Array Insertion at End in DSA Go - Execution Trace

Concept Flow - Array Insertion at End

Start with array

↓

Check if space available?

No→Resize array

Yes↓

Insert element at last index

↓

Update array size

↓

Done

Start with the array, check if there is space to add the new element. If not, resize the array. Then insert the element at the end and update the size.

Execution Sample

DSA Go

package main

import "fmt"

func main() {
    arr := make([]int, 3, 6)
    arr[0] = 1
    arr[1] = 2
    arr[2] = 3
    arr = append(arr, 4)
    fmt.Println(arr)
}

Insert element 4 at the end of array [1, 2, 3] and print the updated array.

Execution Table

Step	Operation	Array Before	Action	Array After	Notes
1	Start	[1, 2, 3]	Check space	[1, 2, 3]	Array has space for new element
2	Insert 4	[1, 2, 3]	Append 4 at end	[1, 2, 3, 4]	Element 4 added at last index
3	Print	[1, 2, 3, 4]	Output array	[1, 2, 3, 4]	Final array printed
4	End	[1, 2, 3, 4]	No more operations	[1, 2, 3, 4]	Insertion complete

💡 Insertion done, array updated with new element at the end

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
arr	[1, 2, 3]	[1, 2, 3]	[1, 2, 3, 4]	[1, 2, 3, 4]	[1, 2, 3, 4]
size	3	3	4	4	4

Key Moments - 3 Insights

Why do we check if there is space before inserting?

Because arrays have fixed capacity; if full, we must resize before adding. Here, step 1 shows the check and confirms space is available.

What happens if the array is full?

We create a bigger array and copy elements before inserting. This is implied in the flow but not shown here since space was available.

Why does append add the element at the end?

Append always adds at the last index after current elements, as shown in step 2 where 4 is added after 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the array after step 2?

A [1, 2, 3, 4]

B [1, 2, 3]

C [4, 1, 2, 3]

D [1, 2]

💡 Hint

Check the 'Array After' column in row for step 2 in execution_table

At which step is the new element actually added to the array?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Operation' and 'Action' columns in execution_table

If the array was full, which step would change in the flow?

A Step 3 would print empty array

B Step 2 would skip insertion

C Step 1 would resize array

D Step 4 would add element

💡 Hint

Refer to concept_flow where resizing happens if no space

Concept Snapshot

Array Insertion at End:
- Check if array has space
- If full, resize array
- Insert element at last index
- Update size
- Append adds element at end

Full Transcript

This visual trace shows how to insert an element at the end of an array. First, we check if there is space in the array. If yes, we add the new element at the last position. If not, we resize the array to make space. The example inserts 4 into [1, 2, 3], resulting in [1, 2, 3, 4]. The variable tracker shows how the array and its size change after each step. Key moments clarify why space check and append behavior matter. The quiz tests understanding of these steps.

Practice

1. What happens when you insert an element at the end of a fixed-size array in Go?

easy

A. You cannot insert because arrays have fixed size

B. The element replaces the first element

C. The element is added and array size increases automatically

D. The array is converted to a slice automatically

Solution

1. Step 1: Understand Go array size

   Arrays in Go have a fixed size defined at declaration and cannot grow.

2. Step 2: Effect of insertion at end

   Since size is fixed, you cannot add new elements beyond the declared size.

3. Final Answer:

   You cannot insert because arrays have fixed size -> Option A

4. Quick Check:

   Fixed size array = no insertion beyond size [OK]

Hint: Arrays have fixed size; no automatic growth on insertion [OK]

Common Mistakes:

• Thinking arrays can grow automatically
• Confusing arrays with slices
• Assuming insertion replaces last element

2. Which of the following is the correct way to append an element 10 to a slice named nums in Go?

easy

A. nums = nums + 10

B. nums.append(10)

C. append(nums, 10)

D. nums = append(nums, 10)

Solution

1. Step 1: Recall Go slice append syntax

   In Go, append returns a new slice with the element added, so assignment is needed.

2. Step 2: Check each option

   nums = append(nums, 10) correctly assigns the result of append to nums. Others are invalid syntax or missing assignment.

3. Final Answer:

   nums = append(nums, 10) -> Option D

4. Quick Check:

   Use append with assignment to add element [OK]

Hint: Always assign append result back to slice variable [OK]

Common Mistakes:

• Not assigning append result back
• Using method call syntax like nums.append()
• Trying to add with + operator

3. What is the output of the following Go code?

nums := []int{1, 2, 3}
nums = append(nums, 4)
fmt.Println(nums)

medium

A. [1 2 3]

B. [4 1 2 3]

C. [1 2 3 4]

D. Compilation error

Solution

1. Step 1: Understand initial slice and append

   nums starts as [1 2 3]. append(nums, 4) adds 4 at the end.

2. Step 2: Print the updated slice

   After append, nums becomes [1 2 3 4], which is printed.

3. Final Answer:

   [1 2 3 4] -> Option C

4. Quick Check:

   Appending 4 adds it at end [OK]

Hint: append adds element at end, print shows updated slice [OK]

Common Mistakes:

• Expecting original slice without new element
• Thinking append adds at start
• Confusing slice with array syntax

4. Identify the error in the following Go code that tries to insert 5 at the end of an array:

arr := [3]int{1, 2, 3}
arr = append(arr, 5)
fmt.Println(arr)

medium

A. Cannot use append on array; append works only on slices

B. Array size is too small; must declare larger array

C. Missing import for fmt package

D. Syntax error in array declaration

Solution

1. Step 1: Check type of arr

   arr is declared as an array [3]int, which has fixed size and no append method.

2. Step 2: Understand append usage

   append works only on slices, not arrays. Using append on array causes error.

3. Final Answer:

   Cannot use append on array; append works only on slices -> Option A

4. Quick Check:

   append requires slice, not array [OK]

Hint: append works only on slices, not fixed-size arrays [OK]

Common Mistakes:

• Trying to append to fixed-size array
• Ignoring difference between array and slice
• Assuming append auto-converts array to slice

5. Given a slice data := []int{2, 4, 6}, which code snippet correctly inserts the value 8 at the end and then doubles all elements?

hard

A. data = append(data, 8)
for _, v := range data {
v *= 2
}
fmt.Println(data)

B. data = append(data, 8)
for i := range data {
data[i] *= 2
}
fmt.Println(data)

C. data = append(data, 8)
for i := 0; i < len(data); i++ {
data[i] = data[i] + 2
}
fmt.Println(data)

D. data = append(data, 8)
for i := range data {
data[i] = data[i] / 2
}
fmt.Println(data)

Solution

1. Step 1: Append 8 to slice

   data = append(data, 8) adds 8 at the end, making data [2 4 6 8].

2. Step 2: Double each element correctly

   Using for i := range data and data[i] *= 2 doubles each element in place.

3. Step 3: Check other options

   data = append(data, 8) for _, v := range data { v *= 2 } fmt.Println(data) doubles copy of value, not original slice. Options C and D do wrong math.

4. Final Answer:

   data = append(data, 8) for i := range data { data[i] *= 2 } fmt.Println(data) -> Option B

5. Quick Check:

   Append then modify elements by index [OK]

Hint: Modify slice elements by index, not by value copy [OK]

Common Mistakes:

• Modifying value copy in range instead of slice element
• Using wrong arithmetic operation
• Not assigning append result back