Section 8: Arrays

Basic Array Declaration & Initialization

Arrays are collections of items stored at contiguous memory locations.

// 1. Standard Declaration
int A[5] = {1, 2, 3, 4, 5};

// 2. Float and Char Arrays
float B[5] = {1.2f, 2.2f, 3.3f, 4.4f};
char C[5] = {'A', 66, 'C', 68}; // 66 is ASCII for 'B', 68 for 'D'

// 3. Partial Initialization
// Remaining elements are automatically set to 0
int d[5] = {2, 4}; // Result: [2, 4, 0, 0, 0]

// 4. Automatic Size Deduction
int e[] = {1, 2, 3, 4}; // Compiler sets size = 4 automatically

// 5. Invalid Declaration
// int a[]; // Error: Size must be known or initialized

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Iterating Arrays

1. Traditional For Loop

for (int i = 0; i < 5; i++) {
    cout << A[i] << endl;
}

2. Range-based For Loop (For Each)

Syntax introduced in C++11. Reads as "for every element x in collection A".

for (int x : A) {
    cout << x << endl; 
}

3. Using References (&)

By default, the range-based loop creates a copy of the element. Use & to modify the original array.

for (int &x : A) {
    ++x; // This modifies the actual values inside array A
}

Note on auto: * We can use auto instead of int if we don't want to specify the type manually. * Example: for (auto x : A) * Note: auto works on collections (vectors, lists, arrays) but does not work on raw pointers in this context.

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Nested Loops

Used to iterate over dimensions or create patterns.

for(int i = 1; i <= 5; i++)
{
    for(int j = 1; j <= 5; j++)
    {
         cout << "(" << i << ", " << j << ") ";
    }
    cout << endl;
}

Output:

(1, 1) (1, 2) (1, 3) (1, 4) (1, 5)
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5)
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5)
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5)
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5)

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Multidimensional Arrays (2D Arrays)

Conceptual View vs. Reality

Visually, we think of a 2D array as a grid (Rows x Columns). Declaration: int A[3][4]; (3 Rows, 4 Columns)

Row/Col	0	1	2	3
0	[0][0]	[0][1]	[0][2]	[0][3]
1	[1][0]	15	...	...
2	...	...	...	...

Reality (Memory): In memory, it is a Single Dimension Array (Row-Major Order). All rows are stored sequentially. [Row 0] [Row 1] [Row 2]

Initialization

// Nested braces (Readable)
int A[2][3] = {
    {2, 5, 9}, 
    {6, 9, 15}
};

// Flat list (Works same as above)
int B[2][3] = {2, 4, 6, 3, 5, 7}; 

Iterating 2D Arrays

1. Using Nested For Loops

for(int i = 0; i < 2; i++) // Iterate Rows
{
    for(int j = 0; j < 3; j++) // Iterate Columns
    {
         cout << A[i][j] << " ";
    }
    cout << endl;
}

2. Using For-Each (Range-Based) Crucial Note: When iterating a 2D array with for-each, you must use a reference (&) for the outer loop. * Why? A 2D array is an "array of arrays". Without &, the inner array decays into a pointer, and you cannot run a range-based loop on a pointer.

// Correct way to iterate 2D array
for (auto &row : A) {      // Access each row by reference
    for (auto col : row) { // Access each element in that row
        cout << col << " ";
    }
    cout << endl;
}