C++ Omit Array Size

📦 C++ Omit Array Size

In C++, you can omit (skip) the array size only when the array is initialized at the time of declaration.
The compiler automatically determines the size based on the number of elements.

🔹 1. Basic Example (Size Omitted)

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

✔ Compiler sets the size to 4

Equivalent to:

int arr[4] = {10, 20, 30, 40};

🔹 2. Accessing Elements

int arr[] = {5, 15, 25};

cout << arr[0]; // 5
cout << arr[2]; // 25

🔹 3. Loop Through an Array with Omitted Size

int arr[] = {2, 4, 6, 8};

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

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

🔹 4. Character Array (String) Example

char name[] = "Cplusplus";

✔ Size automatically includes the null character '\0'.

🔹 5. Omit Size in 2D Arrays (Important Rule)

✔ Allowed only for the first dimension

int matrix[][3] = {
{1, 2, 3},
{4, 5, 6}
};

❌ Not allowed (column size missing):

int matrix[][] = { {1,2}, {3,4} }; // error

🔹 6. When You CANNOT Omit Array Size ❌

❌ Without Initialization

int arr[]; // error

❌ When Size Is Needed Later

int n = 5;
int arr[] = n; // error

🔹 7. Use Case: Cleaner Code

int primes[] = {2, 3, 5, 7, 11};

✔ Cleaner
✔ Less error-prone
✔ Easy to maintain

🔁 Omitted Size vs Explicit Size

Omitted Size Explicit Size
Compiler decides size Programmer decides size
Must initialize Can initialize later
Cleaner syntax More control

📌 Summary

  • Array size can be omitted only with initialization

  • Compiler counts elements automatically

  • In 2D arrays, omit only the first dimension

  • Not allowed without initialization

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