section5.md

Section5: Structure of a C++

Table of Contents

• C++ Keywords
• C++ Preprocessor
• Comments
• The main() function
• Namespaces
• Basic I/O using cin and cout
• cout and <<
• cin and >>
• cout examples
• cout, cin example1 using integers
• cout, cin example2 using integers
• cout, cin example3 using integers
• cout, cin example4 using doubles
• cout, cin example5 using integers and doubles

C++ Keywords

C++ has over 90 keywords. These are words that are reserved and cannot be used for variables, function names etc. We have seen the use of int and return keywords.

Identifiers are names created by programmers, such as a variable names or built-in library objects such as cout, cin or namespaces etc.

Identifiers cannot start with a number or have spaces.

C++ Preprocessor

Preprocessor examples

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

The C++ preprocessor is a program that processes source code before the compiler.

• It strips all comments from the source file, and replaces a comment with a single space.

• It executes any preprocessor directives.

• It removes any preprocessor directives from the source code.

Comments

Single line comment

// This is a single line comment.

Multiple line comment

/* 
This is 
a multiple
line comment.
*/

The main() function

• Every C++ program must have exactly 1 main() function.

• This is the starting point of the program.

• return 0 indicates successful program execution

main() versions

int main()
{
    // code
    return 0;
}

For accepting arguments at program execution via command line.

int main(int argc, char *argv[])
{
    // code
    return  0;
}

Namespaces

Namespaces are used to prevent naming conflicts. std::cout the programmer explicitly declares the namespace std and object count for use in the source code.

cout usage not using a namespace directive

std::cout << "Enter your favorite number between 1 and 100? ";

To save typing std::cout repeatedly, use a namespace directive.

Use all objects from the std(standard) namespace, not recommended.

#include <iostream>

using namespace std;

Use only the objects used from the std(standard) namespace.

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

cout usage using a namespace directive

cout << "Enter your favorite number between 1 and 100? ";

Basic I/O using cin and cout

preprocessor #include <iostream> required.

cout, cin, cerr and clong are objects representing streams.

cout

• standard output stream
• to console

cin

• standard input stream
• default from keyboard, ignores whitespace

<<

• insertion operator
• output to stream

>>

• extraction operator
• input from stream

cout and <<

• Insert the data variable into the cout stream

cout << data;

• Can be chained

cout << "data 1 is " << data1;

• Does not automatically add line breaks

cout << "data 1 is " << data1 << endl>>;
cout << "data 1 is " << data1 << "\n">>;

endl will also flush the stream.

cin and >>

• Extract data from cin stream based on data's type.

cin >> data;

if data is an integer then an integer is stored. if data is an double then a real number is stored.

• can be chained.

cin >> data1 >> data2;

• New lines (pressing enter) terminate the extraction from cin.

• Spaces entered on console, will be ignored.

• It can fail if the type entered cannot be interpreted. For example data is of type int, but a string is entered on the console. data will have an undetermined value.

cout examples

#include <iostream>

using std::cout;
using std::endl;

int main()
{
    cout << "Hello world!"; // No newline
    cout << "Hello world!" << endl;
    cout << "Hello "; // No newline
    cout << "world!" << endl;
    cout << "Hello" << " world!" << endl;
    cout << "Hello" << " world!\n"; // uses \n as newline
    cout << "Hello\nOut\nThere\n"; // uses \n aas newline

    return 0;
}

Output

Hello world!Hello world!
Hello world!
Hello world!
Hello world!
Hello
Out
There

cout, cin example1 using integers

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

int main()
{

    int num1;
    cout << "Enter an integer: ";
    cin >> num1;
    cout << "You entered: " << num1 << endl;

    return 0;
}

Output

Enter an integer:                   7                 
You entered: 7

cout, cin example2 using integers

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

int main()
{

    int num1;
    int num2;

    cout << "Enter an integer: ";
    cin >> num1;

    cout << "Enter a second integer: ";
    cin >> num2;

    cout << "You entered " << num1 << " and " << num2 << "." << endl;

    return 0;
}

Output

Enter an integer:     45
Enter a second integer:     23
You entered 45 and 23.

Here, cin is stored in a buffer, the buffer is not flushed, therefore the second integer is read from the buffer into num2, without the prompt being needed.

Output

Enter an integer:  23           99
Enter a second integer: You entered 23 and 99.

cout, cin example3 using integers

Here, cin is chained to retrieve two integers from the buffer. This saves having to prompt the user multiple times.

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

int main()
{

    int num1;
    int num2;

    cout << "Enter two integers separated by a space: ";
    cin >> num1 >> num2; // cin chained
    cout << "You entered " << num1 << " and " << num2 << "." << endl;

    return 0;
}

Output

Enter two integers separated by a space:      34      78
You entered 34 and 78.

cout, cin example4 using doubles

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

int main()
{

    double num3;

    cout << "Enter a double: ";
    cin >> num3;
    cout << "You entered " << num3 << endl;

    return 0;
}

Here, the user entered a space between the double, so only the first part of the number was stored.

Output

Enter a double:        45    .   78
You entered 45

Output

Enter a double: 45.6
You entered 45.6

Here, a integer was entered and a integer is displayed.

Output

Enter a double: 23
You entered 23

cout, cin example5 using integers and doubles

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

int main()
{

    int num1;
    double num3;

    cout << "Enter an integer: ";
    cin >> num1;

    cout << "Enter a double: ";
    cin >> num3;

    cout << "The integer is: " << num1 << endl;
    cout << "And the double is: " << num3 << endl;

    return 0;
}

Enter an integer: 10
Enter a double: 12.5
The integer is: 10
And the double is: 12.5

Here, a integer 10 and a double .5 is found in the input stream and stored without the prompt need for the double prompt.

Enter an integer: 10.5
Enter a double: The integer is: 10
And the double is: 0.5

Here, a string was entered not a integer, with unpredicted results.

Enter an integer: hello world
Enter a double: The integer is: 0
And the double is: 6.8991e-310

Note: In the real world cin would not be used to read numbers, a string would be read and then cast into a number.