A short introduction to C++ - basic

• Part 0: Setting up C++

• Part 1: Hello world! Your first steps with C++

• Part 2: Files, arrays and functions

• Part 3: Pointers, arguments and classes

Part 0. Setting up C++

OS: Linux / Mac OS / WSL

We are going to have two windows open at the same time:

1. A text editor (vi, emacs, vscode etc).

2. The command line or terminal, where a C++ compiler has already been installed and is working.

1. Getting ready

1. Create a new directory called cpp:

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   mkdir cpp
   cd cpp
   

2. In this chapter, every example can be compiled with:

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   g++ -std=c++17 filename.cpp -o program_name
   

3. Then run with:

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   ./program_name
   

Part 1. Hello world!

2. Hello world!

For cin, cout, endl stuff, we need the <iostream> library. Libraries are like toolboxes.

• New file! helloworld.cpp

​x
#include <iostream>

using namespace std;

int main()
{
    cout << "Hello world!" << endl;
    return 0;
}

• Save as helloworld.cpp.

• Compile:

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  g++ -std=c++17 helloworld.cpp -o hello
  

• Run:

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  ./hello
  

return 0; in the main function means the program executed successfully.

3. Use of variables

• New file! variables.cpp

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#include <iostream>

using namespace std;

int main()
{
    double x = 3.141;
    double y = 1.618;
    auto j = 16;      // j is int
    auto k = 2.3;     // k is double
    auto good = true; // good is bool

    cout << "The value of x is " << x
         << " and the value of y is " << y << endl;
    cout << "j = " << j << ", k = " << k
         << ", good = " << good << endl;
    return 0;
}

• auto declaration type (since C++11): the type is deduced from the initializer.

• Compile and run:

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  g++ -std=c++17 variables.cpp -o variables
  ./variables
  

4. Command-line input

• New file! height.cpp

• We are going to use strings here, which need the <string> library.

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#include <iostream>
#include <string>

using namespace std;

int main()
{
    string name;
    double height;

    cout << "What is your name?" << endl;
    cin >> name;

    cout << "What is your height (in metres)?" << endl;
    cin >> height;

    cout << name << " is " << height << " metres tall." << endl;
    return 0;
}

• Compile and run:

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  g++ -std=c++17 height.cpp -o height
  ./height
  

5. Maths

• New file! calculator.cpp

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#include <cmath>
#include <iostream>

using namespace std;

int main()
{
    double x;

    cout << "Enter a number" << endl;
    cin >> x;

    cout << "That number squared is " << pow(x, 2) << endl;
    if (x >= 0) {
        cout << "The square root of that number is " << sqrt(x) << endl;
    } else {
        cout << "The square root is not a real number." << endl;
    }

    return 0;
}

• Compile and run:

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  g++ -std=c++17 calculator.cpp -o calculator
  ./calculator
  

6. If

• New file! bartender.cpp

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#include <iostream>

using namespace std;

int main()
{
    int age;

    cout << "Welcome to the pub." << endl;
    cout << "What is your age?" << endl;
    cin >> age;

    if (age > 18) {
        cout << "Have a pint!" << endl;
    } else if (age == 18) {
        cout << "Show me your ID and then have a pint!" << endl;
    } else {
        cout << "I'm calling the police." << endl;
    }

    return 0;
}

• Compile and run:

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  g++ -std=c++17 bartender.cpp -o bartender
  ./bartender
  

7. For and while loops

• New file! square_numbers.cpp

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#include <cmath>
#include <iostream>

using namespace std;

int main()
{
    int n = 10;

    cout << "Squares from a for loop:" << endl;
    for (int i = 0; i < n; i++) {
        cout << i << "^2 = " << pow(i, 2) << endl;
    }

    cout << endl;
    cout << "Cubes from a while loop:" << endl;
    int i = 3;
    while (i < n) {
        cout << i << "^3 = " << pow(i, 3) << endl;
        i++;
    }

    return 0;
}

• Compile and run:

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  g++ -std=c++17 square_numbers.cpp -o square_numbers
  ./square_numbers
  

Part 2. Files, arrays and functions

8. Saving and reading data

To save and read files, we need to include the <fstream> library.

