Array Declaration and Initialization in DSA 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 declare an array of 5 integers in Go?

You declare it as: var arr [5]int. This creates an array named arr with 5 integer elements initialized to zero.

beginner

How to initialize an array with values during declaration in Go?

You can initialize like this: arr := [3]int{10, 20, 30}. This creates an array of size 3 with elements 10, 20, and 30.

beginner

What happens if you declare an array without initializing it in Go?

All elements are set to the zero value of the array's type. For integers, this means all elements are 0.

intermediate

How to declare an array without specifying size but initializing it in Go?

Use arr := [...]int{1, 2, 3}. The compiler counts the elements and sets the array size automatically.

How do you declare an array of 4 strings in Go?

A var arr string[4]

B var arr [4]string

C arr := []string{4}

D arr := [4]string{}

Option B correctly declares an array of 4 strings. Option D is also valid but initializes an empty array; C is the standard declaration.

What is the default value of elements in an uninitialized integer array in Go?

A 0

B nil

C undefined

D empty string

Integer arrays default to 0 for all elements if not initialized.

How to declare and initialize an array with values 5, 10, 15 in Go without specifying size?

A arr := [3]int{5, 10, 15}

B arr := []int{5, 10, 15}

C arr := [...]int{5, 10, 15}

D var arr []int = {5, 10, 15}

Option C uses '...' to let the compiler determine the size automatically.

Which of these is NOT a valid way to declare an array in Go?

A var arr [2]int

B arr := [2]int{1, 2}

C var arr []int

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

Option D tries to initialize an array of size 2 with 3 elements, which is invalid.

What does the following code print?

var arr [3]int
fmt.Println(arr)

A [0 0 0]

B [1 2 3]

C [nil nil nil]

D Compilation error

Uninitialized integer arrays have all elements set to 0.

Explain how to declare and initialize an array in Go with examples.

Think about how you create a fixed-size list of items in Go.

What happens if you declare an array but do not assign any values to it in Go?

Consider what Go does to keep arrays predictable and safe.

Practice

1. Which of the following is the correct way to declare an array of 5 integers in Go?

easy

A. var arr []int = make([]int, 5)

B. var arr [5]int

C. arr := [5]int

D. arr := []int{5}

Solution

1. Step 1: Understand array declaration syntax in Go

   Arrays in Go are declared with a fixed size inside square brackets, followed by the type, e.g., var arr [5]int.

2. Step 2: Analyze each option

   var arr [5]int correctly declares an array of 5 integers. arr := [5]int uses shorthand but lacks the array literal {}, causing a syntax error. var arr []int = make([]int, 5) declares a slice, not an array. arr := []int{5} declares a slice with one element 5, not an array of size 5.

3. Final Answer:

   var arr [5]int -> Option B

4. Quick Check:

   Array declaration = var arr [5]int [OK]

Hint: Arrays use fixed size in brackets, slices do not [OK]

Common Mistakes:

• Confusing slices with arrays
• Using shorthand without initialization
• Incorrect size placement

2. Which of the following is the correct syntax to initialize an array with values 1, 2, 3 in Go?

easy

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

B. arr := [3]int(1, 2, 3)

C. var arr [3]int = {1, 2, 3}

D. var arr = [3]int{1, 2, 3}

Solution

1. Step 1: Review array initialization syntax

   In Go, arrays are initialized with curly braces and values inside, e.g., [3]int{1, 2, 3}.

2. Step 2: Check each option

   var arr = [3]int{1, 2, 3} uses correct syntax. arr := [3]int(1, 2, 3) uses parentheses which is invalid. var arr [3]int = {1, 2, 3} misses the type keyword before braces. arr := []int{1, 2, 3} declares a slice, not an array.

3. Final Answer:

   var arr = [3]int{1, 2, 3} -> Option D

4. Quick Check:

   Array init with braces = var arr = [3]int{1, 2, 3} [OK]

Hint: Use curly braces {} for array initialization [OK]

Common Mistakes:

• Using parentheses instead of braces
• Confusing slices with arrays
• Omitting type in declaration

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

package main
import "fmt"
func main() {
  var arr = [4]int{10, 20}
  fmt.Println(arr)
}

medium

A. [10 20 0 0]

B. [10 20]

C. [0 0 0 0]

D. Compilation error

Solution

1. Step 1: Understand partial array initialization

   When an array is partially initialized, the remaining elements are set to zero values (0 for int).

2. Step 2: Analyze the given array

   The array size is 4, but only first two elements are initialized as 10 and 20. The last two elements default to 0.

3. Final Answer:

   [10 20 0 0] -> Option A

4. Quick Check:

   Partial init fills rest with zeros = [10 20 0 0] [OK]

Hint: Uninitialized array elements default to zero [OK]

Common Mistakes:

• Expecting only initialized elements in output
• Thinking partial init causes error
• Confusing slices with arrays

4. Identify the error in the following Go code snippet:

package main
func main() {
  var arr [3]int = [3]int{1, 2}
}

medium

A. No error, code is correct

B. Array size mismatch error

C. Syntax error in array declaration

D. Missing import statement

Solution

1. Step 1: Check array size and initialization

   The array is declared with size 3 but initialized with 2 values. Go allows this and fills the last element with zero.

2. Step 2: Verify syntax and imports

   No syntax error exists. Import is not mandatory if no external packages are used. The code compiles fine.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Partial init allowed, no import needed [OK]

Hint: Partial initialization is allowed; missing imports only if used [OK]

Common Mistakes:

• Assuming size mismatch causes error
• Expecting import for main package
• Confusing slices with arrays

5. You want to declare an array of 5 integers in Go and initialize only the last element to 100, leaving others as zero. Which code snippet achieves this correctly?

hard

A. var arr = [5]int{0: 100}

B. var arr = [5]int{5: 100}

C. var arr = [5]int{4: 100}

D. var arr = [5]int{100}

Solution

1. Step 1: Understand indexed initialization in arrays

   Go allows initializing specific elements by index using syntax like {index: value}. Indexing starts at 0.

2. Step 2: Analyze each option

   var arr = [5]int{4: 100} initializes the element at index 4 (the last element in a 5-element array) to 100, others default to 0. var arr = [5]int{5: 100} uses index 5 which is out of range. var arr = [5]int{100} initializes only the first element. var arr = [5]int{0: 100} initializes index 0 to 100, not the last.

3. Final Answer:

   var arr = [5]int{4: 100} -> Option C

4. Quick Check:

   Use index 4 for last element in 5-size array [OK]

Hint: Index starts at 0; last element is index size-1 [OK]

Common Mistakes:

• Using out-of-range index
• Initializing first element instead of last
• Omitting index for specific element