Array Declaration and Initialization in DSA Go - Execution Trace

Concept Flow - Array Declaration and Initialization

Declare array variable

↓

Specify array size

↓

Initialize elements

↓

Array ready to use

The flow shows declaring an array variable, setting its size, initializing elements, and then the array is ready for use.

Execution Sample

DSA Go

var arr [3]int
arr[0] = 10
arr[1] = 20
arr[2] = 30

This code declares an integer array of size 3 and initializes its elements with values 10, 20, and 30.

Execution Table

Step	Operation	Array State	Pointer Changes	Visual State
1	Declare array variable 'arr' of size 3	[0, 0, 0]	arr points to array start	[0] -> [0] -> [0]
2	Initialize arr[0] = 10	[10, 0, 0]	arr[0] set to 10	[10] -> [0] -> [0]
3	Initialize arr[1] = 20	[10, 20, 0]	arr[1] set to 20	[10] -> [20] -> [0]
4	Initialize arr[2] = 30	[10, 20, 30]	arr[2] set to 30	[10] -> [20] -> [30]
5	Array fully initialized	[10, 20, 30]	No pointer change	[10] -> [20] -> [30]

💡 All elements initialized, array ready for use

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
arr	undefined	[0, 0, 0]	[10, 0, 0]	[10, 20, 0]	[10, 20, 30]	[10, 20, 30]

Key Moments - 3 Insights

Why does the array start with zeros after declaration?

In Go, arrays are automatically initialized with zero values for their type. See execution_table row 1 where arr is [0, 0, 0] after declaration.

Can we assign values beyond the declared size of the array?

No, arrays have fixed size. Trying to assign beyond index 2 (for size 3) causes an error. The execution_table only shows indices 0 to 2 being assigned.

What happens if we do not initialize all elements?

Uninitialized elements remain zero. For example, after step 2, arr is [10, 0, 0] as shown in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the array state after step 3?

A [0, 0, 0]

B [10, 0, 0]

C [10, 20, 0]

D [10, 20, 30]

💡 Hint

Check the 'Array State' column at step 3 in execution_table

At which step does the array become fully initialized?

A Step 2

B Step 5

C Step 4

D Step 3

💡 Hint

Look for the row where all elements have assigned values in execution_table

If we skip initializing arr[1], what would be the array state after step 4?

A [10, 0, 30]

B [10, 20, 30]

C [0, 0, 0]

D [10, 0, 0]

💡 Hint

Refer to variable_tracker and execution_table to see default zero values for uninitialized elements

Concept Snapshot

Array Declaration and Initialization in Go:
- Declare with: var arr [size]type
- Size fixed at declaration
- Elements auto-initialized to zero
- Assign values by index: arr[i] = value
- Access elements by index 0 to size-1

Full Transcript

This concept shows how to declare and initialize arrays in Go. First, declare an array with a fixed size and type. The array elements start with zero values automatically. Then assign values to each element by their index. The array is ready to use after all elements are initialized. The execution table traces each step showing the array's state and pointer changes. Beginners often wonder why arrays start with zeros and why size cannot be changed. The quiz questions help reinforce understanding by referencing the execution steps and variable states.

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