• New file! save_to_file.cpp

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#include <fstream>
#include <iostream>

using namespace std;

int main()
{
    ofstream myfile("greeting.txt");
    if (!myfile) {
        cerr << "Cannot open greeting.txt for writing." << endl;
        return 1;
    }

    myfile << "header line 1" << endl;
    myfile << "header line 2" << endl;
    myfile << "header line 3" << endl;

    int datapoint = 10;
    myfile << datapoint << endl;

    for (int i = 0; i < datapoint; ++i) {
        myfile << i << "  " << 0.01 + 0.2 * i + 0.03 * i * i << endl;
    }

    myfile.close();
    cout << "Data saved to greeting.txt" << endl;
    return 0;
}

• Compile and run:

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  g++ -std=c++17 save_to_file.cpp -o save_to_file
  ./save_to_file
  

Your turn:

• Create another file, for example save_to_file2.cpp, where you print the first 10 squares to a text file.

• Run your program again. What can you say about how it writes to the text file?

• Replace

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  ofstream myfile("square_numbers.txt");
  

  with

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  ofstream myfile("square_numbers.txt", ios_base::app);
  

  if you want to append to the end of a text file, rather than overwrite.

• New file! read_from_file.cpp

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#include <fstream>
#include <iostream>
#include <string>

using namespace std;

int main()
{
    string line;
    ifstream myfile("greeting.txt");
    if (!myfile) {
        cerr << "Cannot open greeting.txt for reading." << endl;
        return 1;
    }

    for (int i = 0; i < 3; i++) {
        getline(myfile, line);
        cout << line << endl;
    }

    int datapoint;
    myfile >> datapoint;
    cout << "point: " << datapoint << endl;

    int x[1000];
    double y[1000];
    int count = 0;

    while (myfile >> x[count] >> y[count]) {
        cout << count << " " << x[count] << " " << y[count] << endl;
        count++;
    }

    if (datapoint != count) {
        cout << "data miss" << endl;
    } else {
        cout << "read all data." << endl;
    }

    myfile.close();
    return 0;
}

• Compile and run:

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  g++ -std=c++17 read_from_file.cpp -o read_from_file
  ./read_from_file
  

Note: this version uses while (myfile >> x[count] >> y[count]), which is safer than checking eof() first.

9. Arrays

• New file! array_square_numbers.cpp

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#include <iostream>

using namespace std;

int main()
{
    int n = 10;
    int square_numbers[10];
    int favourite_numbers[10] = {3, 1, -4, 1};

    for (int i = 0; i < n; i++) {
        square_numbers[i] = i * i;
    }

    cout << "square_numbers:" << endl;
    for (int i = 0; i < n; i++) {
        cout << square_numbers[i] << endl;
    }

    cout << endl;
    cout << "favourite_numbers:" << endl;
    for (int i = 0; i < n; i++) {
        cout << favourite_numbers[i] << endl;
    }

    int magic_square[3][3] = {
        {4, 9, 2},
        {3, 5, 7},
        {8, 1, 6}
    };

    cout << endl;
    cout << "magic_square[1][1] = " << magic_square[1][1] << endl;
    return 0;
}

• Note you cannot directly cout an array.

• We can also declare:

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  int favourite_numbers[] = {3, 1, -4, 1};
  

• Compile and run:

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  g++ -std=c++17 array_square_numbers.cpp -o array_square_numbers
  ./array_square_numbers
  

10. Functions

Writing functions allows us to do more complicated things.

• New file! learning_functions.cpp

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#include <iostream>

using namespace std;

int sign_function(int n);

int main()
{
    int n = -7;
    cout << "sign_function(" << n << ") = " << sign_function(n) << endl;
    return 0;
}

int sign_function(int n)
{
    int sign;

    if (n > 0) {
        sign = 1;
    } else if (n == 0) {
        sign = 0;
    } else {
        sign = -1;
    }

    return sign;
}

• Global and local variables.

• Note: order is important. What happens if you define sign_function after main without a declaration?

• The line

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  int sign_function(int n);
  

  is a forward declaration.

• Compile and run:

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  g++ -std=c++17 learning_functions.cpp -o learning_functions
  ./learning_functions
  

Part 3. Pointers, arguments and classes

11. Pointers

• New file! pointers.cpp

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#include <iostream>

using namespace std;

int main()
{
    int x = 10;
    int *p = &x;

    cout << "Value of x is " << x << endl;
    cout << "Address of x is " << &x << endl;
    cout << "Address stored in p is " << p << endl;
    cout << "Value of *p is " << *p << endl;

    x = 25;
    cout << "After changing x, *p = " << *p << endl;
    return 0;
}

• The & sign indicates an address in memory.

