Common array operations in C++ - Step-by-Step Execution

Concept Flow - Common array operations

Start

↓

Initialize array

↓

Access elements by index

↓

Modify elements

↓

Traverse array with loop

↓

Search for element

↓

Insert or update elements

↓

End

This flow shows how we start with an array, access and change elements, loop through them, search, and update values.

Execution Sample

C++

#include <iostream>
using namespace std;

int main() {
  int arr[5] = {10, 20, 30, 40, 50};
  arr[2] = 35;
  for(int i = 0; i < 5; i++) {
    cout << arr[i] << " ";
  }
  return 0;
}

This code creates an array, changes one element, then prints all elements.

Execution Table

Step	Operation	Index/Variable	Value/Action	Array State	Output
1	Initialize array	-	{10, 20, 30, 40, 50}	{10, 20, 30, 40, 50}
2	Modify element	arr[2]	35	{10, 20, 35, 40, 50}
3	Loop start	i=0	-	{10, 20, 35, 40, 50}
4	Print element	arr[0]	10	{10, 20, 35, 40, 50}	10
5	Increment i	i=1	-	{10, 20, 35, 40, 50}	10
6	Print element	arr[1]	20	{10, 20, 35, 40, 50}	10 20
7	Increment i	i=2	-	{10, 20, 35, 40, 50}	10 20
8	Print element	arr[2]	35	{10, 20, 35, 40, 50}	10 20 35
9	Increment i	i=3	-	{10, 20, 35, 40, 50}	10 20 35
10	Print element	arr[3]	40	{10, 20, 35, 40, 50}	10 20 35 40
11	Increment i	i=4	-	{10, 20, 35, 40, 50}	10 20 35 40
12	Print element	arr[4]	50	{10, 20, 35, 40, 50}	10 20 35 40 50
13	Increment i	i=5	-	{10, 20, 35, 40, 50}	10 20 35 40 50
14	Check loop condition	i < 5	False	{10, 20, 35, 40, 50}	10 20 35 40 50

💡 Loop ends when i reaches 5, condition i < 5 is False

Variable Tracker

Variable	Start	After Step 2	After Step 5	After Step 7	After Step 9	After Step 11	Final
arr	{10,20,30,40,50}	{10,20,35,40,50}	{10,20,35,40,50}	{10,20,35,40,50}	{10,20,35,40,50}	{10,20,35,40,50}	{10,20,35,40,50}
i	-	-	1	2	3	4	5

Key Moments - 3 Insights

Why does arr[2] change from 30 to 35?

Because at step 2 we assign arr[2] = 35, which updates the third element in the array as shown in the execution_table row 2.

Why does the loop stop after i reaches 5?

At step 14, the condition i < 5 becomes False, so the loop ends. This is shown clearly in the execution_table row 14.

Why do we start counting i from 0 in the loop?

Arrays in C++ start at index 0, so to access all elements from first to last, we loop i from 0 up to 4 (less than 5). This is shown in the loop steps 3 to 13.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of arr[2] after step 2?

A 30

B 20

C 35

D 40

💡 Hint

Check the 'Modify element' operation at step 2 in the execution_table.

At which step does the loop condition become false?

A Step 14

B Step 13

C Step 12

D Step 11

💡 Hint

Look at the 'Check loop condition' row in the execution_table.

If we changed the loop to i < 3, how many elements would be printed?

A 2

B 3

C 5

D 4

💡 Hint

Refer to the variable_tracker for i values and loop condition in execution_table.

Concept Snapshot

Common array operations in C++:
- Declare: int arr[5] = {10,20,30,40,50};
- Access: arr[index], index starts at 0
- Modify: arr[2] = 35;
- Loop: for(int i=0; i<5; i++) to traverse
- Arrays have fixed size
- Index out of bounds causes errors

Full Transcript

This visual trace shows common array operations in C++. We start by creating an array with 5 elements. Then we change the third element from 30 to 35. Next, we use a for loop to print all elements from index 0 to 4. The loop stops when the index reaches 5 because the condition i < 5 becomes false. Variables like the array and loop index change step by step, which is tracked in the tables. This helps beginners see exactly how arrays work in C++.

