CS 201 Lecture No 3

  Lecture Handout 
  Introduction to programming
  Lecture No. 3
  Reading Material
  Deitel & Deitel – C++ How to Program    chapter 1 
  1.19, 1.20, 1.21, 1.22 

  First C program
  The best way to learn C is to start coding right away. So here is our very first program in
  # include <iostream.h>
        cout << "Welcome to Virtual University of Pakistan";
  We will look at this code line by line and try  to understand them.
  # include <iostream.h>
  #include: This is a pre-processor directive. It is not part of our program; it is an
  instruction to the compiler. It tells the C compiler to include the contents of a file, in this
  case the system file iostream.h. The compiler knows that it is a system file, and therefore 
  looks for it in a special place.  The features of preprocessor will be discussed later. For
  the time being take this line on faith. You have to write this line. The sign # is known as
  HASH and also called SHARP.
  This is the name of the library definition file for all Input Output Streams. Your program
  will almost certainly want to send stuff to the screen and read things from the keyboard.
  iostream.h is the name of the file in which has code to do that work for you
  The name main is special, in that the main is actually the one which is run when your
  program is used. A C program is made up of a large number of functions. Each of these is
  given a name by the programmer and they refer to each other as the program runs. C
  regards the name "main" as a special case and will run this function first. If you forget to
  have a main function, or mistype the name, the compiler will give you an error.
  Notice that there are parentheses (“( )”, normal brackets) with main. Here the parentheses
  contain nothing. There may be something written inside the parentheses. It will be
  discussed in next lectures.
  { }
  Next, there is a curly bracket also called braces("{ }"). For every open brace there must be
  a matching close. Braces allows to group together pieces of a program. The body of main
  is enclosed in braces. Braces are very important in C; they enclose the blocks of the
  cout <<  “ Welcome to Virtual University of Pakistan”
  This is known as out put stream in C and C++. Stream is a complicated thing, you will
  learn about it later. Think a stream as a door. The data is transferred through stream, cout
  takes data from computer and sends it to the output. For the moment it is a screen of the
  monitor. hence we use cout for output.
  The sign << indicates the direction of data. Here it is towards cout and the function of
  cout is to show data on the screen.
  “ Welcome to Virtual University of Pakistan”
  The thing between the double quotes (“ ”) is known as character string. In C
  programming character strings are written in double quotes. Whatever is written after <<
  and within quotation marks will be direct it to cout,  cout will  display it on the screen.
  There is a semicolon (;) at the end of the above statement. This is very important. All C
  statements end with semicolon (;). Missing of a semicolon (;) at the end of statement is a
  syntax error and compiler will report an error during compilation. If there is only a
  semicolon (;) on a line than it will be called a null statement. i.e. it does nothing. The
  extra semicolons may be put at the end but are useless and aimless. Do not put semicolon
  (;) at a wrong place, it may cause a problem during the execution of the program or may
  cause a logical error.
  In this program we give a fixed character string to cout and the program prints it to the
  screen as:
  During programming we need to store data. This data is stored in variables. Variables are
  locations in memory for storing data. The memory is divided into blocks. It can be
  viewed as pigeon-holes. You can also think of it as PO Boxes. In post offices there are
  different boxes and each has an address. Similarly in memory, there is a numerical
  address for each location of memory (block). It is difficult for us to handle these
  numerical addresses in our programs. So we give a name to these locations. These names
  are variables. We call them variables because they can contain different values at
  different times.
  The variable names in C may be started with a character or an underscore ( _ ). But avoid
  starting a name with underscore ( _ ). C has many libraries which contain variables and
  function names normally starting with underscore ( _ ). So your variable name starting
  with underscore ( _ ) may conflict with these variables or function names.
  In a program every variable has
  o  Name
  o Size
  o Value
  The variables having a name, type and size (type and size will be discussed later) are just
  empty boxes. They are useless until we put some value in them. To put some value in
  these boxes is known as assigning values to variables. In C language, we use assignment
  operator for this purpose.
Assignment Operator
  In C language equal-to-sign (=) is used as assignment operator. Do not confuse the
  algebraic equal-to with the assignment operator. In Algebra X = 2 means the value of X
  is 2, whereas in C language X = 2 (where X is a variable name) means take the value 2
  and put it in the memory location labeled as X, afterwards you can assign some other
  value to X, for example you can write X = 10, that means now the memory location X
  contains the value 10 and the previous value 2 is no more there.
  Assignment operator is a binary operator (a binary operator has two operands). It must
  have variable on left hand side and expression (that evaluates to a single value) on right
  hand side. This operator takes the value on right hand side and stores it to the location
  labeled as the variable on left hand side, e.g. X = 5,  X = 10 + 5, and X = X +1. 
