Array Access and Update at Index in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to access an element at a specific index in an array?

Accessing an element at a specific index means retrieving the value stored at that position in the array. Arrays use zero-based indexing, so the first element is at index 0.

beginner

How do you update the value of an element at a given index in an array?

To update an element, assign a new value to the array at that index using the syntax: array[index] = newValue; This replaces the old value with the new one.

intermediate

What happens if you try to access or update an index outside the array bounds?

Accessing or updating an index outside the array bounds causes an error called ArrayIndexOutOfBoundsException in Java. It means the index is invalid because it is less than 0 or greater than or equal to the array length.

intermediate

Why do arrays use zero-based indexing?

Arrays use zero-based indexing because the first element is stored at the starting memory address of the array. Index 0 points directly to this address, making calculations simpler and faster.

beginner

Example: Given int[] arr = {10, 20, 30, 40}; what is the value of arr[2] and how to change it to 35?

arr[2] is 30 because indexing starts at 0 (arr[0]=10, arr[1]=20, arr[2]=30). To change it to 35, write: arr[2] = 35; After update, arr = {10, 20, 35, 40}.

What is the index of the first element in a Java array?

A -1

B 1

C Depends on array size

D 0

Java arrays use zero-based indexing, so the first element is at index 0.

How do you update the third element of an array named 'numbers' to 50?

A numbers[2] = 50;

B numbers[3] = 50;

C numbers(2) = 50;

D numbers.update(3, 50);

The third element is at index 2 because indexing starts at 0.

What error occurs if you try to access an index outside the array bounds in Java?

A ArrayIndexOutOfBoundsException

B NullPointerException

C ClassCastException

D ArithmeticException

Accessing invalid indices causes ArrayIndexOutOfBoundsException.

If int[] arr = {5, 10, 15}, what is arr[1]?

A 5

B 10

C 15

D Error

arr[1] is the second element, which is 10.

Which of the following is the correct way to access the last element of an array 'arr'?

A arr[0]

B arr[arr.length]

C arr[arr.length - 1]

D arr.length

The last element is at index arr.length - 1 because indexing starts at 0.

Explain how to access and update an element at a specific index in a Java array.

Think about how you get and set values in an array using the index.

What happens if you try to access or update an index outside the bounds of an array? How can you avoid this?

Consider what Java does when you use an invalid 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