Golang iterate over slice in reverse? [SOLVED]

In Golang, iterating over a slice is surprisingly straightforward; In this article, we will learn how to iterate over a slice in reverse in Go.

 

Example 1: Using a loop that counts down the index

There isn't a practical operator to solve this. But we can simply use a normal loop that counts down the index and iterate over it in reverse order. Here is an example of how we can do it in Golang:

package main

import "fmt"

func main() {
	intSlice := []int{12, 23, 9, 32, 182}
	// counting down the index
	for i := len(intSlice) - 1; i >= 0; i-- {
		fmt.Println(intSlice[i])
	}
}

Output:

182
32
9
23
12

 

Example 2: Using a loop that handles the index

In this example, we will try to handle the index when iterating the slice so it can print the slice in reverse order:

package main

import "fmt"

func main() {
	intSlice := []int{12, 23, 9, 32, 182}
	sliceLen := len(intSlice)

	for i := range intSlice {
		// handle the index
		reverseIndex := sliceLen - 1 - i
		fmt.Println(intSlice[reverseIndex])
	}
}

Output:

182
32
9
23
12

 

Example 3: Using generic to write a function for multiple data type slices

In this example, we will use the generics to write a function that takes any data types slices as a parameter and print out all the elements in reverse order:

package main

import "fmt"

func reverseIterate[T any](slice []T) {
	for i := len(slice) - 1; i >= 0; i-- {
		fmt.Printf("%v ", slice[i])
	}
}

func main() {
	intSlice := []int{12, 23, 9, 32, 182}
	reverseIterate(intSlice)
	fmt.Println()

	strSlice := []string{"This", "is", "reverse", "iterating", "example"}
	reverseIterate(strSlice)
}

Output:

182 32 9 23 12 
example iterating reverse is This

 

Example 4: Using a channel to reverse the slice

One method to iterate the slice in reverse order is to use a channel to reverse a slice without duplicating it. Let's take a look at the example below to see how we can use a channel to reverse a slice and print it in the reverse order:

package main

import (
	"fmt"
)

func reverseSlice(slice []string) chan string {
	result := make(chan string)
	go func() {
		for i, _ := range slice {
			result <- slice[len(slice)-1-i]
		}
		close(result)
	}()
	return result
}

func main() {
	strSlice := []string{"a", "b", "c", "d"}
	for element := range reverseSlice(strSlice) {
		fmt.Println(element)
	}
	fmt.Println("Original slice:", strSlice)
}

Output:

d
c
b
a
Original slice: [a b c d]

 

Summary

In this article, I already show you some examples of iterating over a slice in reverse order. We can simply use a normal loop that handles the index order. Another way to print the slice in reverse order is to use a channel to sort the slice first and then iterate through it. With the generics, example 3 shows us how to write a function to sort slices of any data type.

 

References

Is there a way to iterate over a slice in reverse in Go?