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Summary
o First C program
o Variables
o Data Types
o Arithmetic Operators
o Precedence of Operators
o Tips
First C program
The best way to learn C is to start coding right away. So here is our very first
program in
C.
# include <iostream.h>
main()
{
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
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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.
<iostream.h>
This is the name of the 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
main()
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
program.
cout << “ Welcome to Virtual University of Pakistan”
cout:
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.
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“ 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:
Variables
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 Type
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.
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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
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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>
main()
{
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;
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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
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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>
main()
{
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 */
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#include <iostream.h>
main()
{
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>
main()
{
char x;
x = ’a’;
cout << “The character value in x = “;
cout << x;
}
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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
modulus.
ARITHMETIC
OPERATION
ARITHMETIC
OPERATOR
ALGEBRAIC
EXPRESSION
C
EXPRESSION
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
type.
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
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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:
OPERATORS OPERATIONS PRECEDENCE (ORDER OF
EVALUATION)
( ) Parentheses Evaluated first
*, /, or % Multiplication,
Division, Modulus
Evaluated second. If there are
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
expressions.
TIP
o Use spaces in the coding to make it easy to read and understand
o Reserved words can not be used as variable names
o There is always a main( ) in a C program that is the starting point of
execution
o Write one statement per line
o Type parentheses ’( )’ and braces ‘{ }’ in pairs
o Use parentheses for clarification in arithmetic expressions
o Don’t forget semicolon at the end of each statement
o C Language is case sensitive so variable names x and X are two different
variables |
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