C & C++ notes for beginners

#include <stdio.h>      /* input / output library */

int main(void) {          /* program starts here */
    printf("Hello, Study Hub!\n");
    printf("2 + 3 = %d\n", 2 + 3);
    return 0;             /* 0 means "finished successfully" */
}

#include <iostream>       // input / output streams

int main() {
    std::cout << "Hello, Study Hub!" << std::endl;
    std::cout << "2 + 3 = " << 2 + 3 << std::endl;
    return 0;
}

/* after the first few files you can save typing with:
   using namespace std;
   and then write  cout << "text";  directly.  */

# C
gcc hello.c -o hello
./hello                      # Windows: hello.exe

# C++
g++ hello.cpp -o hello
./hello

# turn on warnings - they find real bugs for free
g++ -Wall -Wextra hello.cpp -o hello

#include <iostream>
using namespace std;

int main() {
    int marks = 87;              // whole number
    double percent = 87.5;       // decimal
    char grade = 'A';            // ONE character, single quotes
    bool passed = true;          // C++ only
    auto guess = 42;             // C++ figures out the type
    const double GST = 0.18;     // cannot be changed later

    cout << "Marks: " << marks << endl;
    cout << "Percent: " << percent << endl;
    cout << "Grade: " << grade << endl;

    // integer division drops the fraction - a classic surprise
    cout << 7 / 2 << endl;                       // 3
    cout << 7 / 2.0 << endl;                     // 3.5
    cout << (double)7 / 2 << endl;                // 3.5 by casting

    // uninitialised variables hold garbage - always give a value
    int total = 0;               // never: int total;
    total = marks + 13;
    cout << "Total: " << total << endl;

    return 0;
}

#include <stdio.h>

int main(void) {
    int age;
    double marks;

    printf("Your age: ");
    scanf("%d", &age);        /* & = "the address of age" */

    printf("Your marks: ");
    scanf("%lf", &marks);     /* %lf for a double */

    printf("Next year: %d\n", age + 1);
    printf("Marks: %.2f\n", marks);   /* %.2f = 2 decimal places */

    /* %-10s pads to 10 characters, left aligned */
    printf("|%-10s|%6d|\n", "Maths", 78);

    return 0;
}

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

int main() {
    int age;
    string name;                     // C++ has easy text

    cout << "Your name: ";
    getline(cin, name);              // reads a whole line
    cout << "Your age: ";
    cin >> age;

    cout << "Hello " << name
         << ", next year you will be " << age + 1 << endl;

    return 0;
}

int marks = 78;
string grade;

if (marks >= 90) {
    grade = "A+";
} else if (marks >= 75) {
    grade = "A";
} else if (marks >= 33) {
    grade = "B";
} else {
    grade = "F";
}

// ternary
string label = (marks >= 33) ? "Pass" : "Fail";

// switch needs break, or it falls through
switch (marks / 10) {
    case 10:
    case 9:  grade = "A+"; break;
    case 8:  grade = "A";  break;
    default: grade = "B";
}

// for - the standard counting loop
for (int i = 1; i <= 5; i++) {
    cout << i << " ";
}

// while
int count = 1;
while (count <= 3) {
    cout << "Round " << count << endl;
    count++;                  // forget this = infinite loop
}

// do...while always runs at least once
int choice;
do {
    cout << "1) Notes  2) Exit" << endl;
    cin >> choice;
} while (choice != 2);

// break and continue
for (int n = 1; n <= 10; n++) {
    if (n == 8) break;         // stop the loop
    if (n % 2 == 0) continue;  // skip this round
    cout << n << " is odd" << endl;
}

// the modern C++ range loop
int marks[] = {78, 91, 66};
for (int m : marks) {
    cout << m << endl;
}

#include <iostream>
using namespace std;

// you must state the return type and every parameter type
int add(int a, int b) {
    return a + b;
}

double average(double a, double b) {
    return (a + b) / 2.0;
}

// default arguments must come last
double power(double base, int exponent = 2) {
    double result = 1;
    for (int i = 0; i < exponent; i++) result *= base;
    return result;
}

// void = returns nothing
void printLine(int width = 20) {
    for (int i = 0; i < width; i++) cout << "-";
    cout << endl;
}

// pass by reference: changes the caller's variable (C++)
void bump(int &value) {
    value++;
}

// prototype: declares a function you define further down
int gradePoints(int marks);

int main() {
    cout << add(3, 4) << endl;              // 7
    cout << average(80, 90) << endl;        // 85
    cout << power(2, 10) << endl;           // 1024
    cout << power(5) << endl;              // 25 (default exponent)

    printLine(15);

    int score = 10;
    bump(score);
    cout << score << endl;                  // 11

    cout << gradePoints(78) << endl;
    return 0;
}

int gradePoints(int marks) {
    if (marks >= 90) return 10;
    if (marks >= 75) return 9;
    return 8;
}

