Accessing array elements in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an array in Go?

An array in Go is a fixed-size collection of elements of the same type, stored in contiguous memory locations.

beginner

How do you access the first element of an array named arr in Go?

You access it using arr[0] because Go arrays are zero-indexed, meaning the first element is at index 0.

intermediate

What happens if you try to access an array element with an index out of range in Go?

Go will cause a runtime panic with an 'index out of range' error, stopping the program.

beginner

Given var arr = [3]int{10, 20, 30}, what is the value of arr[2]?

The value is 30, because array indices start at 0, so arr[2] is the third element.

beginner

Can you change the value of an element in a Go array after it is created?

Yes, you can assign a new value to an element by accessing it with its index, like arr[1] = 50.

What is the index of the first element in a Go array?

A 1

B 0

C -1

D Depends on the array size

Go arrays start indexing at 0, so the first element is at index 0.

How do you access the third element of an array named nums?

A nums[2]

B nums[3]

C nums[1]

D nums[0]

Since indexing starts at 0, the third element is at index 2.

What happens if you try to access arr[5] in an array of length 3?

A Compiles but undefined behavior

B Returns zero value

C Returns last element

D Runtime panic: index out of range

Accessing an index outside the array length causes a runtime panic in Go.

Can you assign a new value to an existing element in a Go array?

A Yes, by using its index

B No, arrays are immutable

C Only if the array is a slice

D Only during array declaration

Go arrays allow changing element values by accessing them with their index.

Which of these is a valid way to declare an array of 4 integers in Go?

A arr := [4]int{1,2,3}

B var arr []int

C var arr [4]int

D arr := []int{1,2,3,4}

Option C declares a fixed-size array of 4 integers. Options B and D declare slices, and C has fewer elements than size.

Explain how to access and modify elements in a Go array.

Think about how you get and set values using the index.

Describe what happens if you try to access an array element outside its range in Go.

What does Go do to protect you from invalid access?

Practice

1. What is the index of the first element in a Go array?

easy

A. 1

B. Depends on the array size

C. -1

D. 0

Solution

1. Step 1: Understand Go array indexing

   In Go, arrays start indexing from 0, meaning the first element is at index 0.

2. Step 2: Confirm the first element's index

   The first element is always accessed with index 0, not 1 or any other number.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   First element index = 0 [OK]

Hint: Remember, Go arrays start counting at zero [OK]

Common Mistakes:

• Thinking indexing starts at 1
• Assuming negative indexes are valid
• Believing index depends on array size

2. Which of the following is the correct way to access the third element of an array named arr in Go?

easy

A. arr[3]

B. arr[2]

C. arr(2)

D. arr{2}

Solution

1. Step 1: Recall Go array indexing syntax

   Go uses square brackets with zero-based indexing to access elements, so the third element is at index 2.

2. Step 2: Identify correct syntax

   arr[2] accesses the third element; arr[3] accesses the fourth, arr(2) and arr{2} are invalid syntax.

3. Final Answer:

   arr[2] -> Option B

4. Quick Check:

   Third element = arr[2] [OK]

Hint: Use square brackets and zero-based index [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based index
• Using curly braces for access

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

arr := [4]int{10, 20, 30, 40}
fmt.Println(arr[1])

medium

A. 10

B. 40

C. 20

D. 30

Solution

1. Step 1: Identify the array elements and their indexes

   The array is [10, 20, 30, 40], indexed 0 to 3. Index 1 corresponds to the second element, 20.

2. Step 2: Determine the printed value

   arr[1] accesses the element 20, so fmt.Println prints 20.

3. Final Answer:

   20 -> Option C

4. Quick Check:

   arr[1] = 20 [OK]

Hint: Remember index 1 is second element [OK]

Common Mistakes:

• Printing arr[0] instead
• Confusing index 1 with 2
• Expecting output as 30 or 40

4. What is wrong with this Go code snippet?

arr := [3]int{1, 2, 3}
fmt.Println(arr[3])

medium

A. Index 3 is out of bounds for the array

B. Array declaration syntax is incorrect

C. fmt.Println cannot print array elements

D. The array size should be 4

Solution

1. Step 1: Check array size and valid indexes

   The array has size 3, so valid indexes are 0, 1, and 2 only.

2. Step 2: Identify the error in accessing arr[3]

   Accessing arr[3] tries to get the fourth element, which does not exist, causing an out-of-bounds runtime error.

3. Final Answer:

   Index 3 is out of bounds for the array -> Option A

4. Quick Check:

   Valid indexes: 0 to 2 only [OK]

Hint: Array indexes go from 0 to length-1 [OK]

Common Mistakes:

• Using index equal to array length
• Thinking fmt.Println can't print elements
• Assuming array size auto-expands

5. Given the array arr := [5]int{5, 10, 15, 20, 25}, which code snippet correctly sums the first three elements?

hard

A. sum := arr[0] + arr[1] + arr[2]

B. sum := arr[1] + arr[2] + arr[3]

C. sum := arr[0] + arr[2] + arr[3]

D. sum := arr[2] + arr[3] + arr[4]

Solution

1. Step 1: Identify the first three elements

   The first three elements are at indexes 0, 1, and 2, which are 5, 10, and 15 respectively.

2. Step 2: Check which option sums these elements

   sum := arr[0] + arr[1] + arr[2] sums arr[0], arr[1], and arr[2], which is correct. Other options include elements beyond the first three.

3. Final Answer:

   sum := arr[0] + arr[1] + arr[2] -> Option A

4. Quick Check:

   Sum first three = arr[0]+arr[1]+arr[2] [OK]

Hint: Sum elements at indexes 0, 1, and 2 for first three [OK]

Common Mistakes:

• Starting sum at index 1
• Skipping elements
• Including elements beyond first three