Array traversal in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does array traversal mean in programming?

Array traversal means going through each element of an array one by one to read or process them.

beginner

How do you write a basic for loop to traverse an array in C++?

You use a for loop with an index starting at 0 up to the array size minus one, accessing each element by its index.

beginner

Why is it important to know the size of the array when traversing it?

Knowing the size prevents accessing elements outside the array, which can cause errors or crashes.

beginner

What is the output of this code?

int arr[] = {1, 2, 3};
for(int i = 0; i < 3; i++) {
  std::cout << arr[i] << " ";
}

The output is: 1 2 3
It prints each element of the array separated by spaces.

intermediate

Can you traverse an array backwards? How?

Yes, by starting the loop index at the last element (size - 1) and decreasing it until 0.

What is the starting index for traversing a C++ array?

A Array size

B 1

C -1

D 0

C++ arrays start at index 0, so traversal begins from 0.

Which loop is commonly used for array traversal in C++?

A for loop

B while loop

C do-while loop

D if statement

The for loop is most common because it easily controls the index.

What happens if you access an array element outside its bounds?

A It prints zero

B It automatically resizes the array

C Program may crash or show garbage values

D Nothing happens

Accessing outside bounds leads to undefined behavior, often crashes or wrong data.

How do you know how many times to loop when traversing an array?

A Use the array size

B Loop forever

C Use random number

D Loop once

Looping exactly the array size times ensures all elements are visited.

Which of these is a valid way to traverse an array backwards?

A for(int i = 1; i <= size; i++)

B for(int i = size - 1; i >= 0; i--)

C while(i < size)

D for(int i = 0; i < size; i++)

Starting from the last index and decreasing visits elements in reverse order.

Explain how to traverse an array in C++ using a for loop.

Think about how you count from the first to the last element.

Why must you be careful about array size when traversing?

Imagine walking on a path that ends at a certain point.

Practice

1. What does array traversal mean in C++ programming?

easy

A. Visiting each element of the array one by one

B. Creating a new array with the same elements

C. Sorting the array elements in order

D. Deleting elements from the array

Solution

1. Step 1: Understand the term 'array traversal'

   Array traversal means going through each element in the array in sequence.

2. Step 2: Match the definition to the options

   Visiting each element of the array one by one correctly describes visiting each element one by one.

3. Final Answer:

   Visiting each element of the array one by one -> Option A

4. Quick Check:

   Array traversal = visiting elements in order [OK]

Hint: Traversal means visiting all elements one by one [OK]

Common Mistakes:

• Confusing traversal with sorting
• Thinking traversal means deleting elements
• Mixing traversal with copying arrays

2. Which of the following is the correct syntax to traverse an array arr of size n in C++?

easy

A. for(int i = 1; i <= n; i++) { /* access arr[i] */ }

B. for(int i = n; i > 0; i--) { /* access arr[i] */ }

C. for(int i = 0; i <= n; i++) { /* access arr[i] */ }

D. for(int i = 0; i < n; i++) { /* access arr[i] */ }

Solution

1. Step 1: Identify valid index range for array traversal

   Array indices in C++ start at 0 and go up to n-1, so loop should run from 0 to less than n.

2. Step 2: Check each option's loop condition

   for(int i = 0; i < n; i++) { /* access arr[i] */ } uses i from 0 to i < n, which is correct. Others either start at 1 or go out of bounds.

3. Final Answer:

   for(int i = 0; i < n; i++) { /* access arr[i] */ } -> Option D

4. Quick Check:

   Loop from 0 to n-1 = correct traversal [OK]

Hint: Array indices start at 0, loop till less than size [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= n causing out-of-bounds access
• Decrementing loop without correct index

3. What is the output of the following C++ code?

int arr[] = {2, 4, 6};
for(int i = 0; i < 3; i++) {
    std::cout << arr[i] << " ";
}

medium

A. 0 2 4 6

B. 2 4 6 0

C. 2 4 6

D. 6 4 2

Solution

1. Step 1: Understand the loop and array elements

   The array has elements 2, 4, 6 at indices 0, 1, 2. The loop runs from i=0 to i=2.

2. Step 2: Print elements in order

   Each iteration prints arr[i] followed by a space: 2, then 4, then 6.

3. Final Answer:

   2 4 6 -> Option C

4. Quick Check:

   Print elements in order = 2 4 6 [OK]

Hint: Loop prints elements in order from index 0 to 2 [OK]

Common Mistakes:

• Expecting extra zero at end
• Printing elements in reverse order
• Confusing array indices with values

4. Identify the error in this C++ code snippet for array traversal:

int arr[4] = {1, 2, 3, 4};
for(int i = 0; i <= 4; i++) {
    std::cout << arr[i] << " ";
}

medium

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

B. Array size should be 5 instead of 4

C. Loop variable i should start from 1

D. No error, code is correct

Solution

1. Step 1: Check loop condition against array size

   The array has size 4, valid indices are 0 to 3. Loop runs while i <= 4, which includes i=4 (out of bounds).

2. Step 2: Correct the loop condition

   Change condition to i < 4 to avoid accessing arr[4] which is invalid.

3. Final Answer:

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

4. Quick Check:

   Loop must stay within 0 to size-1 [OK]

Hint: Loop index must be less than array size, not less or equal [OK]

Common Mistakes:

• Using <= instead of < causing out-of-bounds
• Starting loop at 1 instead of 0
• Increasing array size unnecessarily

5. Given an integer array arr of size n, which code snippet correctly calculates the sum of all elements using array traversal?

hard

A. int sum = arr[0];
for(int i = 1; i <= n; i++) {
sum += arr[i];
}

B. int sum = 0;
for(int i = 0; i < n; i++) {
sum += arr[i];
}

C. int sum = 0;
for(int i = 1; i < n; i++) {
sum += arr[i];
}

D. int sum = 0;
for(int i = 0; i <= n; i++) {
sum += arr[i];
}

Solution

1. Step 1: Initialize sum and loop correctly

   Sum should start at 0. Loop must run from i=0 to i < n to include all elements.

2. Step 2: Check each option for correctness

   int sum = 0; for(int i = 0; i < n; i++) { sum += arr[i]; } initializes sum to 0 and loops correctly. Others either start sum wrong or loop out of bounds.

3. Final Answer:

   int sum = 0; for(int i = 0; i < n; i++) { sum += arr[i]; } -> Option B

4. Quick Check:

   Sum all elements from index 0 to n-1 [OK]

Hint: Start sum at 0 and loop from 0 to less than n [OK]

Common Mistakes:

• Starting sum with arr[0] and looping incorrectly
• Looping with i <= n causing out-of-bounds
• Skipping first element by starting at i=1