#include <stdio.h>

int main(void) {
    int marks[5] = {78, 91, 66, 88, 72};   /* fixed size, known at compile time */
    int count = 5;
    int total = 0;

    printf("First: %d  Last: %d\n", marks[0], marks[count - 1]);

    for (int i = 0; i < count; i++) {
        total += marks[i];
    }
    printf("Average: %.1f\n", (double)total / count);

    /* a 2D array: 2 students, 3 subjects each */
    int grid[2][3] = {{78, 91, 66}, {92, 88, 95}};
    for (int r = 0; r < 2; r++) {
        int sum = 0;
        for (int c = 0; c < 3; c++) sum += grid[r][c];
        printf("Student %d average: %.1f\n", r + 1, (double)sum / 3);
    }

    /* text is just an array of characters ending with '\0' */
    char name[] = "Ayush";
    printf("Hello, %s (%d letters)\n", name, (int)strlen(name));

    return 0;
}

#include <stdio.h>

/* a function that CAN change the caller's variables */
void swap(int *a, int *b) {
    int temp = *a;      /* * means "the value at this address" */
    *a = *b;
    *b = temp;
}

int main(void) {
    int marks = 87;

    int *p = &marks;     /* & means "the address of marks" */

    printf("Value: %d\n", marks);    /* 87      */
    printf("Value via pointer: %d\n", *p); /* 87  */
    printf("Address: %p\n", (void *)p);

    *p = 95;                          /* changes marks itself */
    printf("Now marks is %d\n", marks);    /* 95 */

    int x = 1, y = 2;
    swap(&x, &y);
    printf("x=%d y=%d\n", x, y);      /* x=2 y=1 */

    /* dynamic memory: ask for space while the program runs */
    int *nums = malloc(5 * sizeof(int));
    if (nums == NULL) return 1;       /* always check! */
    for (int i = 0; i < 5; i++) nums[i] = (i + 1) * 10;
    printf("%d\n", nums[4]);          /* 50 */
    free(nums);                       /* give it back - no malloc without free */
    nums = NULL;                      /* avoids a dangling pointer */

    return 0;
}

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>     // sort, reverse, max_element
using namespace std;

int main() {
    // ---- string: length, joining and searching are built in ----
    string name = "Ayush Danthta";
    cout << name.length() << endl;             // 13
    cout << name.substr(0, 5) << endl;         // Ayush
    cout << name.find("Danthta") << endl;      // 6  (npos if missing)
    name += " ji";                             // easy appending
    for (char c : name) { /* loop each character */ }

    // ---- vector: an array that grows by itself ----
    vector<int> marks = {78, 91, 66};
    marks.push_back(88);                       // add to the end
    cout << marks.size() << endl;              // 4
    cout << marks[0] << " " << marks.back() << endl;

    sort(marks.begin(), marks.end());          // lowest to highest
    reverse(marks.begin(), marks.end());       // highest to lowest

    int total = 0;
    for (int m : marks) total += m;            // range loop
    cout << "Average: " << (double)total / marks.size() << endl;

    // ---- a class: your own type with data and behaviour ----
    class Student {
        private:
            string name;
            vector<int> marks;

        public:
            Student(string n) : name(n) {}     // constructor

            void addMark(int m) {
                if (m >= 0 && m <= 100) marks.push_back(m);
            }

            double average() const {
                if (marks.empty()) return 0;
                int sum = 0;
                for (int m : marks) sum += m;
                return (double)sum / marks.size();
            }

            void report() const {
                cout << name << " -> " << average() << endl;
            }
    };

    Student s("Ayush");
    s.addMark(78);
    s.addMark(91);
    s.addMark(66);
    s.report();                                // Ayush -> 78.3333

    return 0;
}

#include <iostream>
#include <vector>
#include <string>
#include <iomanip>
using namespace std;

struct Student {
    string name;
    vector<int> marks;
};

double average(const vector<int> &nums) {
    if (nums.empty()) return 0;
    int sum = 0;
    for (int n : nums) sum += n;
    return (double)sum / nums.size();
}

char gradeOf(double avg) {
    if (avg >= 85) return 'A';
    if (avg >= 70) return 'B';
    if (avg >= 50) return 'C';
    return 'D';
}

int main() {
    vector<Student> students = {
        {"Ayush", {78, 91, 66}},
        {"Riya",  {92, 88, 95}},
        {"Karan", {45, 52, 60}},
    };

    cout << left << setw(8) << "Name"
         << setw(10) << "Average"
         << setw(8) << "Grade" << endl;

    cout << string(26, '-') << endl;

    double classTotal = 0;
    for (const Student &s : students) {
        double avg = average(s.marks);
        classTotal += avg;

        cout << left << setw(8) << s.name
             << setw(10) << fixed << setprecision(1) << avg
             << setw(8) << gradeOf(avg) << endl;
    }

    cout << string(26, '-') << endl;

    cout << "Class average: "
         << fixed << setprecision(1)
         << classTotal / students.size() << endl;

    return 0;
}