C++ Array Size

📏 C++ Array Size

In C++, array size means the number of elements an array can store.
Array size is fixed and must be known at compile time (for static arrays).

🔹 1. Declaring Array with Size

int arr[5];

➡ This array can store 5 integers
➡ Index range: 0 to 4

🔹 2. Initialize Array with Size

int arr[5] = {10, 20, 30, 40, 50};

✔ Size = 5
✔ Fully initialized

🔹 3. Partial Initialization

int arr[5] = {1, 2};

➡ Remaining elements are automatically set to 0

Result:

1 2 0 0 0

🔹 4. Omit Array Size (Compiler Decides)

int arr[] = {5, 10, 15, 20};

✔ Compiler sets size = 4

🔹 5. Finding Array Size Using sizeof

Total number of elements

int arr[] = {10, 20, 30, 40};

int size = sizeof(arr) / sizeof(arr[0]);
cout << size;

Output:

4

🔹 6. Using Array Size in a Loop

int arr[] = {1, 2, 3, 4, 5};
int size = sizeof(arr) / sizeof(arr[0]);

for (int i = 0; i < size; i++) {
cout << arr[i] << " ";
}

🔹 7. Size of Character Array (String)

char name[] = "Hello";

✔ Size = 6
➡ Includes null character '\0'

cout << sizeof(name); // 6

🔹 8. 2D Array Size

int matrix[2][3];

➡ Rows = 2
➡ Columns = 3
➡ Total elements = 2 × 3 = 6

❌ Common Mistakes

int arr[0]; // ❌ invalid
int arr[-5]; // ❌ invalid
cout << arr[5]; // ❌ out of bounds

⚠️ Accessing outside the array leads to undefined behavior.

🔁 Array Size vs Vector Size

Array Vector
Fixed size Dynamic size
sizeof needed v.size()
Faster Safer & flexible

📌 Summary

  • Array size is fixed

  • Index starts from 0

  • Use sizeof(arr)/sizeof(arr[0]) to get length

  • Character arrays include '\0'

  • For dynamic size, use vector

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