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Understanding For Loops.

Objective

To understand for loops.

Go offers only "for" loops for doing repetitive tasks, if you come from some other languages there are no "while" or "do-while" loops in Go, instead "for" loops have a flexible syntax and offers all the looping functions.

General syntax is:

for condition {
    do something
}

Structure

Navigate to our code folder

code/basic/

For our program create a new folder '08_for_loops'

code/basic/08_for_loops

And lets create a file 'for_loops.go' in it, finally the structure would look like this:

code/basic/08_for_loops/for_loops.go

Code

The code will be divided into four parts:

1.)

for loops
    1 package main
    2
    3 import "fmt"
    4
    5 func main() {
    6 // Variation 1 - conditional loop
    7 num1 := 0
    8 fmt.Println("Starting for loop...")
    9 for i := 0; i < 11; i++ {
    10 fmt.Println("Num =", num1)
    11 num1++
    12 }
    13 fmt.Println()

Review

Line 9 declared a for loop along with condition

for i := 0; i &lt; 11; i++ {

We initial the variable "i" and set it to "0", then we say loop till "i is less than 11" and after each pass do "i++", i.e increment the value of "i" by 1.

Line 10 prints the value after each pass and on line 11 we increment the value of num

fmt.Println("Num =", num1)
num1++

2.)

for loops
    14 // Variation 2 - Infinite loop
    15 num2 := 20
    16 fmt.Println("Entering infinite loop...")
    17 for {
    18 // break condition
    19 if num2 < 10 {
    20 break
    21 }
    22 fmt.Println("Num =", num2)
    23 num2--
    24 }
    25 fmt.Println()

Review

On line 17 we start a infinite loop, the for loop will keep on executing till it encounters a break condition or runs out of memory

for {

On line 19 we check for a break condition

if num2 &lt; 10

3.)

for loops
    26 // Variation 3 - Optional statements
    27 num3 := 20
    28 fmt.Println("Loop with optional statements...")
    29 for num3 <= 30 {
    30 fmt.Println("Num =", num3)
    31 num3++
    32 }
    33 fmt.Println()

Review

On line 29 we start a loop with operational statements, the loop will break when it fulfills the condition.

for num3 &lt;= 30

Note we have declared and initialized "num3" on line 27

num3 := 20

4.)

for loops
    34 // Variation 4 - Boolean operators
    35 cond := true
    36 num4 := 30
    37 fmt.Println("Loops with boolean operator...")
    38 for cond {
    39 if num4 >= 40 {
    40 cond = false
    41 }
    42 fmt.Println("Num =", num4)
    43 num4++
    44 }
    45 }

Review

We declare a boolean condition on line 35, and start the loop on line 38

for cond

In order to break the loop we set the condition on line 39

if num4 &gt;= 40 {
    cond = false
}

Full Code

Complete For-Loop Code
    package main

    import "fmt"

    func main() {
        // Variation 1 - conditional loop
        num1 := 0
        fmt.Println("Starting for loop...")
        for i := 0; i < 11; i++ {
            fmt.Println("Num =", num1)
            num1++
        }
        fmt.Println()
        // Variation 2 - Infinite loop
        num2 := 20
        fmt.Println("Entering infinite loop...")
        for {
            // break condition
            if num2 < 10 {
                break
            }
            fmt.Println("Num =", num2)
            num2--
        }
        fmt.Println()
        // Variation 3 - Optional statements
        num3 := 20
        fmt.Println("Loop with optional statements...")
        for num3 <= 30 {
            fmt.Println("Num =", num3)
            num3++
        }
        fmt.Println()
        // Variation 4 - Boolean operators
        cond := true
        num4 := 30
        fmt.Println("Loops with boolean operator...")
        for cond {
            if num4 >= 40 {
                cond = false
            }
            fmt.Println("Num =", num4)
            num4++
        }
    }

Run Code

Open your terminal and navigate to our folder

code/basic/08_for_loops

Once in the folder type the following command

go run for_loops.go

Output

Output
Starting for loop...
Num = 0
Num = 1
Num = 2
Num = 3
Num = 4
Num = 5
Num = 6
Num = 7
Num = 8
Num = 9
Num = 10

Entering infinite loop...
Num = 20
Num = 19
Num = 18
Num = 17
Num = 16
Num = 15
Num = 14
Num = 13
Num = 12
Num = 11
Num = 10

Loop with optional statements...
Num = 20
Num = 21
Num = 22
Num = 23
Num = 24
Num = 25
Num = 26
Num = 27
Num = 28
Num = 29
Num = 30

Loops with boolean operator...
Num = 30
Num = 31
Num = 32
Num = 33
Num = 34
Num = 35
Num = 36
Num = 37
Num = 38
Num = 39
Num = 40

Note

Go ships with one more variant know as "for - range", we will study it in the coming chapters.

Github

Just in case you have some errors with your code, you can check out the code at github repo

Github Repo

Golang Playground

You can also run the code at playground

Golang Playground

Next

In the next chapter we will learn about function declaration.

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