  In C language the statement X = X + 1 means that add 1 to the value of X and then store
  the result in X variable. If the value of X is 10 then after the execution of this statement
  the value of X becomes 11. This is a common practice for incrementing the value of the
  variable by ‘one in C language. Similarly you can use the statement X = X - 1 for
  decrementing the value of the variable by one. The statement X = X + 1 in algebra is not
  valid except when X is infinity. So do not confuse assignment operator (=) with equal
  sign (=) in algebra. Remember that assignment operator must have a variable name on
  left hand side unlike algebra in which you can use expression on both sides of equal sign
  (=). For example, in algebra, X +5 = Y + 7 is correct but incorrect in C language. The
  compiler will not understand it and will give error. 
  Data Types
  A variable must have a data type associated with it, for example it can have data types
  like integer, decimal numbers, characters etc.  The variable of type Integer stores integer
  values and a character type variable stores character value. The primary difference
  between various data types is their size in memory. Different data types have different
  size in memory depending on the machine and compilers. These also affect the way they
  are displayed. The ‘cout’ knows how to display a digit and a character. There are few
  data types in C language. These data types are reserved words of C language. The reserve
  words can not be used as a variable name.
  Let’s take a look into different data types that the C language provides us to deal with
  whole numbers, real numbers and character data.
  Whole Numbers
  The C language provides three data types to handle whole numbers. 
  o  int
  o  short
  o  long
  int Data Type
  The data type int is used to store whole numbers (integers). The integer type has a space
  of 4 bytes (32 bits for windows operating system) in memory. And it is mentioned as
  int’ which is a reserved word of C, so we can not use it as a variable name.  
  In programming before using any variable name we have to declare that variable with its
  data type. If we are using an integer variable named as ‘i’, we have to declare it as 
                                    int i ;
  The above line is known as declaration statement. When we declare a variable in this
  way, it reserves some space in memory depending on the size of data type and labels it
  with the variable name. The declaration statement int i ; reserves 4 bytes of memory and
  labels it as ‘i’. This happens at the execution time.
  Sample Program 1
  Let’s consider a simple example to explain int data type. In this example we take two
  integers, add them and display the answer on the screen.  
  The code of the program is written below.
  #include <iostream.h>
  int x; 
              int y; 
              int z; 
  x = 5;
  y = 10;
  z = x + y;
  cout << “x = “;
  cout << x;
  cout << “ y=“;
  cout << y;
  cout  << “ z = x + y = “;
  cout << z;
  The first three lines declare three variables x, y and z as following. 
              int x; 
              int y; 
              int z;
  These three declarations can also be written on one line. C provides us the comma
  separator (,). The above three lines can be written in a single line as below
              int x, y, z; 
  As we know that semicolon (;) indicates the end of the statement. So we can write many
  statements on a single line. In this way we can also write the above declarations in the
  following form 
            int x; int y; int z; 
  For good programming practice, write a single statement on a single line.
  Now we assign values to variables x and y by using assignment operator. The lines x = 5;
  and y = 10 assign the values 5 and 10 to the variables x and y, respectively. These
  statements put the values 5 and 10 to the memory locations labeled as x and y.
  The next statement z = x + y; evaluates the expression on right hand side. It takes values
  stored in variables x and y (which are 5 and 10 respectively), adds them and by using the
  assignment operator (=), puts the value of the result, which is 15 in this case, to the
  memory location labeled as z. 
  Here a thing to be noted is that the values of x and y remains the same after this
  operation. In arithmetic operations the values of variables used in expression on the right
  hand side are not affected. They remain the same. But a statement like x = x + 1; is an
  exceptional case. In this case the value of x is changed.
  The next line cout << “ x = “ ; is simple it just displays ‘ x = ‘ on the screen.
  Now we want to display the value of x after ‘x =’. For this we write the statement 
  cout << x ;
  Here comes the affect of data type on cout. The previous statement cout << “x = “ ; has a
  character string after << sign and   cout simply displays the string. In the statement cout
  << x; there is a variable name x. Now cout will not display ‘x’ but the value of x. The
  cout interprets that x is a variable of integer type, it goes to the location x in the memory
  and takes its value and displays it in integer form, on the screen. The next line cout << ”y
  =”; displays ‘ y = ‘ on the screen. And line cout << y; displays the value of y on the
  screen. Thus we see that when we write something in quotation marks it is displayed as it
  is but when we use a variable name it displays the value of the variable not name of the
  variable. The next two lines cout << “z = x + y = ”; and cout << z; are written to display
  ‘z = x + y = ’ and the value of z that is 15. 
  Now when we execute the program after compiling, we get the following output.
  x = 5 y = 10 z = x + y = 15
  short Data type
  We noted that the integer occupies four bytes in memory. So if we have to store a small
  integer like 5, 10 or 20 four bytes would be used. The C provides another data type for
  storing small whole numbers which is called short. The size of short is two bytes and it
  can store numbers in range of -32768 to 32767. So if we are going to use a variable for
  which we know that it will not increase from 32767, for example the age of different
  people, then we use the data type short for age. We can write the above sample program
  by using short instead of int. 
