Go Loops

Loops are fundamental programming constructs that allow developers to execute a code block repeatedly. In Go, looping is streamlined and efficient. Unlike many programming languages offering multiple loops, Go uses a single looping construct: the for loop. However, this loop is flexible enough to cover the functionality of traditional while, do-while, and for loops in other languages.

for Loop

In Go, for loops are defined with a three-component syntax similar to what is used in for loops for other languages like C or Java:

for init; condition; post {
  // Statements
}

In the syntax:

• init: The statement initializes an index variable.
• condition: The condition used to check the index variable against a boolean expression before every loop. As long as it’s true, the iteration will continue, and the statements inside the loop will be executed.
• post: The statement that executes after every loop, usually to increment/decrement the index variable.

Example

This example uses the for loop to count down from 10 to 1. The init statement sets the variable i to 10, the condition ends the loop when i is no longer greater than 0, and the post statement decrements i by one after each pass:

package main
import "fmt"

func main() {
  // Counts down from 10 to 1
  for i := 10; i > 0; i-- {
    fmt.Println(i)
  }
}

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Here is the output:

10
9
8
7
6
5
4
3
2
1

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for Loop as a While Loop

In Go, the while loop is implemented with a for loop by omitting the init and post statements:

for condition {
  // Statements
}

The loop will execute as long as condition remains true.

Example

This example behaves the same way as the for loop example, except the variable countdown is initialized outside the loop, and is decremented inside the loop itself. The condition still runs the loop as long as countdown is still greater than 0:

package main
import "fmt"

func main() {
  countdown := 10

  // Counts down from 10 to 1
  for countdown > 0 {
    fmt.Println(countdown)

    countdown--
  }
}

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Here is the output:

10
9
8
7
6
5
4
3
2
1

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Infinite Loops

If the condition statement is omitted along with the post and init statements in a for loop, the loop will execute indefinitely unless a break statement is encountered. This type of loop is known as an infinite loop:

for {
  // Statements
}

Example

This example will print the given string until the program is halted externally:

package main
import "fmt"

func main() {
  // Executes forever
  for {
    fmt.Println("Help! I'm trapped in a loop!")
  }
}

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Here is the output:

Help! I'm trapped in a loop!
Help! I'm trapped in a loop!
Help! I'm trapped in a loop!
Help! I'm trapped in a loop!
Help! I'm trapped in a loop!
...

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for...range Loop

By using the range keyword, a for loop can step through the items in a collection such as an array, map, slice, channel, or string. This type of loop is known as a for...range loop:

for index, value = range collection {
  // Statements
}

In the syntax:

• index: A variable containing the index of the collection.
• value: A variable used to step through the values in collection.
• collection: The collection that the loop is stepping through.

Example

In this example, a for...range loop is used to step through the elements of the slice numbers and print the index-value pair for each element:

package main
import "fmt"

func main() {
  numbers := []string{"One","Two","Three"}

  // Loop through the slice
  for i, n := range numbers {
    fmt.Println(i, n)
  }
}

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Here is the output:

0 One
1 Two
2 Three

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break and continue Statements

The break and continue statements work in Go as they do in C and Java:

• The break statement halts execution of a loop and continues with the next statement after the loop.
• The continue statement skips execution to the next iteration of the loop.

Example

This example demonstrates the usage of break and continue statements in Go:

package main
import "fmt"

func main() {
  for i := 0; i < 20; i++ {
    if i % 2 == 0 {
      continue
    }

    if i == 10 {
      break
    }

    fmt.Println(i)
  }
}

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Here is the output:

1
3
5
7
9
11
13
15
17
19

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Frequently Asked Questions

1. Can you nest Go for loops?

Yes, you can nest Go for loops just like in other languages:

for i := 0; i < 3; i++ {
  for j := 0; j < 2; j++ {
    fmt.Println(i, j)
  }
}

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2. How can you label Go for loops?

Go allows labeled for loops for better control in breaking out of nested loops:

outer:
for i := 0; i < 3; i++ {
  for j := 0; j < 3; j++ {
    if i == j {
      break outer
    }
  }
}

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3. Is do-while loop supported in Go?

No, Go does not have a do-while loop construct. You can simulate it with an infinite loop and a break:

for {
  fmt.Println("Do something")

  if condition {
    break
  }
}

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