First and Last Occurrence of Element in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C

int firstOccurrence(int arr[], int n, int target) {
    for (int i = 0; i < n; i++) {
        if (arr[i] == [1]) {
            return i;
        }
    }
    return -1;
}

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

Ai

Btarget

Cn

Darr[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using the loop index i instead of the target in the comparison.

Comparing arr[i] to arr[i] which is always true.

We compare each element arr[i] with the target to find the first occurrence.

2fill in blank

medium

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

DSA C

int lastOccurrence(int arr[], int n, int target) {
    for (int i = n - 1; i >= 0; i--) {
        if (arr[i] == [1]) {
            return i;
        }
    }
    return -1;
}

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

An

Bi

Ctarget

Darr[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using the loop index i instead of the target in the comparison.

Comparing arr[i] to arr[i] which is always true.

We compare each element arr[i] with the target starting from the end to find the last occurrence.

3fill in blank

hard

Fix the error in the code to correctly find the first occurrence of the target element.

DSA C

int firstOccurrence(int arr[], int n, int target) {
    int i = 0;
    while (i < n) {
        if (arr[i] == [1]) {
            return i;
        }
        i++;
    }
    return -1;
}

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

Aarr[i]

Bi

Cn

Dtarget

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '==' in the if condition.

Comparing with the wrong variable.

The error is using assignment '=' instead of comparison '=='. Replace with '==' and compare arr[i] with target.

4fill in blank

hard

Fill both blanks to create a function that returns the first and last occurrence indices of the target element.

DSA C

void firstAndLastOccurrence(int arr[], int n, int target, int *first, int *last) {
    *first = -1;
    *last = -1;
    for (int i = 0; i < n; i++) {
        if (arr[i] == [1]) {
            if (*first == -1) {
                *first = i;
            }
            *last = [2];
        }
    }
}

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

Atarget

Bi

Cn

D*first

Attempts:

3 left

💡 Hint

Common Mistakes

Updating *last with *first instead of i.

Comparing arr[i] with the wrong variable.

We compare arr[i] with target and update *last with the current index i.

5fill in blank

hard

Fill all three blanks to create a function that returns a struct with first and last occurrence of the target element.

DSA C

typedef struct {
    int first;
    int last;
} Occurrence;

Occurrence findOccurrences(int arr[], int n, int target) {
    Occurrence result = [1];
    result.first = -1;
    result.last = -1;
    for (int i = 0; i < n; i++) {
        if (arr[i] == [2]) {
            if (result.first == -1) {
                result.first = i;
            }
            result.last = [3];
        }
    }
    return result;
}

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

A{}

Btarget

Ci

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Not initializing the struct properly.

Updating result.last with the wrong value.

Comparing arr[i] with the wrong variable.

Initialize result with empty braces {}, compare arr[i] with target, and update result.last with i.

Practice

1. What does the term first occurrence of an element in an array mean?

easy

A. The last index where the element appears

B. The total count of the element in the array

C. The index of the first time the element appears in the array

D. The sum of all elements before the element

Solution

1. Step 1: Understand the meaning of first occurrence

   The first occurrence means the earliest position where the element is found in the array, starting from index 0.

2. Step 2: Compare options with definition

   The index of the first time the element appears in the array matches this definition exactly, while others describe different concepts.

3. Final Answer:

   The index of the first time the element appears in the array -> Option C

4. Quick Check:

   First occurrence = first index found [OK]

Hint: First occurrence means earliest index found [OK]

Common Mistakes:

• Confusing first occurrence with count of element
• Mixing first occurrence with last occurrence
• Thinking it means sum or total

2. Which of the following C code snippets correctly finds the first occurrence of x in array arr of size n?

easy

A. for(int i=0; i

B. for(int i=n-1; i>=0; i--) { if(arr[i] == x) return i; } return -1;

C. int i=0; while(arr[i] != x) i++; return i;

D. for(int i=0; i

Solution

1. Step 1: Analyze each code snippet

   for(int i=0; i

2. Step 2: Check other options for correctness

   for(int i=n-1; i>=0; i--) { if(arr[i] == x) return i; } return -1; searches from end, so it finds last occurrence. int i=0; while(arr[i] != x) i++; return i; may cause out-of-bound error if x not found. for(int i=0; i

3. Final Answer:

   for(int i=0; i -> Option A

4. Quick Check:

   Start to end loop with match check = for(int i=0; i

Hint: Loop from start, return index on first match [OK]

Common Mistakes:

• Looping backward for first occurrence
• Not handling element not found case
• Returning wrong index when element differs

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

int arr[] = {2, 4, 6, 4, 8};
int n = 5, x = 4;
int first = -1, last = -1;
for(int i=0; i

medium

A. 1 3

B. 3 1

C. 0 4

D. -1 -1

Solution

1. Step 1: Trace the loop to find first and last occurrence

   Array: [2,4,6,4,8], x=4. Loop i=0 to 4:
   i=0: arr[0]=2 !=4, skip
   i=1: arr[1]=4 ==4, first=-1 so first=1, last=1
   i=2: arr[2]=6 !=4, skip
   i=3: arr[3]=4 ==4, first=1 (unchanged), last=3
   i=4: arr[4]=8 !=4, skip

2. Step 2: Print first and last

   first=1, last=3

3. Final Answer:

   1 3 -> Option A

4. Quick Check:

   First=1, Last=3 indexes for 4 [OK]

Hint: Track first when unset, update last on every match [OK]

Common Mistakes:

• Swapping first and last values
• Not updating last occurrence
• Ignoring first occurrence initialization

4. Identify the error in the following code that tries to find the last occurrence of x in array arr of size n:

int lastOccurrence(int arr[], int n, int x) {
  int last = -1;
  for(int i=0; i

medium

A. Loop should run from n-1 to 0 for last occurrence

B. Using assignment (=) instead of comparison (==) in if condition

C. Variable last should be initialized to 0

D. Return statement should be inside the loop

Solution

1. Step 1: Check the if condition inside the loop

   The condition uses single '=' which is assignment, not comparison. This causes wrong behavior.

2. Step 2: Correct the condition

   It should be 'if(arr[i] == x)' to compare values properly.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in if condition -> Option B

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Use '==' for comparison, '=' is assignment [OK]

Common Mistakes:

• Using '=' instead of '==' in conditions
• Loop direction confusion for last occurrence
• Wrong initialization of variables

5. Given a sorted array arr of size n with possible duplicates, which approach is the most efficient to find the first and last occurrence of element x?

hard

A. Sort the array again and then find occurrences

B. Traverse the array from start to end once to find both occurrences

C. Use linear search twice: once from start, once from end

D. Use two binary searches: one for first occurrence, one for last occurrence

Solution

1. Step 1: Understand the array is sorted

   Since the array is sorted, binary search can be used to find positions efficiently.

2. Step 2: Use binary search twice

   One binary search finds the first occurrence by moving left on matches, another finds last occurrence by moving right on matches.

3. Final Answer:

   Use two binary searches: one for first occurrence, one for last occurrence -> Option D

4. Quick Check:

   Sorted array + binary search = fastest method [OK]

Hint: Use binary search twice on sorted array [OK]

Common Mistakes:

• Using linear search on sorted array (slow)
• Sorting array again unnecessarily
• Not using binary search for efficiency