Count Occurrences of Element in Sorted Array in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the first occurrence of the target in the sorted array.

DSA C

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

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' causes infinite loop or wrong index.

Not updating high pointer after finding target.

When the target is found, we move the high pointer to mid - 1 to continue searching the left half for the first occurrence.

2fill in blank

medium

Complete the code to find the last occurrence of the target in the sorted array.

DSA C

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

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' causes infinite loop or wrong index.

Not updating low pointer after finding target.

When the target is found, move the low pointer to mid + 1 to continue searching the right half for the last occurrence.

3fill in blank

hard

Fix the error in the countOccurrences function to correctly calculate the count of target in the array.

DSA C

int countOccurrences(int arr[], int n, int target) {
    int first = firstOccurrence(arr, n, target);
    if (first == -1) return 0;
    int last = lastOccurrence(arr, n, target);
    return last [1] first + 1;
}

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

A/

B+

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' causes wrong count.

Forgetting to add 1 after subtraction.

The count is last index minus first index plus one, so we subtract first from last and add 1.

4fill in blank

hard

Fill both blanks to complete the binary search condition and update pointers correctly.

DSA C

while (low [1] high) {
    int mid = low + (high - low) / 2;
    if (arr[mid] == target) {
        result = mid;
        high = mid [2] 1;
    } else if (arr[mid] < target) {
        low = mid + 1;
    } else {
        high = mid - 1;
    }
}

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

A<=

B-

C<

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '<=' in loop condition causes missing last element.

Using '+' instead of '-' when updating high pointer.

The loop continues while low is less than or equal to high, and when target is found, high moves to mid - 1 to search left.

5fill in blank

hard

Fill both blanks to create a dictionary-like structure in C that maps each unique element to its count.

DSA C

typedef struct {
    int key;
    int value;
} Pair;

void countElements(int arr[], int n, Pair result[], int *size) {
    *size = 0;
    for (int i = 0; i < n; i++) {
        int found = 0;
        for (int j = 0; j < *size; j++) {
            if (result[j].key == arr[i]) {
                result[j].value [1] 1;
                found = 1;
                break;
            }
        }
        if (!found) {
            result[*size].key = arr[i];
            result[*size].value = 1;
            (*size)[2];
        }
    }
}

int main() {
    int arr[] = {1, 2, 2, 3, 3, 3};
    int n = sizeof(arr) / sizeof(arr[0]);
    Pair result[10];
    int size = 0;
    countElements(arr, n, result, &size);
    for (int i = 0; i < size; i++) {
        printf("%d -> %d\n", result[i].key, result[i].value);
    }
    return 0;
}

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

A++

B+=

C--

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--' or '-=' decreases counts or size incorrectly.

Using '+' alone without '=' does not update the variable.

To increment the count, use '+='; to increase size, use '++'. The first blank increments value by 1, the second increments size by 1.

Practice

1. What is the main advantage of using a binary search approach to count occurrences of an element in a sorted array?

easy

A. It uses less memory than linear search

B. It sorts the array automatically

C. It reduces the search time from linear to logarithmic

D. It works only on unsorted arrays

Solution

1. Step 1: Understand binary search efficiency

   Binary search divides the search space in half each time, reducing time complexity to O(log n).

2. Step 2: Compare with linear search

   Linear search checks each element one by one, taking O(n) time, which is slower for large arrays.

3. Final Answer:

   It reduces the search time from linear to logarithmic -> Option C

4. Quick Check:

   Binary search = faster search [OK]

Hint: Binary search halves search space each step [OK]

Common Mistakes:

• Thinking binary search sorts the array
• Confusing memory usage with speed
• Assuming binary search works on unsorted arrays

2. Which of the following C code snippets correctly declares a function to count occurrences of an integer in a sorted array?

easy

A. void countOccurrences(int arr[], int n, int x);

B. int countOccurrences(int arr[], int n, int x);

C. int countOccurrences(int arr, int n, int x);

D. int countOccurrences(int arr[], int x);

Solution

1. Step 1: Check function return type and parameters

   The function should return an int (count), take array, size, and target element as parameters.

2. Step 2: Validate parameter types

   Array parameter must be int arr[] or int *arr, size as int n, and target as int x.

3. Final Answer:

   int countOccurrences(int arr[], int n, int x); -> Option B

4. Quick Check:

   Correct function signature = int countOccurrences(int arr[], int n, int x); [OK]

Hint: Function returns count as int, array passed as int[] [OK]

Common Mistakes:

• Using void return type instead of int
• Passing array as int instead of int[]
• Missing array size parameter

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

int arr[] = {1, 2, 2, 2, 3, 4, 5};
int n = 7;
int x = 2;
int count = 0;
for (int i = 0; i < n; i++) {
    if (arr[i] == x) count++;
}
printf("%d", count);

medium

A. 1

B. 2

C. 4

D. 3

Solution

1. Step 1: Iterate through the array and count matches

   Elements equal to 2 are at indices 1, 2, and 3, so count increments 3 times.

2. Step 2: Print the count

   The final count is 3, so printf outputs 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Count of 2 in array = 3 [OK]

Hint: Count elements equal to x by scanning array once [OK]

Common Mistakes:

• Counting only first occurrence
• Off-by-one errors in loop
• Printing count before loop ends

4. Identify the error in the following C function intended to count occurrences of x in a sorted array using binary search:

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

medium

A. Incrementing mid inside the loop does not change low or high

B. The loop condition should be low < high

C. count should be initialized to -1

D. The function should return void

Solution

1. Step 1: Analyze loop variables and updates

   Incrementing mid does not affect low or high, so loop may become infinite or incorrect.

2. Step 2: Understand binary search update rules

   To move search window, low or high must be updated, not mid.

3. Final Answer:

   Incrementing mid inside the loop does not change low or high -> Option A

4. Quick Check:

   mid++ does not move search window [OK]

Hint: Update low/high, not mid, to move binary search window [OK]

Common Mistakes:

• Changing mid instead of low/high
• Wrong loop condition causing infinite loop
• Incorrect return type

5. Given a sorted array arr = {1, 2, 2, 2, 3, 3, 4, 5, 5, 5, 5}, write a function that uses binary search to count occurrences of x = 5. What is the count returned?

hard

A. 4

B. 6

C. 5

D. 3

Solution

1. Step 1: Use binary search to find first occurrence of 5

   First 5 is at index 7.

2. Step 2: Use binary search to find last occurrence of 5

   Last 5 is at index 10.

3. Step 3: Calculate count as last - first + 1

   Count = 10 - 7 + 1 = 4.

4. Final Answer:

   4 -> Option A

5. Quick Check:

   Count of 5 in array = 4 [OK]

Hint: Count = last index - first index + 1 using binary search [OK]

Common Mistakes:

• Counting total elements instead of occurrences
• Off-by-one errors in count calculation
• Using linear search instead of binary search