• A pointer is a variable whose value is the address of another variable, and is notated with a *.

• New file! pointer_array.cpp

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#include <iostream>

using namespace std;

int main()
{
    int primes[3] = {2, 3, 5};
    int *p = primes;

    for (int i = 0; i < 3; i++) {
        cout << "Address of primes[" << i << "] is " << p << endl;
        cout << "Value of primes[" << i << "] is " << *p << endl;
        p++;
    }

    cout << "*(primes + 2) = " << *(primes + 2) << endl;
    return 0;
}

• Compile and run:

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  g++ -std=c++17 pointers.cpp -o pointers
  ./pointers
  
  g++ -std=c++17 pointer_array.cpp -o pointer_array
  ./pointer_array
  

12. Main function with arguments

If you want to access command-line arguments, you can declare main() as follows.

• New file! main_arg.cpp

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#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{
    cout << "argc == " << argc << '\n';
    for (int i = 0; i < argc; i++) {
        cout << "argv[" << i << "] == " << argv[i] << '\n';
    }
    return 0;
}

• Compile and run:

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  g++ -std=c++17 main_arg.cpp -o main_arg
  ./main_arg table_in.dat table_out.dat
  

Typical output:

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argc == 3
argv[0] == ./main_arg
argv[1] == table_in.dat
argv[2] == table_out.dat

This form will be useful later when you want to read input files and write output files from the terminal.

13. Classes

Classes are one of the main things that separate C++ from C. Classes are ways for us to set up objects that have properties and functions which are particular to that type of object. A class is something like a user-defined data type.

• New file! rectangles.cpp

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#include <iostream>

using namespace std;

class Rectangle
{
  public:
    double width;
    double height;

    double area() { return width * height; }
    double perimeter() { return 2.0 * (width + height); }
    void set_dimensions(double x, double y);
};

void Rectangle::set_dimensions(double x, double y)
{
    width = x;
    height = y;
}

int main()
{
    Rectangle Rec1;
    Rectangle Rec2;

    Rec1.set_dimensions(5.0, 6.0);
    Rec2.set_dimensions(10.0, 12.0);

    cout << "Area of Rec1: " << Rec1.area() << endl;
    cout << "Perimeter of Rec1: " << Rec1.perimeter() << endl;
    cout << "Area of Rec2: " << Rec2.area() << endl;
    cout << "Perimeter of Rec2: " << Rec2.perimeter() << endl;
    return 0;
}

• area() and perimeter() are member functions.

• set_dimensions() shows another way of defining a member function outside the class body.

• Compile and run:

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  g++ -std=c++17 rectangles.cpp -o rectangles
  ./rectangles
  

If you want to use Rectangle in many files, it is annoying to define it every time. In actual usage, the declaration of the class usually goes into a header file, and the definitions of member functions go into a source file.

• rectangle.h

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#ifndef RECTANGLE_H
#define RECTANGLE_H

class Rectangle
{
  public:
    Rectangle();
    Rectangle(double w, double h);

    double area() const;
    double perimeter() const;
    void set_dimensions(double x, double y);

  private:
    double width;
    double height;
};

#endif

• rectangle.cc

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#include "rectangle.h"

Rectangle::Rectangle() : width(0.0), height(0.0) {}

Rectangle::Rectangle(double w, double h) : width(w), height(h) {}

double Rectangle::area() const
{
    return width * height;
}

double Rectangle::perimeter() const
{
    return 2.0 * (width + height);
}

void Rectangle::set_dimensions(double x, double y)
{
    width = x;
    height = y;
}

• rectangles_module.cpp

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#include <iostream>
#include "rectangle.h"

using namespace std;

int main()
{
    Rectangle Rec1;
    Rectangle Rec2(10.0, 12.0);

    Rec1.set_dimensions(5.0, 6.0);

    cout << "Area of Rec1: " << Rec1.area() << endl;
    cout << "Perimeter of Rec1: " << Rec1.perimeter() << endl;
    cout << "Area of Rec2: " << Rec2.area() << endl;
    cout << "Perimeter of Rec2: " << Rec2.perimeter() << endl;
    return 0;
}

• Compile and run:

  g++ -std=c++17 rectangles_module.cpp rectangle.cc -o rectangles_module
  ./rectangles_module