C++ overview, Input, Output, Operations

Comments

2 types of formats for comments are allowed in C++

/* This is the traditional way carried over from the C programming language */

// C++ introduced this new way
// of writing comments

Anything that follows // is a C++ comment and the compiler does not translate it into machine language 



.....
.....

float	fah, cel;		// Variable declarations

....

C style comments (using /* and */) are still useful in commenting large blocks 

/* If you want to write comments
that span multiple lines
and go on and on
then this is the way to do it */
Compiler Directives 

#include <iostream>

iostream is the standard class for most of the C++ input and output

This class is defined in file named iostream.h

The above directive can also be replaced by 

#include <iostream.h>

The #include directive tells the compiler to include code from the iostream class when compiling our program

Namespaces 

using	namespace	std;

The above line tells the compiler that we are going to use objects defined in the standard namespace (std)

The main Function

Every C++ program must have a main function

Program execution begins with the function main 

return_type function_name(argument_list)
{
	Function Body
}

return_type for the main function is int This indicates that the main function returns an integer. Hence we have the statement 

return 0;

towards the end of the main function

Declaration Statements

Used to declare variables that will be used in our program

Consist of the data type and the variable name

Other keywords like const, static etc... can be a part of them

Executable Statements

These statements are the place where your program DOES something like doing some computation, taking input from user, displaying output etc... 

cout << "Enter temperature in Fahrenheit : ";

is an output statement 

cin >> fah;

is an input statement 

cel = (fah - 32) * 5 / 9 ;

is a statement that does some arithmetic operations

Identifiers and Reserved Words

Reserved words are used in C++ for some pre determined purposes and hence cannot be used for any other purposes. Appendix B in the book lists the C++ reserved words.

Identifiers are used to represent data elements and objects in a program. An identifier MUST consist of letters, digits or underscores only, it must begin with either a letter or the underscore symbol. Any reserved word cannot be used as an identifier. 

first_name
num1
temp_1_f
.....
are all valid identifier names.

Identifiers are case sensitive. 

AVERAGE, average, AveRage
are all distinct identifiers. Choose identifiers in a way that relates the name to the identifier's purpose.
Data Types

Integers, Floating Point, Characters and Boolean are the pre-defined standard data types

int

  • Positive or negative whole numbers
  • short, int, long are the various ways to represent it. Compilers pick different number of bytes to represent them. Normally represented as 4 bytes.
  • 23, -46, 1875, 0, -562, 20453 are all integers

    float

  • Real numbers
  • float, double, long double are the various ways to represent it and compilers use different number of bytes to represent them.
  • These are stored as a mantissa and an integer exponent (which is a power of 2)
  • 23.5, -3.6, 0.0, -290.45, 4567.1234 are all floating point numbers

    bool

  • Can hold 2 values. true or false

    char

  • Stores one character. Uses one byte of memory
  • 'a', '2', '4', '$' are all char
  • Each charaacter has an integer representation. According to the American Standard Code for Information Interchange (ASCII), the character 'A' represents the integer 65. It is possible to do integer arithmetic using characters. 'A' + 1 = 'B' 
    string
    class

    Strings in C++ can be represented as a sequence of characters. This is the preferred way of doing string representation.

    There is a pre-defined string class that can be used to manipulate strings in C++. 

    #include <string>
    must be included as a compiler directive

    "CS 120", "a", "123", "Ball State" are all string objects. "a" is a string whereas 'a' is a character. "123" is a string whereas 123 is an integer.

    Variables

    Can take on different values (hence the name)

    Variables must follow the rules for naming identifiers since a variable name is essentially an identifier. 

    data_type identifier;
data_type identifier_list;
data_type identifier = initial_value;
    Here are some examples 
    int num1;
int avg = 0;
float gcd, lcm;
string first_name = "John";
    Constants

    Never change value

    Used for representing data that will always have the same value anywhere in the program. Multiplying factor in a formula, conversion factor etc... 

    const data_type identifier = initial_value;
const float CE_TO_FA = 1.8;
fah = (cel * CE_TO_FA) + 32;

