README.md

Working with loops

This chapter covers working with loops in Go. Loops are used to repeat statements in your code.

Introduction

This chapter will cover:

• Loop statements with for.
• Device conditions on when to break a loop.
• Apply range to iterate over an array.
• Control the loop with continue and break.

The case for looping statements

You are likely to want to repeat a set of instructions. For example, you might have a list of orders where you need to process each order. Or you have a file that you need to read line by line or there might be some other calculation. Regardless of your situation, you are likely to need a looping construct, so what are your options in Go?

You are using the for loop. There are three major ways you can use it:

• increment steps. With the help of a variable, you define a start point, a condition when to stop and an incrementation step. This is your "classic" for-loop. Here's what it looks like:

  for i := 0; i < 10; i++ {
    // run these statements
  } 

• while. In many programming languages you have a while keyword. Go doesn't have that, but what you can do is use the for-loop similarly. You omit the initialization step and increment step and get this code:

  for <condition> {
    // run these statements
  }

• for each. lastly, you have the for-each like construct that operates on an array-like sequence. It uses the range keyword to function:

  for i,s := range array {
    // run these statements
  }

The for loop

The conventional for-loop has three different parts:

• initialization, here you want to create a variable and assign it a starter value like so:

  for i:= 0;

  Note the use of ;, you usually don's use semicolons, but for this construct, you need it.

• condition. The next step is evaluating whether you should continue incrementing or not. You define a Boolean expression here, that if true, continues to loop:

  for i := 0; i< 10     

  i < 10, as long as a value is between 0 and 10 (becomes 10, then loop breaks), then it returns true, and the loop continues.

• increment, in this step, the loop variable i is updated, updating it by 1 is common but you can add any increment size, negative or positive.

  for i := 0; i< 10; i++ {
  
  }

  Here, i is updated by 1. This loop will run ten times.

Repeat until the condition is met with while

A simplified version of this loop can omit the initialization and increment steps. You are then left with the condition step only. This step tests whether a variable is true or false and the loop exits on false. Here's an example:

i := 1
for i < 10 {
  i++
  // do something
}

In this case, we are declaring i outside of the loop. Within the loop, we need to change the value to something that will make the loop expression evaluate to false or it will loop forever.

Here's another code, using the same idea, but this time we ask for input from the user:

var keepGoing = true
answer := ""
for keepGoing {
  fmt.Println("Type command: ")
  fmt.Scan(&answer)
  if answer == "quit" {
    keepGoing = false
  }
}
fmt.Println("program exit")

An example run of the program could look like so:

Type command: test
Type command: something
Type command: quit
program exit

Using for-each over a range

For this next loop construct, the idea is to operate over an array or known sequence. For each iteration you can get the index as well as the next item in the loop. Here's some example code:

arr := []string{"arg1", "arg2", "arg3"}
 for i, s := range arr {
  fmt.Printf("index: %d, item: %s \n", i, s)
 }

arr is defined as an array and then the range construct is used to loop over the array. For each iteration, the current index is assigned to i and the array item is assigned to s. An output of the above code will look like so:

index: 0, item: arg1
index: 1, item: arg2
index: 2, item: arg3

Controlling the loop with continue and break

So far, you've seen three ways you can use the for construct. There are also ways to control the loop. You can control the loop with the following keywords:

• break, this will exit the loop construct

  arr = []int{-1,2}
  for i := 0; i< 2; i++ {
    fmt.Println(arr[i])
    if arr[i] < 0 {
      break;
    }
  }     

  The output will be:

  -1
  

  it won't output 2 as the loop exits after the first iteration.

• continue, this will move on to the next iteration. If break exits the loop, continue skips the current iteration:

  arr = []int{-1,2,-1, 3}
  for i := 0; i< 4; i++ {
    if arr[i] < 0 {
      break;
    }
    fmt.Println(arr[i])
  } 

  The output will be:

  2
  3
  

Assignment - create a command line loop

When creating console apps, you often want to read user input. The user input could be data used in the program or it can be the user typing a command to do something like "save", "print", "backup" etc. We will build a program for the latter case.

1. Create a file main.go and give it the following content:

   package main
   
   import "fmt"
   
   func main() {
   
   }

2. Add the following code to the main() method:

   var keepGoing = true
   answer := ""
   for keepGoing {
     fmt.Println("Type command: ")
     fmt.Scan(&answer)
     if answer == "quit" {
       keepGoing = false
     }
   }
   fmt.Println("program exit")

3. Run the code by typing go run main.go:

   go run main.go

   You should see an output like so:

   Type command: command
   Type command: quit
   program exit 
   

🚀 Challenge

• Add a command "print" that ends up outputting "printing file".

• See if you can use a break instead of the variable keepGoing to break the loop when the user types"quit".

Solution

package main

import "fmt"

func main() {
  var keepGoing = true
   answer := ""
   for keepGoing {
     fmt.Println("Type command: ")
     fmt.Scan(&answer)
     if answer == "quit" {
       keepGoing = false
     }
   }
   fmt.Println("program exit")
}