This is the chapter 10 of the golang comprehensive tutorial series. Refer to this link for other chapters of the series – Golang Comprehensive Tutorial Series

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Overview

When it comes to loop, golang has:

for loop

for-range loop

In this tutorial, we will be learning about the for loop only. The while loop is missing from go but a while loop can be implemented using a for loop as we will see later in this tutorial.

for loop in GO basically has three parts as shown below in the format

init part

condition part

post part

for init_part; condition_part; post_part {
   ...
}

Here is the sequence of execution of the three parts:

The init part is executed first before the first iteration

The condition part is executed before every iteration. If the condition is false the loop will exit otherwise the loop will continue to iterate.

The post part is executed after every iteration. After this the condition is check, if it is true then the loop is continued otherwise loop exists.

Some points to note about for loop:

The parenthesis is not necessary around for loop but the curly braces around the body is necessary.

The init and post part is optional.

The init part can be any statement with a short declaration, function call or assignment. If the init part has the variable declaration, then the scope of that variable is limited to within the for loop.

The post part can be any statement but generally contains the increment logic. The post part cannot contain initialization. Compiler will raise error in case we add any initialization logic to the post part

Examples

Simple for loop

package main

import "fmt"

func main() {
    for i := 0; i < 5; i++ {
        fmt.Println(i)
    }
}

Output:

For loop with only condition

package main

import "fmt"

func main() {
    i := 0
    for i < 5 {
        fmt.Println(i)
        i++
    }
}

Output:

For Infinite loop

package main

import (
    "fmt"
    "time"
)

func main() {
    i := 0
    for {
        fmt.Println(i)
        i++
        time.Sleep(time.Second * 1)
    }
}

Output: Above program prints a infinite loop

Break Statement in For loop

break statement help exit out of the for loop. None of the statement after break is executed inside the for loop.

package main

import (
    "fmt"
)

func main() {
    i := 0
    for {
        fmt.Println(i)
        i++
        if i >= 5 {
            break
        }
    }
}

Output:

Continue Statement in For loop

continue statement help skip the current iteration of the for loop. None of the statement after the continue is executed and the execution reaches the start again with next iteration. The use case is when you want to only operate on certain elements of the for loop.

Let's see an exampleIn below program we only want to print non-multiples of 3.

package main

import "fmt"

func main() {
    for i := 1; i < 10; i++ {
        if i%3 == 0 {
            continue
        }
        fmt.Println(i)
    }
}

Output:

Nested For Loop

for loop can also be nested as well. See below example

package main

import "fmt"

func main() {
    for i := 0; i < 3; i++ {
        fmt.Printf("Outer loop iteration %d\n", i)
        for j := 0; j < 2; j++ {
            fmt.Printf("i= %d j=%d\n", i, j)
        }
    }
}

Output:

Outer loop iteration 0
i= 0 j=0
i= 0 j=1
Outer loop iteration 1
i= 1 j=0
i= 1 j=1
Outer loop iteration 2
i= 2 j=0
i= 2 j=1

Function Call and Assignment in Init part

The below code is an example that we can also have function call or assignment in init part too.

package main

import "fmt"

func main() {
    i := 1
    //Function call in the init part in for loop
    for test(); i < 3; i++ {
        fmt.Println(i)
    }

    //Assignment in the init part in for loop
    for i = 2; i < 3; i++ {
        fmt.Println(i)
    }
}
func test() {
    fmt.Println("In test function")
}

Output

In test function
1
2
2

Implementing while loop using for loop

Go doesn't have the while keyword. Instead it has the for keyword only. However for keyword can be used to simulate the functionality same as while.

for loop can be implemented to behave the same as while if initialization_part and increment_part can be skipped. Here is an example:

package main

import "fmt"

func main() {
    i := 1
    for i <= 5 {
        fmt.Println(i)
        i++
    }
}

Output:

Conclusion

This is all about for loop in golang. Please share feedback/improvements/mistakes in comments

Next Tutorial – For Range loop
Previous Tutorial – Constants

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