  /*This program uses short data type to store values */
  #include <iostream.h>
  short x; 
              short y; 
              short z;
  x = 5;
  y = 10;
  z = x + y;
  cout << “x = “;
  cout << x;
  cout << “ y=“;
  cout << y;
  cout  << “ z = x + y = “;
  cout << z;
  long Data Type
  On the other side if we have a very large whole number that can not be stored in an int
  then we use the data type long provided by C. So when we are going to deal with very big
  whole numbers in our program, we use long data type. We use it in program as:
  long x = 300500200;
  Real Numbers
  The C language provides two data types to deal with real numbers (numbers with decimal
  points e.g. 1.35, 735.251). The real numbers are also known as floating point numbers.
  o  float
  o  double
  float Data Type
  To store real numbers, float data type is used. The float data type uses four bytes to store
  a real number. Here is program that uses float data types.
  /*This program uses short data type to store values */
  #include <iostream.h>
  float x; 
              float y; 
              float z; 
  x = 12.35;
  y = 25.57;
  z = x + y;
  cout << “ x = “;
  cout << x;
  cout << “ y = “;
  cout << y;
  cout << “ z = x + y = “;
  cout << z;
  double Data Type
  If we need to store a large real number which cannot be store in four bytes, then we use
  double data type. Normally the size of double is twice the size of float. In program we
  use it as:
  double x = 345624.769123;
  char Data Type
  So far we have been looking on data types to store numbers, In programming we do need
  to store characters like a,b,c etc. For storing the character data C language provides char
  data type. By using char data type we can store characters in variables. While assigning a
  character value to a char type variable single quotes are used around the character as ‘a’. 
  /* This program uses short data type to store values */
  #include <iostream.h>
  char x; 
              x = ’a’;
  cout << “The character value in x =  “;
  cout << x;
  Arithmetic Operators 
  In C language we have the usual arithmetic operators for addition, subtraction,
  multiplication and division. C also provides a special arithmetic operator which is called
  modulus. All these operators are binary operators which means they operate on two
  operands. So we need two values for addition, subtraction, multiplication, division and
  Addition   +   x + y   x + y
  Subtraction   -   x – y   x - y
  Multiplication   *   Xy   x * y
  Division   /   x ÷ y, x / y   x / y
  Modulus   %   x mod y   x % y
  Addition, subtraction and multiplication are same as we use in algebra. 
  There is one thing to note in division that when we use integer division (i.e. both
  operands are integers) yields an integer result. This means that if, for example, you are
  dividing 5 by 2 (5 / 2) it will give integer result as 2 instead of actual result 2.5. Thus in
  integer division the result is truncated to the whole number, the fractional part (after
  decimal) is ignored. If we want to get the correct result, then we should use float data
  The modulus operator returns the remainder after division. This operator can only be used
  with integer operands. The expression x % y returns the remainder after x is divided by
  y. For example, the result of 5 % 2  will be 1, 23 % 5 will be 3 and 107%10 will be 7.
  Precedence of Operators
  The arithmetic operators in an expression are evaluated according to their precedence.
  The precedence means which operator will be evaluated first and which will be evaluated
  after that and so on. In an expression, the parentheses ( ) are used to force the evaluation
  order. The operators in the parentheses ( ) are evaluated first. If there are nested
  parentheses then the inner most is evaluated first.
  The expressions are always evaluated from left to right. The operators *, / and % have the
  highest precedence after parentheses. These operators are evaluated before + and –
  operators. Thus + and – operators has the lowest precedence. It means that if there are *
  and + operators in an expression then first the * will be evaluated and then its result will
  be added to other operand. If  there are * and / operators in an expression (both have the
  same precedence) then the operator which occurs first from left will be evaluated first and
  then the next, except you force any operator to evaluate by putting parentheses around it.
  The following table explains the precedence of the arithmetic operators:       
  ( )   Parentheses   Evaluated first
  *, /, or %   Multiplication,   Evaluated second.  If there are
  Division, Modulus   several, they are evaluated from left
  to right
  + or -   Addition, Subtraction   Evaluated last. If there are several,
  they are evaluated from left to right
  Lets look some examples.
  What is the result of   10 + 10 * 5 ?
  The answer is 60 not 100. As * has higher precedence than + so 10 * 5 is evaluated first
  and then the answer 50 is added to 10 and we get the result 60. The answer will be 100 if
  we force the addition operation to be done first by putting 10 + 10 in parentheses. Thus
  the same expression rewritten as (10 + 10) * 5 will give the result 100. Note that how the
  parentheses affect the evaluation of an expression.
  Similarly the expression 5 * 3 + 6 / 3 gives the answer 17, and not 7. The evaluation of
  this expression can be clarified by writing it with the use of parentheses as (5 * 3) + (6 /
  3) which gives 15 + 2 = 17. Thus you should be careful while writing arithmetic

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