Array Declaration and Initialization in DSA C++ - Build from Scratch

Array Declaration and Initialization

📖 Scenario: You are helping a small store keep track of the number of items sold each day in a week.

🎯 Goal: Create an array to store daily sales numbers, set a target sales number, calculate how many days met or exceeded the target, and print the result.

📋 What You'll Learn

Declare an integer array named dailySales with exactly 7 elements

Initialize dailySales with these exact values: 12, 15, 9, 20, 18, 7, 14

Create an integer variable target and set it to 15

Use a for loop with variable i to count how many days had sales >= target

Store the count in an integer variable count

Print the value of count

💡 Why This Matters

🌍 Real World

Stores and businesses often track daily sales or other data using arrays to analyze performance.

💼 Career

Understanding arrays and loops is fundamental for programming tasks like data processing, reporting, and automation.

1

Declare and initialize the sales array

Declare an integer array called dailySales with 7 elements and initialize it with these exact values: 12, 15, 9, 20, 18, 7, 14

DSA C++

// Declare and initialize the dailySales array
// Your code here

Hint

Use the syntax int arrayName[size] = {values}; to declare and initialize an array.

2

Set the target sales number

Create an integer variable called target and set it to 15

DSA C++

int dailySales[7] = {12, 15, 9, 20, 18, 7, 14};
// Create the target variable and set it to 15
// Your code here

Hint

Use int target = 15; to create and set the variable.

3

Count days meeting or exceeding the target

Create an integer variable called count and set it to 0. Then use a for loop with variable i from 0 to 6 to check each element in dailySales. Inside the loop, if dailySales[i] is greater than or equal to target, increase count by 1.

DSA C++

int dailySales[7] = {12, 15, 9, 20, 18, 7, 14};
int target = 15;
// Initialize count to 0
// Use a for loop with i from 0 to 6
// Inside loop, if dailySales[i] >= target, increment count
// Your code here

Hint

Use a for loop to go through each day and an if statement to check the sales.

4

Print the count of days meeting the target

Print the value of the variable count using std::cout

DSA C++

int dailySales[7] = {12, 15, 9, 20, 18, 7, 14};
int target = 15;
int count = 0;
for (int i = 0; i = target) {
        count++;
    }
}
// Print the count variable
// Your code here

Hint

Use std::cout << count << std::endl; to print the count.

Practice

1. Which of the following is the correct way to declare an integer array of size 5 in C++?

easy

A. int arr = new int[5];

B. array<int> arr(5);

C. int arr[5];

D. int arr();

Solution

1. Step 1: Understand basic array declaration syntax

   In C++, to declare a fixed-size array, we use the syntax type name[size];.

2. Step 2: Check each option

   int arr[5]; uses int arr[5]; which is the correct syntax for declaring an integer array of size 5. int arr = new int[5]; uses dynamic allocation syntax which is valid but not a declaration. array<int> arr(5); uses a non-standard syntax. int arr(); declares a function, not an array.

3. Final Answer:

   int arr[5]; -> Option C

4. Quick Check:

   Array declaration = int arr[5]; [OK]

Hint: Use type name[size]; for fixed-size arrays [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Confusing dynamic allocation with declaration
• Using non-standard syntax

2. Which of the following is the correct syntax to initialize an array of 3 integers with values 1, 2, and 3 in C++?

easy

A. int arr[3] = {1, 2, 3};

B. int arr = {1, 2, 3};

C. int arr[3] = (1, 2, 3);

D. int arr[3] = [1, 2, 3];

Solution

1. Step 1: Recall array initialization syntax

   In C++, arrays are initialized using curly braces with values separated by commas, like int arr[size] = {values};.

2. Step 2: Evaluate each option

   int arr[3] = {1, 2, 3}; correctly uses curly braces with values. int arr = {1, 2, 3}; misses the size in declaration. int arr[3] = (1, 2, 3); uses parentheses which is invalid. int arr[3] = [1, 2, 3]; uses square brackets which is incorrect for initialization.

3. Final Answer:

   int arr[3] = {1, 2, 3}; -> Option A

4. Quick Check:

   Array init uses curly braces = int arr[3] = {1, 2, 3}; [OK]

Hint: Initialize arrays with curly braces and commas [OK]

Common Mistakes:

• Omitting array size in declaration
• Using parentheses or square brackets for initialization
• Missing commas between values

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

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

medium

A. 10 20 0 20

B. 10 20 0 0

C. 10 20 10 20

D. 10 20 garbage garbage

Solution

1. Step 1: Understand partial array initialization

   When an array is partially initialized, the remaining elements are set to zero automatically in C++.

2. Step 2: Trace the loop output

   The first two elements are 10 and 20 as given. The last two elements are 0 by default. So the output prints: 10 20 0 0

3. Final Answer:

   10 20 0 0 -> Option B

4. Quick Check:

   Partial init fills rest with 0 = 10 20 0 0 [OK]

Hint: Uninitialized array elements default to 0 if partially initialized [OK]

Common Mistakes:

• Assuming uninitialized elements have garbage values
• Confusing partial initialization with full initialization
• Misreading loop bounds

4. Identify the error in the following C++ code snippet:

int arr[];
arr = {1, 2, 3};

medium

A. Array size must be specified during declaration

B. Array elements must be initialized with parentheses

C. Missing semicolon after array declaration

D. Cannot assign values to array after declaration like this

Solution

1. Step 1: Check array declaration rules

   In C++, arrays must have size specified or be initialized at declaration. Here, int arr[]; is invalid because size or initializer is missing.

2. Step 2: Understand assignment to arrays

   Arrays cannot be assigned values after declaration using = { ... }. This syntax is invalid. Initialization must happen at declaration.

3. Final Answer:

   Cannot assign values to array after declaration like this -> Option D

4. Quick Check:

   Arrays can't be assigned with braces after declaration [OK]

Hint: Initialize arrays only at declaration, no later assignment [OK]

Common Mistakes:

• Trying to assign array values after declaration
• Omitting array size without initializer
• Confusing array assignment with pointer assignment

5. You want to declare and initialize a 2D integer array with 2 rows and 3 columns, where all elements are zero. Which is the best way to do this in C++?

hard

A. int arr[2][3] = {0};

B. int arr[2][3] = {{0,0,0},{0,0,0}};

C. int arr[2][3]; memset(arr, 0, sizeof(arr));

D. int arr[2][3] = {0, 0, 0, 0, 0, 0};

Solution

1. Step 1: Understand zero initialization of multi-dimensional arrays

   In C++, initializing the first element to zero with {0} sets all elements to zero by default.

2. Step 2: Compare options for simplicity and correctness

   int arr[2][3] = {0}; uses {0} which zero-initializes all elements. int arr[2][3] = {{0,0,0},{0,0,0}}; explicitly lists zeros but is longer. int arr[2][3]; memset(arr, 0, sizeof(arr)); uses memset which works but is less clean. int arr[2][3] = {0, 0, 0, 0, 0, 0}; lists all zeros but is verbose.

3. Final Answer:

   int arr[2][3] = {0}; -> Option A

4. Quick Check:

   Use {0} to zero initialize entire array [OK]

Hint: Use {0} to zero initialize all elements in arrays [OK]

Common Mistakes:

• Not knowing {0} zero initializes entire array
• Using memset without including <cstring>
• Writing verbose initializers unnecessarily