    One of the advantages is that it makes changes to code easier

    Conventionally declared with all uppercase letters

    Input and Output 
    cin, cout
     are used for standard input and output respectively 
    cout << stuff_to_output;
     
    cin >> variable_to_store_input;

    cout directs output to standard output (normally the screen)

    cout is able to determine the data type of the object and outputs it accordingly 

    cout << " First Name is "; 
    
cout << name; 
    
cout << "Salary is "; 
    
cout << salary;
    name, salary are string and int variables repsectively. 
    cout << "This is CS 120 \n";       // Outputs a newline character towards the end 
    
cout << "This is CS 120 " << endl;		 // Signifies end of line 
    
cout << "First name is "<< name <<" Salary is "<< salary << endl; // Chaining is allowed 
    

cout << "This is the "; 
    
cout << "the same line "; 
    
cout << "but uses multiple "; 
    
cout << "cout's for output.";

    cin maps to standard input (normally the keyboard)

    cin is able to determine the data type of the variable and stores data appropriately 

    cin >> num1;				// Stores data in num1 
    
cin >> num1 >> num2;				// Chaining is allowed

    cin reads input till the next whitespace character (space, tab, newline).

    Type mismatches are sometimes handled but one needs to be careful

    Basic Operations

    Add +

    Subtract -

    Multiple *

    Divide /

    Remainder % 

    speed = distance / time;
    
salary = no_of_hours * rate;
    
17 / 5 = 3 
    
17.0 / 5 = 3.4 
    
17 / 5.0 = 3.4 
    
6 + 3 = 9 
    
6 + 3.0 = 9
    Result is float if any of the operands is float. 
    24 % 7 = 3 
    
30 % 3 = 0
    Order of evaluation is
  • Parentheses
  • Unary operators
  • *, /, % left to right
  • +, - left to right

    In order to avoid confusion, use parentheses whenever needed

    ++ and -- are respectively increment and decrement operators 

    ....
int x = 4;
cout << x << endl;
x++;
cout << x << endl;
    will output 
    4
    5
    

    x++			// x = x + 1;
y--			// y = y - 1;
    x++ is post increment ++x is pre increment 
    int p = 4;
int q = 10;
int y, z;
y = p++;		// p = 5, y = 4
z = ++q;		// q = 11, z = 11


    x += 3;			// Addition assignment operator. x = x + 3
x *= 3;			// Multiplication assignment operator. x = x * 3
x -= 3;			// Subtraction assignment operator. x = x - 3
x /= 3;			// Division assignment operator. x = x / 3
    Input and Output Redirection

    cout sends output to screen and cin gets input from the keyboard.

    It is possible to send output to a file and read input from a file 

    a.out > outputfile			// Send output to outputfile
    
a.out < inputfile 				// Take input from inputfile
    
a.out < inputfile > outputfile		// Send output to outputfile
							// and take input from inputfile
    Examples 
    // Add 2 integers
#include <iostream>
using 	namespace	std;

int main(){
	int	num1,	num2;
	cin >> num1 >> num2;
	cout << "Sum is " << (num1 + num2) << endl;
	return 0;
}

    
// Expression with multiple operations
#include <iostream>
using	namespace	std;

int main(){
	float	num1,	num2,	num3,	num4,	num5;
	cin >> num1 >> num2 >> num3 >> num4 >> num5;
	float	ans;
	ans = (num1 * num2 * (num3 - num4 )) / num5;
	cout << ans;
	return 0;
}

    
// Division and Remainder
#include <iostream>
using	namespace	std;

int main(){
	int	num1,	num2;
	cin >> num1 >> num2;
	cout << "Quotient is " << (num1 / num2) << endl;
	cout << "Remainder is " << (num1 % num2) << endl;
	return 0;
}

    
// Assignment Operators
#include <iostream>
using	namespace	std;

int main(){
	float num1,	num2;
	cin >> num1 >> num2;
	num1 += 20;
	num1 *= 5;
	cout << num1 << endl;
	num2 -= num1;
	cout << num2 << endl;
	return 0;
}

    
// Strings
#include <iostream>
#include <string>

using	namespace	std;

int main(){
	string	str1 = "Bill";
	string	str2;
	str2 = "Gates";
	cout << str1 << " " << str2 << endl;
	return 0; 
}

    
// Characters and Integers
#include <iostream>

using	namespace	std;

int main(){
	char	ch1,	ch2;
	cin >> ch1 >> ch2;
	cout << ch2 << endl;
	cout << ch1 + 5 << endl;
	return 0; 
}