Array Access and Update at Index in DSA Go - Execution Trace

Concept Flow - Array Access and Update at Index

Start

↓

Access array at index i

↓

Check if index i is valid

↓

Read or Update

↓

Finish

The flow shows accessing or updating an array element by index, checking if the index is valid before proceeding.

Execution Sample

DSA Go

arr := [5]int{10, 20, 30, 40, 50}
index := 2
value := arr[index]
arr[index] = 99

This code accesses the element at index 2 of the array, reads it, then updates that element to 99.

Execution Table

Step	Operation	Index	Value Accessed/Updated	Array State	Notes
1	Access element	2	30	[10, 20, 30, 40, 50]	Index 2 is valid, value read is 30
2	Update element	2	99	[10, 20, 99, 40, 50]	Index 2 is valid, value updated to 99
3	Access element	5	N/A	[10, 20, 99, 40, 50]	Index 5 invalid, out of bounds error

💡 Execution stops when index is out of bounds or after update operation completes.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
arr	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 99, 40, 50]	[10, 20, 99, 40, 50]
index	2	2	2	5
value	N/A	30	30	N/A

Key Moments - 3 Insights

Why do we check if the index is valid before accessing or updating?

Because accessing or updating an invalid index causes an error or crash. The execution_table row 3 shows an out of bounds error when index 5 is used.

Does updating the array element change the array size?

No, updating changes the value at that index only. The array size stays the same as shown in execution_table rows 1 and 2 where array length is constant.

What happens if we try to read a value after updating it?

We get the updated value. In execution_table row 2, the value at index 2 is updated to 99, so reading it afterward would return 99.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value accessed at step 1?

A 30

B 20

C 10

D 99

💡 Hint

Check the 'Value Accessed/Updated' column in execution_table row 1.

At which step does the array get updated?

A Step 1

B Step 3

C Step 2

D No update occurs

💡 Hint

Look at the 'Operation' and 'Array State' columns in execution_table row 2.

If index was set to 4 instead of 5 at step 3, what would happen?

A Out of bounds error

B Access value 50 successfully

C Update value 50 to 99

D No change, index 4 is invalid

💡 Hint

Array length is 5, valid indices are 0 to 4, see concept_flow and execution_table.

Concept Snapshot

Array Access and Update at Index:
- Use arr[index] to read or update
- Check index is within 0 and length-1
- Reading returns value at index
- Updating changes value at index
- Invalid index causes error

Full Transcript

This concept shows how to access and update an array element by index. First, we check if the index is valid to avoid errors. Then we read or update the value at that index. The array size does not change when updating. Trying to access an invalid index causes an error and stops execution.

Practice

1. What does the expression arr[2] do in an array arr?

easy

A. Accesses the element at index 3 of the array

B. Accesses the element at index 2 of the array

C. Updates the element at index 2 of the array

D. Deletes the element at index 2 of the array

Solution

1. Step 1: Understand array indexing

   Array indexing starts at 0, so index 2 means the third element.

2. Step 2: Identify the operation

   Using arr[2] accesses (reads) the element at index 2 without changing it.

3. Final Answer:

   Accesses the element at index 2 of the array -> Option B

4. Quick Check:

   arr[2] reads element at index 2 [OK]

Hint: Remember arrays start at 0 index [OK]

Common Mistakes:

• Thinking index 2 is the second element
• Confusing access with update
• Assuming indexing starts at 1

2. Which of the following is the correct syntax to update the element at index 1 in a Go array named nums to 10?

easy

A. nums[1] = 10

B. nums(1) = 10

C. nums[1] := 10

D. nums.update(1, 10)

Solution

1. Step 1: Recall Go array update syntax

   In Go, to update an element, use array[index] = value.

2. Step 2: Check each option

   nums[1] = 10 uses correct syntax. nums(1) = 10 uses parentheses which is invalid. nums[1] := 10 uses := which is for declaration, not update. nums.update(1, 10) uses a method that does not exist.

3. Final Answer:

   nums[1] = 10 -> Option A

4. Quick Check:

   Use square brackets and = for update [OK]

Hint: Use square brackets and = to update array element [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Using := instead of = for update
• Assuming arrays have update methods

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

package main
import "fmt"
func main() {
  arr := [4]int{5, 10, 15, 20}
  arr[2] = 25
  fmt.Println(arr)
}

medium

A. [5 10 25 20]

B. [5 10 15 20]

C. [25 10 15 20]

D. [5 25 15 20]

Solution

1. Step 1: Understand initial array and update

   Initial array is [5, 10, 15, 20]. The code updates element at index 2 (third element) to 25.

2. Step 2: Apply the update and print

   After update, array becomes [5, 10, 25, 20]. Printing shows this updated array.

3. Final Answer:

   [5 10 25 20] -> Option A

4. Quick Check:

   Index 2 updated to 25 [OK]

Hint: Index 2 means third element update [OK]

Common Mistakes:

• Confusing index 2 with index 1 or 3
• Not updating the array before print
• Expecting commas in printed array

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

package main
func main() {
  arr := [3]int{1, 2, 3}
  arr[3] = 4
}

medium

A. Cannot assign to arr[3] because arrays are immutable

B. Syntax error due to missing import

C. Index out of range error at arr[3]

D. No error, code runs fine

Solution

1. Step 1: Check array size and index used

   Array has size 3 with valid indices 0, 1, 2. Accessing arr[3] is invalid.

2. Step 2: Identify error type

   Accessing arr[3] causes runtime panic: index out of range error.

3. Final Answer:

   Index out of range error at arr[3] -> Option C

4. Quick Check:

   Index must be less than array length [OK]

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

Common Mistakes:

• Assuming arr[3] is valid for size 3 array
• Thinking arrays are immutable in Go
• Ignoring runtime errors

5. Given an array arr := [5]int{1, 2, 3, 4, 5}, which code snippet correctly doubles the value at each index using array access and update?

hard

A. for i := 1; i <= len(arr); i++ { arr[i] = arr[i] * 2 }

B. for i := 0; i < len(arr); i++ { arr[i] *= 2; arr[i+1] = arr[i] }

C. for i := 0; i < len(arr); i++ { arr[i+1] = arr[i] * 2 }

D. for i := 0; i < len(arr); i++ { arr[i] = arr[i] * 2 }

Solution

1. Step 1: Understand array length and valid indices

   Array length is 5, valid indices are 0 to 4. Loop must run from 0 to 4.

2. Step 2: Analyze each loop snippet

   for i := 0; i < len(arr); i++ { arr[i] = arr[i] * 2 } loops correctly and updates each element by doubling. for i := 1; i <= len(arr); i++ { arr[i] = arr[i] * 2 } uses i from 1 to 5, arr[5] is invalid index. for i := 0; i < len(arr); i++ { arr[i+1] = arr[i] * 2 } updates arr[i+1], which causes out of range at last iteration. for i := 0; i < len(arr); i++ { arr[i] *= 2; arr[i+1] = arr[i] } updates arr[i+1] inside loop, causing out of range and incorrect logic.

3. Final Answer:

   for i := 0; i < len(arr); i++ { arr[i] = arr[i] * 2 } -> Option D

4. Quick Check:

   Loop from 0 to len-1 and update arr[i] [OK]

Hint: Loop indices 0 to len-1 for safe access [OK]

Common Mistakes:

• Looping beyond array length causing index error
• Updating wrong indices inside loop
• Using <= instead of < in loop condition