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

Concept Flow - Array Access and Update at Index

Start

↓

Access array at index i

↓

Read value at index i

→Use value

↓

Update value at index i

→Store new value

↓

End

This flow shows how we access or update a value at a specific index in an array by directly reaching that position.

Execution Sample

DSA Java

int[] arr = {10, 20, 30, 40, 50};
int val = arr[2];
arr[3] = 100;
System.out.println(java.util.Arrays.toString(arr));

This code reads the value at index 2 and updates the value at index 3 in the array.

Execution Table

Step	Operation	Index Accessed	Value Read	Value Updated	Array State
1	Initialize array	-	-	-	[10, 20, 30, 40, 50]
2	Read value at index 2	2	30	-	[10, 20, 30, 40, 50]
3	Update value at index 3	3	-	100	[10, 20, 30, 100, 50]
4	Print array	-	-	-	[10, 20, 30, 100, 50]

💡 All operations completed; array updated at index 3.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
arr	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 100, 50]	[10, 20, 30, 100, 50]
val	undefined	30	30	30

Key Moments - 3 Insights

Why does reading arr[2] give 30 and not 20 or 40?

Because arrays start indexing at 0, so index 2 points to the third element, which is 30 (see execution_table step 2).

What happens to the array after updating arr[3] = 100?

The value at index 3 changes from 40 to 100, updating the array state as shown in execution_table step 3.

Can we access or update an index outside the array length?

No, accessing or updating an invalid index causes an error. This example only uses valid indices within array bounds.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'val' after step 2?

A 20

B 30

C 40

D 100

💡 Hint

Check the 'Value Read' column at step 2 in execution_table.

At which step does the array change its content?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Array State' column and see when the value at index 3 changes.

If we update arr[0] = 99 instead of arr[3], what would be the array state after that update?

A [10, 20, 30, 40, 50]

B [10, 20, 30, 100, 50]

C [99, 20, 30, 40, 50]

D [10, 20, 99, 40, 50]

💡 Hint

Updating index 0 changes the first element; check how array state changes in execution_table step 3.

Concept Snapshot

Array Access and Update:
- Access: arr[index] reads value at that position.
- Update: arr[index] = newValue changes value at that position.
- Indexing starts at 0.
- Must use valid indices within array length.
- Direct access makes arrays fast for read/write.

Full Transcript

This lesson shows how to access and update values in an array by using an index. Arrays start counting at zero, so the first element is at index 0. Reading arr[2] gets the third element. Updating arr[3] changes the fourth element. The execution table tracks each step, showing the array before and after changes. Variables like 'val' store read values. Remember, accessing outside the array size causes errors. Arrays allow quick direct access and updates by index.

Practice

1. What does the expression arr[2] do in a Java array named arr?

easy

A. Deletes the element at index 2

B. Adds a new element at index 2

C. Accesses the element at index 2 of the array

D. Returns the length of the array

Solution

1. Step 1: Understand array indexing

   In Java, arrays use zero-based indexing, so arr[2] accesses the third element.

2. Step 2: Identify the operation

   The expression reads the value stored at index 2 without modifying the array.

3. Final Answer:

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

4. Quick Check:

   arr[2] means get element at index 2 [OK]

Hint: Remember arrays start at index 0, so arr[2] is third element [OK]

Common Mistakes:

• Thinking arr[2] adds or deletes elements
• Confusing index with array length
• Assuming indexing starts at 1

2. Which of the following is the correct syntax to update the element at index 1 of an integer array nums to 10?

easy

A. nums(1) = 10;

B. nums[1] = 10;

C. nums{1} = 10;

D. nums.1 = 10;

Solution

1. Step 1: Recall Java array update syntax

   In Java, array elements are accessed and updated using square brackets: array[index] = value;

2. Step 2: Match correct syntax

   Only nums[1] = 10; uses correct brackets and assignment.

3. Final Answer:

   nums[1] = 10; -> Option B

4. Quick Check:

   Use square brackets for array index update [OK]

Hint: Use square brackets [] to access or update array elements [OK]

Common Mistakes:

• Using parentheses () instead of brackets []
• Using curly braces {} which are invalid for arrays
• Trying to access with dot notation

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

int[] arr = {5, 10, 15, 20};
arr[2] = 25;
System.out.println(arr[2]);

medium

A. 25

B. 15

C. 20

D. 10

Solution

1. Step 1: Understand initial array values

   The array arr starts as [5, 10, 15, 20].

2. Step 2: Update element at index 2

   We set arr[2] = 25; changing the third element from 15 to 25.

3. Step 3: Print updated element

   System.out.println(arr[2]); prints the updated value 25.

4. Final Answer:

   25 -> Option A

5. Quick Check:

   arr[2] updated to 25, so output is 25 [OK]

Hint: Remember updates change the element before printing [OK]

Common Mistakes:

• Printing old value before update
• Confusing index 2 with index 3
• Assuming array is unchanged

4. Identify the error in the following code snippet:

int[] numbers = {1, 2, 3};
numbers[3] = 4;

medium

A. ArrayIndexOutOfBoundsException at runtime

B. Syntax error due to missing semicolon

C. Compilation error: invalid array declaration

D. No error, code runs fine

Solution

1. Step 1: Check array size and valid indices

   The array numbers has length 3, valid indices are 0, 1, 2.

2. Step 2: Analyze index used for update

   Attempting numbers[3] = 4; accesses index 3, which is out of bounds.

3. Step 3: Identify error type

   This causes a runtime ArrayIndexOutOfBoundsException.

4. Final Answer:

   ArrayIndexOutOfBoundsException at runtime -> Option A

5. Quick Check:

   Index 3 invalid for length 3 array [OK]

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

Common Mistakes:

• Assuming index 3 is valid for length 3 array
• Confusing compile-time and runtime errors
• Ignoring zero-based indexing

5. Given the array int[] data = {2, 4, 6, 8, 10};, which code snippet correctly doubles the value at index 3 and then prints the entire array?

hard

A. data[4] = data[4] * 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

B. data[3] *= 2;
for(int i = 1; i <= data.length; i++) {
System.out.print(data[i] + " ");
}

C. data[3] = data[3] + 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

D. data[3] = data[3] * 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

Solution

1. Step 1: Identify correct index and operation

   Index 3 corresponds to the fourth element (value 8). Doubling means multiplying by 2.

2. Step 2: Check loop correctness for printing

   The loop must run from 0 to data.length - 1 to print all elements without error.

3. Step 3: Verify each option

   data[3] = data[3] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } correctly doubles data[3] and loops from 0 to length-1. data[3] *= 2; for(int i = 1; i <= data.length; i++) { System.out.print(data[i] + " "); } uses invalid loop bounds and will cause runtime error. data[3] = data[3] + 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } adds 2 instead of doubling. data[4] = data[4] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } doubles wrong index (4).

4. Final Answer:

   data[3] = data[3] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } -> Option D

5. Quick Check:

   Correct index and loop bounds ensure proper update and print [OK]

Hint: Use index 3 for 4th element; loop from 0 to length-1 [OK]

Common Mistakes:

• Using wrong index for update
• Incorrect loop bounds causing errors
• Adding instead of multiplying