Return inside loops in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the first even number from the array.

C++

int firstEven(int arr[], int size) {
    for (int i = 0; i < size; i++) {
        if (arr[i] % 2 == 0) {
            return [1];
        }
    }
    return -1;
}

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

Aarr[i]

Bi

C0

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the index i instead of the element arr[i].

Returning size or 0 which are unrelated.

We return the actual even number found, which is arr[i].

2fill in blank

medium

Complete the code to return true if any number in the array is negative.

C++

bool hasNegative(int arr[], int size) {
    for (int i = 0; i < size; i++) {
        if (arr[i] < 0) {
            return [1];
        }
    }
    return false;
}

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

Afalse

Btrue

Carr[i]

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning false inside the loop when a negative is found.

Returning the element arr[i] instead of a boolean.

Return true immediately when a negative number is found.

3fill in blank

hard

Fix the error in the code to return the index of the first zero in the array, or -1 if none.

C++

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

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

A-1

Barr[i]

C0

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Returning arr[i] which is zero instead of the index.

Returning 0 or -1 incorrectly inside the loop.

We return the index i where zero is found, not the element arr[i].

4fill in blank

hard

Fill both blanks to return the sum of positive numbers in the array, stopping early if a zero is found.

C++

int sumPositives(int arr[], int size) {
    int sum = 0;
    for (int i = 0; i < size; i++) {
        if (arr[i] == 0) {
            return [1];
        }
        if (arr[i] > 0) {
            sum [2] arr[i];
        }
    }
    return sum;
}

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

Asum

B+=

C-=

Darr[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Returning arr[i] instead of sum.

Using -= instead of += to add values.

Return the current sum when zero is found, and add positive numbers to sum using +=.

5fill in blank

hard

Complete the code to return the first character in the string that is a vowel, or '\0' if none.

C++

char firstVowel(const char* str) {
    for (int i = 0; str[i] != '\0'; i++) {
        char c = str[i];
        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
            return [1];
        }
    }
    return '\0';
}

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

A[

B]

Cc

Dstr

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of brackets to access characters.

Returning str instead of the character c.

Use brackets [ ] to access characters by index and return the character c when it is a vowel.

Practice

1.

What happens when a return statement is executed inside a loop in a C++ function?

easy

A. The loop runs twice before the function stops.

B. Only the current loop iteration stops, and the loop continues.

C. The function immediately stops and returns the value.

D. The loop skips to the next iteration without stopping the function.

Solution

1. Step 1: Understand the role of return in functions

   The return statement immediately ends the function and sends back a value.

2. Step 2: Analyze return inside a loop

   When return is inside a loop, it stops the entire function, not just the loop.

3. Final Answer:

   The function immediately stops and returns the value. -> Option C

4. Quick Check:

   Return inside loop = function stops [OK]

Hint: Return inside loop ends function immediately [OK]

Common Mistakes:

• Thinking return only stops the loop
• Confusing return with continue or break
• Assuming code after return runs

2.

Which of the following is the correct syntax to return the first even number found in a vector nums using a loop?

for (int i = 0; i < nums.size(); i++) {
    if (nums[i] % 2 == 0) {
        // return here
    }
}

easy

A. return nums[i];

B. return i;

C. return nums;

D. return i + 1;

Solution

1. Step 1: Identify what to return

   The question asks to return the first even number, so we return the value at index i.

2. Step 2: Match the correct return statement

   return nums[i]; returns the actual even number found.

3. Final Answer:

   return nums[i]; -> Option A

4. Quick Check:

   Return value, not index [OK]

Hint: Return the value, not the index, when asked for element [OK]

Common Mistakes:

• Returning the index instead of the value
• Returning the whole vector
• Returning incorrect arithmetic on index

3.

What is the output of the following C++ function when called with findFirstPositive({-3, -1, 0, 4, 5})?

int findFirstPositive(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] > 0) {
            return nums[i];
        }
    }
    return -1;
}

medium

A. -1

B. 4

C. 0

D. 5

Solution

1. Step 1: Trace the loop to find first positive

   The loop checks each element: -3 (no), -1 (no), 0 (no), 4 (yes, positive).

2. Step 2: Return the first positive number found

   At 4, the function returns immediately with 4.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   First positive = 4 [OK]

Hint: Return stops at first positive number found [OK]

Common Mistakes:

• Returning -1 instead of first positive
• Confusing 0 as positive
• Returning last positive instead of first

4.

Identify the error in this function that tries to return the first negative number from a vector nums:

int findNegative(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] < 0) {
            return nums[i];
        } else {
            return 0;
        }
    }
    return -1;
}

medium

A. The loop condition is incorrect.

B. The function never returns any value.

C. The return type should be void.

D. The function returns 0 too early inside the loop.

Solution

1. Step 1: Analyze the else return inside the loop

   The else block returns 0 immediately if the first element is not negative, stopping the function early.

2. Step 2: Understand why this is wrong

   This prevents checking all elements; function should only return 0 after full loop if no negative found.

3. Final Answer:

   The function returns 0 too early inside the loop. -> Option D

4. Quick Check:

   Return in else inside loop causes early stop [OK]

Hint: Avoid return in else inside loop; return after loop instead [OK]

Common Mistakes:

• Returning inside else causes premature exit
• Thinking loop runs fully before return
• Misunderstanding return type requirements

5.

Write a function that returns the index of the first zero in a vector nums. If no zero is found, return -1. Which of the following implementations is correct?

Option A:
int findZero(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            return i;
        }
    }
    return -1;
}

Option B:
int findZero(std::vector<int> nums) {
    int index = -1;
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            index = i;
        }
    }
    return index;
}

Option C:
int findZero(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            break;
        }
    }
    return i;
}

Option D:
int findZero(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            return nums[i];
        }
    }
    return -1;
}

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Verify the correct logic for finding first zero index

   The implementation that returns i immediately when nums[i] == 0 and -1 after the loop is correct.

2. Step 2: Why others fail

   The one setting a variable continues the loop, returning the last zero's index. The break version doesn't return -1 if no zero found and i goes out of scope. The last returns the value 0 instead of index.

3. Final Answer:

   Option A -> Option A

4. Quick Check:

   Return index immediately on first zero [OK]

Hint: Return index immediately when found, else after loop return -1 [OK]

Common Mistakes:

• Returning value instead of index
• Returning last zero index instead of first
• Using break without returning causes scope errors