Go For Each Loops

Venter C.

—November 15, 2024

The ProblemThe Problem

I don’t know how to use a for-each loop to iterate through a slice or an array in Go.

The SolutionThe Solution

Go does not have a native for-each loop like some other languages, but you can achieve the same functionality using the range keyword. The range keyword allows you to iterate over arrays, slices, maps, and strings.

Iterating Over Arrays and SlicesIterating Over Arrays and Slices

An array is a fixed-size collection of elements, while a slice is a more flexible, dynamic-sized collection in Go. The range keyword allows you to loop over these collections and access each element.

Create a for loop with the range keyword to iterate over an array or slice:

package main

import "fmt"

func main() {
    nums := []int{10, 20, 30, 40, 50}

    for index, value := range nums {
        fmt.Printf("Index: %d, Value: %d\n", index, value)
    }
}

In each iteration of the loop, the index variable is assigned the value of the current element’s index, and the value is assigned the element itself.

The output will be:

Index: 0, Value: 10
Index: 1, Value: 20
Index: 2, Value: 30
Index: 3, Value: 40
Index: 4, Value: 50

Ignoring the IndexIgnoring the Index

Sometimes, you may not need the index and only care about the values in the collection. You can ignore the index using an underscore _:

for _, value := range nums {
    fmt.Println(value)
}

The output will be:

10
20
30
40
50

Iterating Over MapsIterating Over Maps

Maps in Go store key-value pairs. Using range allows you to loop over both the keys and the values. This is equivalent to a for-each loop that iterates over dictionaries in other languages.

package main

import "fmt"

func main() {
    fruits := map[string]int{
        "apple":  5,
        "banana": 7,
        "cherry": 12,
    }

    for key, value := range fruits {
        fmt.Printf("Fruit: %s, Quantity: %d\n", key, value)
    }
}

On each iteration through the fruits map, the key variable is assigned the map key (for example, “apple”), and value is assigned the corresponding value (for example, 5)

The output will be:

Fruit: apple, Quantity: 5
Fruit: banana, Quantity: 7
Fruit: cherry, Quantity: 12

Iterating Only Over Keys or ValuesIterating Only Over Keys or Values

If you only need the keys or values, you can use an underscore _ to ignore the part you don’t need:

// Iterating only over keys
for key := range fruits {
    fmt.Println(key)
}

// Iterating only over values
for _, value := range fruits {
    fmt.Println(value)
}

The keys-only output will be:

apple
banana
cherry

The values-only output will be:

5
7
12

Further readingFurther reading

• The Go slices package
• The Strings, bytes, runes and characters in Go blog post