Iterating over arrays in Go

Introduction

We use iteration to look at each item in an array one by one. This helps us do something with every item, like printing or changing it.

When you want to print all items in a list of names.

When you need to add 1 to every number in a list of scores.

When you want to find the biggest number in a list of ages.

When you want to check if a certain word is in a list of words.

Syntax

Go

package main

import "fmt"

type ArrayExample struct {
    items [5]int
}

func (a *ArrayExample) Iterate() {
    for index, value := range a.items {
        fmt.Printf("Index %d has value %d\n", index, value)
    }
}

The range keyword helps us go through each item in the array easily.

You get both the index and the value of each item during iteration.

Examples

Iterate over an array with 3 numbers, printing index and value.

Go

package main

import "fmt"

func main() {
    var numbers [3]int = [3]int{10, 20, 30}
    for i, v := range numbers {
        fmt.Println(i, v)
    }
}

Iterating over an empty array does nothing, no output.

Go

package main

import "fmt"

func main() {
    var emptyArray [0]int
    for i, v := range emptyArray {
        fmt.Println(i, v) // This will not print anything
    }
}

Iterate over an array with one string element.

Go

package main

import "fmt"

func main() {
    var singleElement [1]string = [1]string{"hello"}
    for i, v := range singleElement {
        fmt.Println(i, v)
    }
}

Check and print only the first and last elements in the array.

Go

package main

import "fmt"

func main() {
    var letters [4]string = [4]string{"a", "b", "c", "d"}
    for i, v := range letters {
        if i == 0 || i == len(letters)-1 {
            fmt.Println("Edge element:", v)
        }
    }
}

Sample Program

This program creates an array of fruits and prints each fruit with its position.

Go

package main

import "fmt"

func main() {
    var fruits [5]string = [5]string{"apple", "banana", "cherry", "date", "elderberry"}

    fmt.Println("Before iteration:", fruits)

    fmt.Println("Iterating over fruits array:")
    for index, fruit := range fruits {
        fmt.Printf("Fruit at index %d is %s\n", index, fruit)
    }
}

Important Notes

Time complexity is O(n) because we visit each item once.

Space complexity is O(1) extra space since we only use a few variables.

Common mistake: forgetting that arrays have fixed size in Go, so you cannot add or remove items during iteration.

Use iteration when you want to process or check every item. For changing size, use slices instead.

Summary

Iteration lets you visit each item in an array one by one.

Use for index, value := range array to loop through arrays in Go.

Remember arrays have fixed size, so iteration is for reading or updating items, not changing size.

Practice

1. What does the range keyword do when used with an array in Go?

easy

A. It creates a new array with copied elements.

B. It changes the size of the array during iteration.

C. It lets you loop over each element and get its index and value.

D. It sorts the array elements automatically.

Solution

1. Step 1: Understand the role of range in loops

   The range keyword in Go is used to iterate over elements in arrays, slices, maps, and more, providing both the index and the value of each element.

2. Step 2: Identify what range does with arrays

   When used with an array, range returns the index and the value of each element one by one, allowing you to process or read each item.

3. Final Answer:

   It lets you loop over each element and get its index and value. -> Option C

4. Quick Check:

   range loops with index and value [OK]

Hint: Remember: range gives index and value for each element [OK]

Common Mistakes:

• Thinking range changes array size
• Assuming range sorts elements
• Believing range copies the array

2. Which of the following is the correct syntax to iterate over an array named nums in Go?

easy

A. for i, v := range nums { fmt.Println(i, v) }

B. for i in nums { fmt.Println(i) }

C. foreach i, v in nums { fmt.Println(i, v) }

D. for (i, v) in nums { fmt.Println(i, v) }

Solution

1. Step 1: Recall Go's for-range loop syntax

   In Go, the correct way to iterate over arrays or slices is using for index, value := range array syntax.

2. Step 2: Match the syntax with the options

   for i, v := range nums { fmt.Println(i, v) } uses for i, v := range nums, which is the correct Go syntax. Other options use invalid or non-Go syntax.

3. Final Answer:

   for i, v := range nums { fmt.Println(i, v) } -> Option A

4. Quick Check:

   Go uses for i, v := range array [OK]

Hint: Use 'for i, v := range array' for Go array loops [OK]

Common Mistakes:

• Using 'for i in nums' like Python
• Using 'foreach' which is not Go syntax
• Using parentheses in for loop header

3. What will be the output of this Go code?

nums := [3]int{10, 20, 30}
for i, v := range nums {
    fmt.Println(i, v)
}

medium

A. 0 0
1 1
2 2

B. 10 0
20 1
30 2

C. Syntax error

D. 0 10
1 20
2 30

Solution

1. Step 1: Understand the array and loop

   The array nums has 3 elements: 10 at index 0, 20 at index 1, and 30 at index 2. The loop uses range to get index and value.

2. Step 2: Trace the loop output

   Each iteration prints the index and the value: first line prints "0 10", second "1 20", third "2 30".

3. Final Answer:

   0 10 1 20 2 30 -> Option D

4. Quick Check:

   Index and value printed in order [OK]

Hint: range returns index then value in order [OK]

Common Mistakes:

• Swapping index and value in print
• Expecting values as indexes
• Thinking code has syntax error

4. Identify the error in this Go code snippet:

nums := [3]int{1, 2, 3}
for i, v = range nums {
    fmt.Println(i, v)
}

medium

A. Missing colon in 'for i, v = range nums' should be ':='

B. Using '=' instead of '==' in loop condition

C. Array declaration syntax is wrong

D. fmt.Println cannot print two variables

Solution

1. Step 1: Check the for loop variable declaration

   In Go, when declaring variables in a for-range loop, you must use ':=' to declare and assign variables, not '='.

2. Step 2: Identify the syntax error

   The code uses '=' instead of ':=' in for i, v = range nums, which causes a syntax error.

3. Final Answer:

   Missing colon in 'for i, v = range nums' should be ':=' -> Option A

4. Quick Check:

   Use ':=' to declare variables in for-range loops [OK]

Hint: Use ':=' not '=' in for-range variable declaration [OK]

Common Mistakes:

• Using '=' instead of ':=' in for loop
• Confusing assignment with comparison
• Incorrect array syntax blamed

5. Given an array nums := [5]int{1, 2, 3, 4, 5}, how can you create a new slice containing only the even numbers using iteration?

hard

A. Use a for loop to change the array size and remove odd numbers.

B. Use a for-range loop to check each value and append even numbers to a slice.

C. Use range to multiply all numbers by 2 and store in a new array.

D. Use a for loop with index only and ignore values.

Solution

1. Step 1: Understand the goal and array

   The goal is to create a new slice with only even numbers from the array nums. Arrays have fixed size, so we cannot remove elements from it.

2. Step 2: Use for-range loop and append even values

   We iterate over nums using for _, v := range nums, check if v % 2 == 0, and append those values to a new slice.

3. Final Answer:

   Use a for-range loop to check each value and append even numbers to a slice. -> Option B

4. Quick Check:

   Filter with for-range and append even numbers [OK]

Hint: Filter with for-range and append to slice [OK]

Common Mistakes:

• Trying to change array size directly
• Ignoring values in loop
• Multiplying instead of filtering