Array size and bounds in C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an array of 5 integers.

C

int numbers[[1]];

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

A5

B10

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as size which creates an empty array.

Using a size larger than needed.

The array size must be 5 to hold exactly 5 integers.

2fill in blank

medium

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

C

int third = numbers[[1]];

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

A2

B3

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as index which is out of bounds for third element.

Confusing element number with index.

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

3fill in blank

hard

Fix the error in the code to avoid accessing out of bounds.

C

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

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

A0

B5

C10

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Using i <= 5 which tries to access numbers[5] out of bounds.

Using a larger number than array size.

The loop should run while i is less than 5 to stay within array bounds.

4fill in blank

hard

Fill both blanks to create an array of 4 integers and assign 10 to the last element.

C

int arr[[1]];
arr[[2]] = 10;

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

A4

B3

C5

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 4 which is out of bounds for size 4.

Using size 3 which is too small.

The array size is 4, so the last element index is 3 (since indexing starts at 0).

5fill in blank

hard

Fill all three blanks to declare an array of 6 integers, assign 20 to the first element, and print it.

C

int nums[[1]];
nums[[2]] = 20;
printf("%d\n", nums[[3]]);

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

A6

B0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 or 5 instead of 0 for first element.

Using wrong array size.

The array size is 6. The first element is at index 0, so assign and print nums[0].

Practice

1. What is the valid range of indexes for an array declared as int arr[10]; in C?

easy

A. 0 to 9

B. 1 to 10

C. 0 to 10

D. 1 to 9

Solution

1. Step 1: Understand array size declaration

   The array arr is declared with size 10, meaning it has 10 elements.

2. Step 2: Determine valid index range

   In C, array indexing starts at 0, so valid indexes are from 0 up to size-1, which is 9.

3. Final Answer:

   0 to 9 -> Option A

4. Quick Check:

   Array indexes = 0 to size-1 = 0 to 9 [OK]

Hint: Array indexes start at 0 and end at size minus one [OK]

Common Mistakes:

• Assuming array indexes start at 1
• Using size as the last index
• Confusing size with maximum index

2. Which of the following is the correct way to declare an array of 5 integers in C?

easy

A. int arr(5);

B. int arr[5];

C. int arr{5};

D. int arr = 5;

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays are declared using square brackets with the size inside, like int arr[5];.

2. Step 2: Check each option

   int arr[5]; uses correct syntax. Options A and C use parentheses or braces which are invalid for arrays. int arr = 5; assigns an int, not an array.

3. Final Answer:

   int arr[5]; -> Option B

4. Quick Check:

   Array declaration uses square brackets [OK]

Hint: Use square brackets [] to declare array size in C [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using braces instead of brackets
• Assigning a single value instead of declaring array

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

int arr[3] = {10, 20, 30};
printf("%d", arr[3]);

medium

A. Undefined behavior (may print garbage or crash)

B. 0

C. Compilation error

D. 30

Solution

1. Step 1: Analyze array size and indexing

   The array arr has size 3, valid indexes are 0, 1, 2. Accessing arr[3] is out of bounds.

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

   Accessing outside array bounds in C causes undefined behavior. It may print garbage value, cause crash, or unpredictable output.

3. Final Answer:

   Undefined behavior (may print garbage or crash) -> Option A

4. Quick Check:

   Out-of-bounds access = undefined behavior [OK]

Hint: Accessing index >= size causes undefined behavior [OK]

Common Mistakes:

• Assuming arr[3] prints last element
• Expecting zero instead of garbage
• Thinking compiler catches out-of-bounds

4. Identify the error in the following code snippet:

int arr[4];
for(int i = 0; i <= 4; i++) {
    arr[i] = i * 2;
}

medium

A. Initialization of i is incorrect

B. Array size should be 5 instead of 4

C. Loop condition should be i < 4, not i <= 4

D. Multiplication by 2 is invalid

Solution

1. Step 1: Check loop bounds against array size

   The array size is 4, so valid indexes are 0 to 3. The loop runs while i <= 4, which includes i=4, an invalid index.

2. Step 2: Correct the loop condition

   Changing the loop condition to i < 4 ensures indexes 0 to 3 are used, avoiding out-of-bounds access.

3. Final Answer:

   Loop condition should be i < 4, not i <= 4 -> Option C

4. Quick Check:

   Loop index must be less than array size [OK]

Hint: Use < not <= for loop index to avoid out-of-bounds [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming array size includes index equal to size
• Ignoring off-by-one errors

5. Given the array declaration int nums[5] = {1, 2, 3, 4, 5};, which code snippet correctly sums all elements without causing out-of-bounds errors?

hard

A. for(int i = 0; i < 6; i++) sum += nums[i];

B. for(int i = 1; i < 5; i++) sum += nums[i];

C. for(int i = 0; i <= 5; i++) sum += nums[i];

D. for(int i = 0; i < 5; i++) sum += nums[i];

Solution

1. Step 1: Understand array size and valid indexes

   The array nums has size 5, so valid indexes are 0 to 4.

2. Step 2: Evaluate each loop condition

   for(int i = 0; i <= 5; i++) sum += nums[i]; uses i <= 5, which accesses nums[5] (out-of-bounds). for(int i = 1; i < 5; i++) sum += nums[i]; starts at 1, missing nums[0]. for(int i = 0; i < 5; i++) sum += nums[i]; uses i < 5, correctly iterating 0 to 4. for(int i = 0; i < 6; i++) sum += nums[i]; uses i < 6, accessing nums[5] (out-of-bounds).

3. Final Answer:

   for(int i = 0; i < 5; i++) sum += nums[i]; -> Option D

4. Quick Check:

   Loop from 0 to size-1 to avoid errors [OK]

Hint: Loop from 0 to less than array size for safe access [OK]

Common Mistakes:

• Using <= size causing out-of-bounds
• Starting loop at 1 missing first element
• Looping beyond array size