Arrays

Memory, indexing, and bounds

• Elements of a 1D array sit in contiguous memory; a[i] is roughly *(a + i).
• A 2D array a[R][C] is stored row-major: row 0’s columns, then row 1’s columns, and so on.
• Going outside 0 .. size-1 is **undefined behaviour** — no automatic bounds check on C arrays.
• Prefer std::array / std::vector and .at(i) when you want bounds-checked access.
• When passing a C array to a function it **decays to a pointer** — pass the size separately.

void print1D(const int a[], int n) {
    for (int i = 0; i < n; i++) cout << a[i] << " ";
}

// For 2D, columns size must be known in the parameter type:
void print2D(int a[][4], int rows) {
    for (int r = 0; r < rows; r++) {
        for (int c = 0; c < 4; c++) cout << a[r][c] << " ";
        cout << "\n";
    }
}

Common 1D operations

• Sum / average, find max/min, search a value, reverse, count even/odd.
• **Real-life:** highest temperature in a week; total monthly expense; find a roll number in a list.

int a[5] = {80, 70, 90, 60, 85};
int maxv = a[0];
for (int i = 1; i < 5; i++)
    if (a[i] > maxv) maxv = a[i];
cout << "Highest marks = " << maxv << "\n";

Common 2D operations

• Row sum (per student total), column sum (per subject total), transpose, diagonal of a square matrix.
• **Real-life:** total sales per product (row), total sales per month (column).
• Count free seats in a cinema grid; find first booked seat with nested loops + flag.

int sales[2][3] = {{10, 20, 15}, {5, 8, 12}};
// Column totals = sales per month
for (int m = 0; m < 3; m++) {
    int col = 0;
    for (int p = 0; p < 2; p++) col += sales[p][m];
    cout << "Month " << (m + 1) << " total = " << col << "\n";
}

1D — print and sum marks

#include <iostream>
using namespace std;

int main() {
    int marks[5] = {80, 70, 90, 60, 85};
    int sum = 0;
    for (int i = 0; i < 5; i++) {
        cout << marks[i] << " ";
        sum += marks[i];
    }
    cout << "\nSum = " << sum << "\n";
    return 0;
}

1D — weekly temperature average

#include <iostream>
using namespace std;

int main() {
    int temp[7] = {32, 31, 33, 30, 29, 34, 32};
    int sum = 0, maxv = temp[0];
    for (int i = 0; i < 7; i++) {
        sum += temp[i];
        if (temp[i] > maxv) maxv = temp[i];
    }
    cout << "Avg = " << (sum / 7.0) << "\n";
    cout << "Hottest = " << maxv << "\n";
    return 0;
}

1D — search roll number

#include <iostream>
using namespace std;

int main() {
    int rolls[6] = {101, 104, 108, 110, 115, 120};
    int key = 110, found = -1;
    for (int i = 0; i < 6; i++) {
        if (rolls[i] == key) { found = i; break; }
    }
    if (found >= 0) cout << "Found at index " << found << "\n";
    else cout << "Not found\n";
    return 0;
}

2D — cinema free/booked seats

#include <iostream>
using namespace std;

int main() {
    int seats[3][4] = {
        {1, 1, 0, 1},
        {1, 0, 0, 1},
        {1, 1, 1, 0}
    };
    int freeCount = 0;
    for (int r = 0; r < 3; r++) {
        for (int c = 0; c < 4; c++) {
            cout << (seats[r][c] ? "F " : "B ");
            if (seats[r][c]) freeCount++;
        }
        cout << "\n";
    }
    cout << "Free seats = " << freeCount << "\n";
    return 0;
}

2D — student × subject totals

#include <iostream>
using namespace std;

int main() {
    int marks[3][4] = {
        {80, 70, 90, 60},
        {75, 85, 80, 70},
        {90, 88, 92, 85}
    };
    for (int s = 0; s < 3; s++) {
        int total = 0;
        for (int sub = 0; sub < 4; sub++) total += marks[s][sub];
        cout << "Student " << (s + 1) << " total = " << total
             << " avg = " << (total / 4.0) << "\n";
    }
    return 0;
}

2D — matrix row and column sums

#include <iostream>
using namespace std;

int main() {
    int a[2][3] = {{1, 2, 3}, {4, 5, 6}};
    for (int r = 0; r < 2; r++) {
        int rs = 0;
        for (int c = 0; c < 3; c++) rs += a[r][c];
        cout << "Row " << r << " sum = " << rs << "\n";
    }
    for (int c = 0; c < 3; c++) {
        int cs = 0;
        for (int r = 0; r < 2; r++) cs += a[r][c];
        cout << "Col " << c << " sum = " << cs << "\n";
    }
    return 0;
}

std::array — playlist durations

#include <array>
#include <iostream>
using namespace std;

int main() {
    array<int, 4> sec = {210, 185, 240, 195};
    int total = 0;
    for (int t : sec) total += t;
    cout << "Playlist length = " << total << " seconds\n";
    return 0;
}

Initialization styles demo

#include <iostream>
using namespace std;

int main() {
    int full[4] = {1, 2, 3, 4};
    int partial[4] = {1, 2}; // -> 1 2 0 0
    int zeros[4] = {};
    int m[2][3] = {{10, 20}, {30}}; // rest 0
    for (int i = 0; i < 4; i++) cout << partial[i] << " ";
    cout << "\n";
    for (int r = 0; r < 2; r++) {
        for (int c = 0; c < 3; c++) cout << m[r][c] << " ";
        cout << "\n";
    }
    return 0;
}

Memory layout — print addresses (1D & 2D)

#include <iostream>
using namespace std;

int main() {
    int a[4] = {11, 22, 33, 44};
    cout << "1D contiguous addresses:\n";
    for (int i = 0; i < 4; i++)
        cout << "&a[" << i << "] = " << &a[i] << "\n";

    int m[2][3] = {{1, 2, 3}, {4, 5, 6}};
    cout << "2D row-major addresses:\n";
    for (int r = 0; r < 2; r++)
        for (int c = 0; c < 3; c++)
            cout << "&m[" << r << "][" << c << "] = " << &m[r][c] << "\n";

    cout << "sizeof(a)=" << sizeof(a)
         << " sizeof(m)=" << sizeof(m) << "\n";
    return 0;
}