Is there a foreach loop in Go? - go

Question

Is there a foreach construct in the Go language?
Can I iterate over a slice or array using a for?

Answer 1

From For statements with range clause:
A "for" statement with a "range" clause iterates through all entries
of an array, slice, string or map, or values received on a channel.
For each entry it assigns iteration values to corresponding iteration
variables and then executes the block.
As an example:
for index, element := range someSlice {
// index is the index where we are
// element is the element from someSlice for where we are
}
If you don't care about the index, you can use _:
for _, element := range someSlice {
// element is the element from someSlice for where we are
}
The underscore, _, is the blank identifier, an anonymous placeholder.

Answer 2

Go has a foreach-like syntax. It supports arrays/slices, maps and channels.
Iterate over an array or a slice:
// index and value
for i, v := range slice {}
// index only
for i := range slice {}
// value only
for _, v := range slice {}
Iterate over a map:
// key and value
for key, value := range theMap {}
// key only
for key := range theMap {}
// value only
for _, value := range theMap {}
Iterate over a channel:
for v := range theChan {}
Iterating over a channel is equivalent to receiving from a channel until it is closed:
for {
v, ok := <-theChan
if !ok {
break
}
}

Answer 3

Following is the example code for how to use foreach in Go:
package main
import (
"fmt"
)
func main() {
arrayOne := [3]string{"Apple", "Mango", "Banana"}
for index,element := range arrayOne{
fmt.Println(index)
fmt.Println(element)
}
}
This is a running example https://play.golang.org/p/LXptmH4X_0

Answer 4

Yes, range:
The range form of the for loop iterates over a slice or map.
When ranging over a slice, two values are returned for each iteration. The first is the index, and the second is a copy of the element at that index.
Example:
package main
import "fmt"
var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
func main() {
for i, v := range pow {
fmt.Printf("2**%d = %d\n", i, v)
}
for i := range pow {
pow[i] = 1 << uint(i) // == 2**i
}
for _, value := range pow {
fmt.Printf("%d\n", value)
}
}
You can skip the index or value by assigning to _.
If you only want the index, drop the , value entirely.

Answer 5

The following example shows how to use the range operator in a for loop to implement a foreach loop.
func PrintXml (out io.Writer, value interface{}) error {
var data []byte
var err error
for _, action := range []func() {
func () { data, err = xml.MarshalIndent(value, "", " ") },
func () { _, err = out.Write([]byte(xml.Header)) },
func () { _, err = out.Write(data) },
func () { _, err = out.Write([]byte("\n")) }} {
action();
if err != nil {
return err
}
}
return nil;
}
The example iterates over an array of functions to unify the error handling for the functions. A complete example is at Google´s playground.
PS: it shows also that hanging braces are a bad idea for the readability of code. Hint: the for condition ends just before the action() call. Obvious, isn't it?

Answer 6

You can in fact use range without referencing its return values by using for range against your type:
arr := make([]uint8, 5)
i,j := 0,0
for range arr {
fmt.Println("Array Loop", i)
i++
}
for range "bytes" {
fmt.Println("String Loop", j)
j++
}
https://play.golang.org/p/XHrHLbJMEd

Answer 7

This may be obvious, but you can inline the array like so:
package main
import (
"fmt"
)
func main() {
for _, element := range [3]string{"a", "b", "c"} {
fmt.Print(element)
}
}
outputs:
abc
https://play.golang.org/p/gkKgF3y5nmt

Answer 8

I'm seeing a lot of examples using range. Just a heads up that range creates a copy of whatever you're iterating over. If you make changes to the contents in a foreach range you will not be changing the values in the original container, in that case you'll need a traditional for loop with an index you increment and deference indexed reference. E.g.:
for i := 0; i < len(arr); i++ {
element := &arr[i]
element.Val = newVal
}

Answer 9

I have just implemented this library: https://github.com/jose78/go-collection.
This is an example of how to use the Foreach loop:
package main
import (
"fmt"
col "github.com/jose78/go-collection/collections"
)
type user struct {
name string
age int
id int
}
func main() {
newList := col.ListType{user{"Alvaro", 6, 1}, user{"Sofia", 3, 2}}
newList = append(newList, user{"Mon", 0, 3})
newList.Foreach(simpleLoop)
if err := newList.Foreach(simpleLoopWithError); err != nil{
fmt.Printf("This error >>> %v <<< was produced", err )
}
}
var simpleLoop col.FnForeachList = func(mapper interface{}, index int) {
fmt.Printf("%d.- item:%v\n", index, mapper)
}
var simpleLoopWithError col.FnForeachList = func(mapper interface{}, index int) {
if index > 1{
panic(fmt.Sprintf("Error produced with index == %d\n", index))
}
fmt.Printf("%d.- item:%v\n", index, mapper)
}
The result of this execution should be:
0.- item:{Alvaro 6 1}
1.- item:{Sofia 3 2}
2.- item:{Mon 0 3}
0.- item:{Alvaro 6 1}
1.- item:{Sofia 3 2}
Recovered in f Error produced with index == 2
ERROR: Error produced with index == 2
This error >>> Error produced with index == 2
<<< was produced
Try this code in playGrounD.