Common array operations - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an integer array of size 5.

C

int arr[[1]];

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

A5

B10

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero as the size, which is invalid.

Forgetting to specify the size.

The code declares an integer array named arr with 5 elements.

2fill in blank

medium

Complete the code to access the third element of the array.

C

int value = arr[[1]];

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

A1

B0

C3

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 instead of 2 for the third element index.

Using 1 which is the second element.

Array indexes start at 0, so the third element is at index 2.

3fill in blank

hard

Fix the error in the loop to print all elements of the array of size 4.

C

for(int i = 0; i < [1]; i++) {
    printf("%d\n", arr[i]);
}

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

A4

B3

C5

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Using 5 causes out-of-bounds access.

Using 3 misses the last element.

The loop should run while i is less than the array size, which is 4.

4fill in blank

hard

Fill both blanks to create a loop that sums all elements in an array named nums of size 6.

C

int sum = 0;
for(int [1] = 0; [1] < [2]; [1]++) {
    sum += nums[[1]];
}

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

Ai

B6

Cn

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not declared in the loop.

Using a wrong loop limit causing out-of-bounds access.

Variable i is commonly used as a loop counter. The loop runs while i < 6 to cover all elements.

5fill in blank

hard

Fill both blanks to declare an array, initialize it, and print its first element.

C

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

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

Aint

Carr[0]

Dfloat

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types for declaration and initialization.

Trying to print the whole array instead of one element.

The array is declared as int arr[3], initialized with values, and the first element arr[0] is printed.

Practice

1. What does the following C code do?
int arr[5] = {1, 2, 3, 4, 5};

easy

A. Creates an array of 5 integers with values 1 to 5

B. Creates a pointer to an integer

C. Declares a single integer variable

D. Creates a 2D array of integers

Solution

1. Step 1: Understand the declaration

   The code declares an integer array named arr with size 5.

2. Step 2: Check the initialization

   The array is initialized with values 1, 2, 3, 4, and 5 in order.

3. Final Answer:

   Creates an array of 5 integers with values 1 to 5 -> Option A

4. Quick Check:

   Array declaration with initialization = Creates an array of 5 integers with values 1 to 5 [OK]

Hint: Array size and values match initialization count [OK]

Common Mistakes:

• Confusing array with pointer declaration
• Thinking it's a single variable, not an array
• Assuming it's a 2D array

2. Which of the following is the correct syntax to access the third element of an array named arr?

easy

A. arr(3)

B. arr[2]

C. arr{3}

D. arr[3]

Solution

1. Step 1: Recall array indexing in C

   Arrays in C use zero-based indexing, so the first element is arr[0].

2. Step 2: Identify the third element index

   The third element is at index 2, so arr[2] accesses it.

3. Final Answer:

   arr[2] -> Option B

4. Quick Check:

   Third element index = 2 [OK]

Hint: Remember C arrays start at index 0 [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing (arr[3])
• Using curly braces for access

3. What is the output of this C code?

int arr[4] = {10, 20, 30, 40};
int sum = 0;
for(int i = 0; i < 4; i++) {
    sum += arr[i];
}
printf("%d", sum);

medium

A. 100

B. 90

C. Syntax error

D. 40

Solution

1. Step 1: Sum all elements in the array

   The loop adds arr[0]=10, arr[1]=20, arr[2]=30, arr[3]=40 to sum.

2. Step 2: Calculate the total sum

   10 + 20 + 30 + 40 = 100.

3. Final Answer:

   100 -> Option A

4. Quick Check:

   Sum of array elements = 100 [OK]

Hint: Add all elements using loop index 0 to size-1 [OK]

Common Mistakes:

• Off-by-one error in loop causing wrong sum
• Confusing sum with last element value
• Syntax errors from missing semicolons

4. Identify the error in this code snippet:

int arr[3] = {1, 2, 3};
for(int i = 0; i <= 3; i++) {
    printf("%d ", arr[i]);
}

medium

A. Array size should be 4

B. printf format specifier is wrong

C. Loop condition should be i < 3, not i <= 3

D. Variable i should be declared outside the loop

Solution

1. Step 1: Analyze loop condition

   The loop runs from i=0 to i=3 inclusive, so it accesses arr[3].

2. Step 2: Check array bounds

   Array arr has size 3, valid indices are 0,1,2. Accessing arr[3] causes out-of-bounds error.

3. Final Answer:

   Loop condition should be i < 3, not i <= 3 -> Option C

4. Quick Check:

   Array index must be less than size [OK]

Hint: Loop index must be less than array size [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming array size auto-expands
• Misunderstanding printf format specifier

5. You want to find the largest number in an integer array arr of size n. Which code snippet correctly does this?

hard

A. int max = arr[n]; for(int i = 0; i < n; i++) { if(arr[i] > max) max = arr[i]; }

B. int max = 0; for(int i = 0; i <= n; i++) { if(arr[i] > max) max = arr[i]; }

C. int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] < max) max = arr[i]; }

D. int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] > max) max = arr[i]; }

Solution

1. Step 1: Initialize max with first element

   Set max = arr[0] to start comparison from first element.

2. Step 2: Loop from second element to end

   Loop i from 1 to n-1, compare arr[i] with max, update max if arr[i] is larger.

3. Step 3: Check other options for errors

   int max = 0; for(int i = 0; i <= n; i++) { if(arr[i] > max) max = arr[i]; } uses max=0 which fails if all numbers are negative; loop condition i <= n causes out-of-bounds; int max = arr[n]; for(int i = 0; i < n; i++) { if(arr[i] > max) max = arr[i]; } initializes max with arr[n] which is invalid index; int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] < max) max = arr[i]; } finds minimum instead of maximum.

4. Final Answer:

   int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] > max) max = arr[i]; } -> Option D

5. Quick Check:

   Initialize max with first element and compare correctly [OK]

Hint: Start max with arr[0], loop i from 1 to n-1 [OK]

Common Mistakes:

• Using wrong loop bounds causing out-of-bounds access
• Initializing max with 0 ignoring negative values
• Confusing max with min logic