All the Ways to Write for Loops in Go

A for loop executes a block of code repeatedly, and in Golang, there are several different ways to write one.

  1. The standard three-component loop
  2. For-range loop
    1. Range over slice
    2. Range over map
    3. Range over channel
    4. Range over string
  3. While loop
  4. Optional components loop
  5. Infinite loop
  6. Break from a loop
  7. Continue (skip to the next iteration) in a loop

#1 The standard three-component loop

Go has fairly standard syntax for the three-component loop you’re used to from C, Java, or JavaScript. The big difference is the lack of parentheses surrounding the components.

for i := 0; i < 100; i++ {
    sum += i
}

The three components are:

  • The init statement, i := 0
  • The condition, i < 100
  • The post statement, i++

The compiler executes the for-loop in the following manner:

  1. The init statement executes and variables declared there are made available to the scope of the loop’s body.
  2. The condition is computed. If it evaluates to true then the body runs, otherwise the loop is complete.
  3. The post statement runs.
  4. Back to step #2

#2 For-range loop

More often than not, you’ll be looping over a collection of items like a map, slice, channel, or string. While you can use a traditional three-component loop, Go makes it easier by providing the range keyword.

Range over a slice in Go

fruits := []string{"apple", "banana", "pear"}
for i, fruit := range fruits {
    fmt.Println(i, s)
}
// prints:
// 0 apple
// 1 banana
// 2 pear

Range over a map in Go

ages := map[string]int{
    "lane":    26,
    "preston": 28,
    "rory":    21,
}
for name, age := range ages {
    fmt.Println(name, age)
}
// prints:
// lane 26
// preston 28
// rory 21

Range over a channel in Go

ch := make(chan int)
go func() {
    for i := 0; i < 3; i++ {
        ch <- i
    }
    close(ch)
}()

// loop ends when channel is close
for value := range ch {
    fmt.Println(value)
}
fmt.Println("channel closed")

// prints:
// 0
// 1
// 2
// channel closed

Range over a string in Go

name := "lane"
for i, char := range name {
     // cast the rune to a string for printing 
     fmt.Println(i, string(char))
}

// prints
// 0 l
// 1 a
// 2 n
// 3 e

#3 While loop

By using one component in a for-loop signature rather than three, we can effectively build a while-loop in Golang. There is no while keyword in Go.

sum := 1
for sum < 10 {
    sum += sum
}
fmt.Println(sum)

#4 Optional components loop

Building on the idea of a flexible for-loop, we can omit the init or post statements of the three-component loop as we please.

i := 0
for ; sum < 1000; i++ {
    sum += i
}

for i := 0; sum < 1000; {
    sum += i
    i++
}

This can be a useful pattern when you want something like a do-while, or an immediate first tick from a ticker.

#5 Infinite loop

Infinite loops are useful within goroutines when you have a worker or process that should continue perpetually.

sum := 0
for {
    sum++ // repeated forever
}
// never reached, loops continues on forever

#6 Break from a loop

Breaking early from a loop can be useful, especially in a forever loop. The break keywords will exit the loop immediately.

sum := 0
for {
    sum++
    if sum >= 1000 {
        break
    }
}
fmt.Println(sum)

// prints:
// 1000

#7 Continue (skip to the next iteration) in a loop

It can be useful to skip to the next iteration of a loop early. This can be a good pattern for guard clauses within a loop.

for i := 0; i < 10; i++{
    if i % 2 == 0 {
        continue
    }
    fmt.Println(i, "is odd")
}

// prints
// 1 is odd
// 3 is odd
// 5 is odd
// 7 is odd
// 9 is odd

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