Array Access and Update at Index in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the element at index 2 in the array.

DSA C

int arr[] = {10, 20, 30, 40, 50};
int value = arr[[1]];
printf("%d\n", value);

Drag options to blanks, or click blank then click option'

A2

B3

C1

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 or 4 which accesses wrong elements.

Forgetting that array indexes start at 0.

The index 2 accesses the third element in the array, which is 30.

2fill in blank

medium

Complete the code to update the element at index 1 to 100.

DSA C

int arr[] = {5, 10, 15, 20};
arr[[1]] = 100;
printf("%d\n", arr[1]);

Drag options to blanks, or click blank then click option'

A2

B0

C3

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Updating the wrong index like 0 or 2.

Confusing index with the value.

Index 1 corresponds to the second element, which we update to 100.

3fill in blank

hard

Fix the error in the code to correctly update the last element of the array.

DSA C

int arr[] = {1, 2, 3, 4, 5};
int n = 5;
arr[[1]] = 50;
printf("%d\n", arr[4]);

Drag options to blanks, or click blank then click option'

A5

Bn - 1

C4

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using n as index which is out of bounds.

Using 5 as index which is invalid.

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

4fill in blank

hard

Fill both blanks to print the updated value at index 3 after setting it to 99.

DSA C

int arr[] = {7, 8, 9, 10, 11};
arr[[1]] = 99;
printf("%d\n", arr[[2]]);

Drag options to blanks, or click blank then click option'

A3

B4

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using different indexes for update and print.

Using out of range indexes.

Index 3 is updated to 99 and then printed from the same index.

5fill in blank

hard

Fill all three blanks to update the element at index 0 to 42, then print it.

DSA C

int arr[] = {0, 1, 2, 3};
arr[[1]] = [2];
printf("%d\n", arr[[3]]);

Drag options to blanks, or click blank then click option'

A0

B42

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong index for update or print.

Using wrong value for update.

We update index 0 to 42 and then print the value at index 0.

Practice

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

easy

A. Accesses the element at index 0 of the array

B. Accesses the element at index 2 of the array

C. Updates the element at index 2 to zero

D. Deletes the element at index 2

Solution

1. Step 1: Understand array indexing

   Arrays use zero-based indexing, so index 2 means the third element.

2. Step 2: Access element at index 2

   The expression arr[2] retrieves the value stored at the third position in the array.

3. Final Answer:

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

4. Quick Check:

   arr[2] means third element access [OK]

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

Common Mistakes:

• Thinking index starts at 1 instead of 0
• Confusing access with update or delete
• Assuming arr[2] changes the array

2. Which of the following is the correct syntax to update the 4th element of an integer array arr to 10 in C?

easy

A. arr[3] = 10;

B. arr(4) = 10;

C. arr[4] == 10;

D. arr{3} = 10;

Solution

1. Step 1: Identify correct array indexing syntax

   In C, arrays use square brackets with zero-based indexing, so the 4th element is at index 3.

2. Step 2: Use assignment operator correctly

   To update, use single equals =, so arr[3] = 10; is correct.

3. Final Answer:

   arr[3] = 10; -> Option A

4. Quick Check:

   Use arr[index] = value; with zero-based index [OK]

Hint: Use square brackets and single = for update, zero-based index [OK]

Common Mistakes:

• Using parentheses or braces instead of brackets
• Using double equals (==) instead of assignment (=)
• Using wrong index number for 4th element

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

int arr[5] = {1, 2, 3, 4, 5};
arr[1] = arr[3] + 2;
printf("%d", arr[1]);

medium

A. 6

B. 4

C. 5

D. 7

Solution

1. Step 1: Evaluate the expression arr[3] + 2

   arr[3] is the 4th element, which is 4. Adding 2 gives 6.

2. Step 2: Assign and print arr[1]

   arr[1] is updated to 6, then printed, so output is 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   arr[3]=4 + 2 = 6 printed [OK]

Hint: Remember arr[3] is 4, add 2, assign to arr[1], print arr[1] [OK]

Common Mistakes:

• Confusing index 1 with index 3 values
• Adding incorrectly or forgetting zero-based index
• Printing wrong array element

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

int arr[3] = {5, 10, 15};
arr[3] = 20;
printf("%d", arr[3]);

medium

A. Syntax error in array declaration

B. Missing semicolon after assignment

C. Array index out of bounds error

D. Incorrect printf format specifier

Solution

1. Step 1: Check array size and valid indices

   Array size is 3, valid indices are 0, 1, 2. Index 3 is out of bounds.

2. Step 2: Understand consequences of out of bounds access

   Accessing or assigning arr[3] causes undefined behavior or runtime error.

3. Final Answer:

   Array index out of bounds error -> Option C

4. Quick Check:

   Index 3 invalid for size 3 array [OK]

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

Common Mistakes:

• Using index equal to array size
• Ignoring zero-based indexing
• Assuming no error for out of bounds

5. Given an integer array arr of size 6 initialized as {2, 4, 6, 8, 10, 12}, which code snippet correctly doubles the value at index 4 and then prints the entire array?

hard

A. arr[4] = arr[4] + 2;
for(int i=0; i<5; i++) printf("%d ", arr[i]);

B. arr[5] = arr[4] * 2;
for(int i=0; i<=6; i++) printf("%d ", arr[i]);

C. arr[4] *= 2;
for(int i=1; i<6; i++) printf("%d ", arr[i]);

D. arr[4] = arr[4] * 2;
for(int i=0; i<6; i++) printf("%d ", arr[i]);

Solution

1. Step 1: Correctly update value at index 4

   Index 4 is the 5th element. Doubling means multiply by 2. Both arr[4] = arr[4] * 2; and arr[4] *= 2; work, but check loop next.

2. Step 2: Print entire array correctly

   Array size is 6, so loop must run from 0 to 5 (i<6). arr[4] = arr[4] * 2; for(int i=0; i<6; i++) printf("%d ", arr[i]); uses correct loop range and prints all elements.

3. Final Answer:

   arr[4] = arr[4] * 2; for(int i=0; i<6; i++) printf("%d ", arr[i]); -> Option D

4. Quick Check:

   Update index 4, loop 0 to 5 to print all [OK]

Hint: Use zero-based index and loop from 0 to size-1 [OK]

Common Mistakes:

• Using wrong index for update
• Looping beyond array size causing errors
• Printing partial array missing elements