs is the Less function

func (a ByAge) Less(i, j int) bool {
	return a[i].Age < a[j].Age
}

The Less function has a receiver type with an identifier of a and of type ByAge

The Less function has two parameters, i and j, both of type int

The Less function returns a value of type bool

Note: The purpose of this sorting function is to sort from youngest to oldest based on the value of the Age field

On the return statement we see that we are writing an expression that checks if a[j].Age is greater than a[i].Age

This is the last step, and helps the Sort function determine which values should be swapped using the Swap function

Inside of func main we declare 4 variables using the short declaration operator, all of these variables are assigned a value using a composite literal and are of type person

me := Person{"martin", 29}
brother := Person{"noah", 20}
sisterOne := Person{"miranda", 26}
sisterTwo := Person{"alexis", 23}

We create a new variable with the identifier family which will be a type of a slice of values that are of type Person

We assign values to family using a composite literal

family := []Person{me, brother, sisterOne, sisterTwo}

Next, we convert the family variable to be of our custom type ByAge, this way we can make use of our functions we created that all have receiver types of ByAge

We pass this ByAge(family) value into the Sort function as the only argument

sort.Sort(ByAge(family))

On the next line, we are using the fmt package to print the value of family

fmt.Println(family)
// [{noah 20} {alexis 23} {miranda 26} {martin 29}]

And there it is! Now our data is sorted from youngest to oldest.

In Summary

Sorting data is an extremely common occurrence; therefore, knowing how to sort data effectively and efficiently in Go is paramount. I hope you learned something about using the sort package or about writing your own custom sorting function. Next week I will be talking about Concurrency in Go. See you then!