Practice

1. What does the following C++ code do?
int arr[5] = {1, 2, 3, 4, 5};

easy

A. Creates an array of 5 integers with values 1 to 5

B. Creates a vector of 5 integers with values 1 to 5

C. Creates a pointer to an integer array

D. Declares an uninitialized array of size 5

Solution

1. Step 1: Understand array declaration

   The code declares an integer array named arr with size 5.

2. Step 2: Check initialization

   The array is initialized with values 1, 2, 3, 4, 5 in order.

3. Final Answer:

   Creates an array of 5 integers with values 1 to 5 -> Option A

4. Quick Check:

   Array declaration with initialization = Creates array with values [OK]

Hint: Array size and values in braces means initialized array [OK]

Common Mistakes:

• Confusing array with vector
• Thinking it's a pointer declaration
• Assuming array is uninitialized

2. Which of the following is the correct syntax to access the third element of an array named arr in C++?

easy

A. arr[3];

B. arr(3);

C. arr{3};

D. arr[2];

Solution

1. Step 1: Recall zero-based indexing

   Arrays in C++ start at index 0, so the third element is at index 2.

2. Step 2: Identify correct syntax for access

   Array elements are accessed using square brackets, so arr[2] is correct.

3. Final Answer:

   arr[2]; -> Option D

4. Quick Check:

   Third element index = 2, use square brackets [OK]

Hint: Remember arrays start at 0, so third element is index 2 [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing
• Using curly braces for access

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

int arr[4] = {10, 20, 30, 40};
int sum = 0;
for(int i = 0; i < 4; i++) {
    sum += arr[i];
}
std::cout << sum;

medium

A. 100

B. 90

C. 40

D. 10

Solution

1. Step 1: Identify array elements and loop

   The array has elements 10, 20, 30, 40 and the loop runs from i=0 to 3.

2. Step 2: Calculate sum of elements

   sum = 10 + 20 + 30 + 40 = 100

3. Final Answer:

   100 -> Option A

4. Quick Check:

   Sum of all elements = 100 [OK]

Hint: Add all elements in loop to get sum [OK]

Common Mistakes:

• Stopping loop early
• Using wrong indices
• Printing sum before loop

4. Find the error in this C++ code that tries to print all elements of an array:

int arr[3] = {5, 10, 15};
for(int i = 1; i <= 3; i++) {
    std::cout << arr[i] << " ";
}

medium

A. Loop index starts at 1 instead of 0

B. Array size is incorrect

C. Using <= causes out-of-bounds access

D. Missing semicolon after cout statement

Solution

1. Step 1: Analyze loop indices

   The loop runs i from 1 to 3 inclusive, so i=3 is included.

2. Step 2: Check array bounds

   Array indices are 0,1,2. Accessing arr[3] is out-of-bounds and causes error.

3. Final Answer:

   Using <= causes out-of-bounds access -> Option C

4. Quick Check:

   Loop must use i < size, not <= [OK]

Hint: Use i < size, not i <= size in loops [OK]

Common Mistakes:

• Starting loop at 1 instead of 0 (not error but unusual)
• Ignoring out-of-bounds access
• Assuming array size is wrong

5. You have an array int arr[6] = {2, 4, 6, 8, 10, 12};. How do you update the element with value 8 to 9 using a loop?

hard

A. arr[3] = 9;

B. for(int i=0; i<6; i++) { if(arr[i] == 8) arr[i] = 9; }

C. for(int i=1; i<=6; i++) { if(arr[i] == 8) arr[i] = 9; }

D. arr[4] = 9;

Solution

1. Step 1: Understand the problem

   We want to find the element with value 8 and update it to 9 using a loop.

2. Step 2: Check loop and condition

   Loop from i=0 to 5 checks each element; if arr[i] == 8, assign arr[i] = 9.

3. Final Answer:

   for(int i=0; i<6; i++) { if(arr[i] == 8) arr[i] = 9; } -> Option B

4. Quick Check:

   Loop with condition to find and update element [OK]

Hint: Loop through array, check value, then update [OK]

Common Mistakes:

• Using wrong indices in loop
• Assigning wrong array index
• Not using loop to find value