Floor and Ceil in Sorted Array in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the floor of a target in a sorted array.

DSA C

int floorInSortedArray(int arr[], int n, int target) {
    int low = 0, high = n - 1, floor = -1;
    while (low <= high) {
        int mid = low + (high - low) / 2;
        if (arr[mid] == target) {
            return mid;
        } else if (arr[mid] < target) {
            floor = mid;
            low = [1];
        } else {
            high = mid - 1;
        }
    }
    return floor;
}

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

Alow + 1

Bmid - 1

Cmid + 1

Dhigh - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting low to mid - 1 causes infinite loop or wrong search direction.

When arr[mid] is less than target, we move low to mid + 1 to search the right half.

2fill in blank

medium

Complete the code to find the ceil of a target in a sorted array.

DSA C

int ceilInSortedArray(int arr[], int n, int target) {
    int low = 0, high = n - 1, ceil = -1;
    while (low <= high) {
        int mid = low + (high - low) / 2;
        if (arr[mid] == target) {
            return mid;
        } else if (arr[mid] > target) {
            ceil = mid;
            high = [1];
        } else {
            low = mid + 1;
        }
    }
    return ceil;
}

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

Amid + 1

Blow - 1

Chigh + 1

Dmid - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting high to mid + 1 causes wrong search direction.

When arr[mid] is greater than target, we move high to mid - 1 to search the left half.

3fill in blank

hard

Fix the error in the binary search loop condition to avoid infinite loops.

DSA C

while ([1]) {
    int mid = low + (high - low) / 2;
    // rest of code
}

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

Alow < high

Blow <= high

Clow == high

Dlow > high

Attempts:

3 left

💡 Hint

Common Mistakes

Using low < high misses checking when low equals high.

The loop should run while low is less than or equal to high to cover all elements.

4fill in blank

hard

Fill both blanks to create a dictionary of word lengths for words longer than 3 characters.

DSA C

int lengths[] = {0};
for (int i = 0; i < n; i++) {
    if ([1] > 3) {
        lengths[i] = [2];
    }
}

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

Astrlen(words[i])

Bwords[i]

Ci

Dstrlen(words)

Attempts:

3 left

💡 Hint

Common Mistakes

Using words[i] directly instead of its length.

Using strlen(words) which is incorrect.

We check the length of each word and assign its length to lengths array.

5fill in blank

hard

Fill all three blanks to create a map of keys to values where values are positive.

DSA C

typedef struct {
    char key[20];
    int value;
} Pair;

Pair result[100];
int count = 0;
for (int i = 0; i < n; i++) {
    if (data[i].[1] > 0) {
        strcpy(result[count].[2], data[i].[3]);
        result[count].value = data[i].value;
        count++;
    }
}

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

Avalue

Bkey

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up key and value fields.

Not copying the key string properly.

Check if data[i].value is positive, then copy data[i].key to result[count].key.

Practice

1. What is the floor of a number in a sorted array?

easy

A. The smallest element strictly less than the number

B. The smallest element greater than or equal to the number

C. The average of all elements less than the number

D. The greatest element less than or equal to the number

Solution

1. Step 1: Understand floor definition in sorted array

   The floor of a number is the greatest element in the array that is less than or equal to the given number.

2. Step 2: Compare options with definition

   The greatest element less than or equal to the number matches the definition exactly, others describe ceil or incorrect concepts.

3. Final Answer:

   The greatest element less than or equal to the number -> Option D

4. Quick Check:

   Floor = greatest ≤ number [OK]

Hint: Floor is always ≤ number, pick greatest such element [OK]

Common Mistakes:

• Confusing floor with ceil
• Thinking floor is strictly less than number
• Mixing floor with average or median

2. Which of the following is the correct C syntax to find the middle index in binary search for array of size n?

easy

A. int mid = (0 + n) / 2;

B. int mid = 0 + n - 1 / 2;

C. int mid = (0 + (n - 1)) >> 1;

D. int mid = n / 2;

Solution

1. Step 1: Understand middle index calculation in binary search

   For array indices 0 to n-1, middle is (0 + n-1) / 2 or equivalent.

2. Step 2: Analyze options for correct syntax and efficiency

   int mid = (0 + (n - 1)) >> 1; uses bitwise right shift for division by 2, which is efficient and correct for integer mid calculation.

3. Final Answer:

   int mid = (0 + (n - 1)) >> 1; -> Option C

4. Quick Check:

   Mid = (start + end) >> 1 [OK]

Hint: Use (start + end) >> 1 for efficient mid calculation [OK]

Common Mistakes:

• Using n instead of n-1 as end index
• Forgetting parentheses causing wrong precedence
• Using floating division instead of integer

3. Given the sorted array arr = {1, 3, 5, 7, 9}, what is the floor of 6?

medium

A. 7

B. 5

C. 6

D. -1

Solution

1. Step 1: Identify elements less than or equal to 6

   Elements ≤ 6 are 1, 3, 5.

2. Step 2: Find greatest among these

   The greatest element ≤ 6 is 5.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Floor(6) = 5 [OK]

Hint: Floor is largest ≤ target in sorted array [OK]

Common Mistakes:

• Choosing 7 which is greater than 6
• Assuming floor equals the number if not present
• Returning -1 incorrectly when floor exists

4. Consider this code snippet to find ceil in a sorted array int ceil = -1; for(int i=0; i= x) { ceil = arr[i]; break; } }. What is the main issue?

medium

A. It may return wrong value if array is not sorted

B. It does not handle empty array

C. It returns first element ≥ x, which is correct for ceil

D. It finds floor instead of ceil

Solution

1. Step 1: Understand the code logic

   The code returns the first element in array ≥ x, which is correct if array is sorted.

2. Step 2: Identify assumptions and issues

   If array is not sorted, first element ≥ x may not be the smallest such element, so ceil is incorrect.

3. Final Answer:

   It may return wrong value if array is not sorted -> Option A

4. Quick Check:

   Ceil requires sorted array for correctness [OK]

Hint: Ceil logic needs sorted array to work correctly [OK]

Common Mistakes:

• Ignoring array sorting requirement
• Confusing ceil with floor
• Not checking for empty array before loop

5. You have a sorted array arr = {2, 4, 6, 8, 10}. If you want to find the ceil of 11 using binary search, what should be the returned value?

hard

A. -1

B. 11

C. 10

D. Error

Solution

1. Step 1: Understand ceil definition and array elements

   Ceil is smallest element ≥ target. Here, target is 11, which is greater than all elements.

2. Step 2: Check if any element ≥ 11 exists

   No element in array ≥ 11, so ceil does not exist, return -1.

3. Final Answer:

   -1 -> Option A

4. Quick Check:

   No element ≥ 11 means ceil = -1 [OK]

Hint: If target > max element, ceil is -1 (not found) [OK]

Common Mistakes:

• Returning last element as ceil incorrectly
• Returning target itself if not found
• Not handling no ceil case properly