CHAPTER 1
INTRODUCTION TO C
Introduction
C was developed in the early 1970s at Bell Labs, by Brian Kenninghan and Dennis
Ritchie. Ritchie with Ken Thompson had also developed Unix Operating system.
Kenninghan and Richie developed C from another language called B, which in turn was
derived from language called BPCL ( a system programming language ).
Developers of proprietary packaged software are very interested in C because it its
considered more transportable than other languages. That is , it is relatively machineindependent.
A C program written for one type of computer can be run on another type with little or no
modifications.
Learning Process of C
C language should be learned in the following phases.
Phase I :
Alphabets, Digits and Special symbols.
Phase II:
Constants, Variables, and Keywords.
PhaseIII :
Instructions
Phase IV :
Program
Let us read about them one by one.
Phase I : Alphabets , Digits and Special Symbols.
Like all other programming languages, C also supports ,
Alphabets
a to z
Numeric
0 to 9
Special characters , .( * )etc.
White spaces
Blank space, horizontal tab, carriage return , new line , form
fed, etc.
Other characters
C can process any of the 256 ASCII characters as data or as
literals.
Phase II :
Constants, Variables, and Keywords
Let us read them one by one.
Constants
C Supports two type of constants.
Primary Constants
Secondary Constants
Let see how each one of them can be represented in C.
PRIMARY CONSTANTS
They in turn are of 3 types. They are :
Character constants
Integer Constants
Real Constants
Character Constants
A character constant is one or more character enclosed in single quotes, as in ‘z’ . The
rule for writing character constant is given below :
A character constant in C must contain one or more characters and must be enclosed in
quotation marks with both of them pointing towards left.
Integer Constants
Integer constants are whole numbers without any fractional part. The method of writing
integer constants has been specified in the following rule.
An integer constant must have at least one digit and must not contain any decimal point.
It may contain either + or-sing. A number with no sign is assumed to be positive.
Commas cannot appear in an integer constants. C allows three types of integer
constants :
Decimal Integer constants ( base 10 )
An integer constant consisting of a sequence of digits is taken to be decimal integer
constant unless it beings with 0 (digit zero). For instance , 1234 , 41 , +97 , -17 are
decimal integer constants.
Octal integer constants ( base 8 )
A sequence of digits starting with 0 ( digit zero ) is taken to be an octal integer. For
instance decimal integer 8 will be written as 010 as octal integer. For example, 8 10= 108
and decimal integer 12 will be written as 014 as octal integer, for example 12 10 = 148 .
Hexadecimal Integer Constants ( base 16 )
A sequence of digits preceded by 0x or 0X is taken to be an hexadecimal integer. For
instance decimal 12 will be written as 0XC as hexadecimal integer.
Real Constants
They are often called floating constants. Real constants are numbers having fractional
parts. These may be written in one of the two forms called fractional form or the
exponent form.
A real constant in fraction form consists of signed or unsigned digits including a decimal
point between digits. The rule for writing a real constant in fractional form is given below
:
A real constant in fractions form must have at least one digit before a decimal point and
at least one digit after the decimal pint. It my also have either + or – sign preceding it. A
real constant with no sign is assumed to be positive.
The following are valid real constants in fractional form
2.0 , 17.5 , -13.0 , -0.00625
The following are invalid real constants.
7
(No decimal point)
7.
(No digit after decimal point)
+ 17/2
(/-illegal symbol)
17,250.26.2 (Two decimal points)
17,250.262 (comma not allowed)
A real constant is exponent form consists of two parts : mantissa nd exponent. For
instance 5.8 can be written as 0.58 x 101. = 0.58 E01 where mantissa part is 0.58 ( the
part appearing before E) and exponent part is 1 ( the part appearting after E). E01
represents 101 . The rule for writing a real constant in exponent for is given below :
A real constant in exponent form has two parts : a mantissa and an exponent The
mantissa : must be either an integer or a proper real constant.
The mantissa is followed by a letter E or e and the exponent. The exponent must be an
integer.
The following are the valid real constants in exponent form :
152E05 .1.52E07 , 0.1523E08, 152.0E08 , 152E+8, 1520E04, E-0.172E-3
The following are invalid real constants in exponent form
1. 172.E5
( At least a digit must follow the decimal point)
2. 1.7E ( No digit specified for exponent)
3. 0.17E2.3 (Exponent cannot have fractional part)
4. 17,225E02 (No comma allowed)
5. .28E-7 (No preceding digits before decimal point)
SECONDARY Constants
They in turn are of 5 types. They are :
Array constants
Pointer constants
Structure constants
Union constants
Union constants
Enum constants etc.
Variables
These are the various names given to location in the memory of the computer where
different constants are stored. All the various types of constants discussed above can
be stored there. Both name and type of constants are unique to that particular location.
This means that you cannot have 2 variables with different type of variables.
For a variable it is safe to have it of 8 characters through some compilers allow variable
names of more than 8. Variable names must begin with an alphabet. No commas or
blanks are allowed within a variable name however, only symbol that you can use is the
underscore ( _ ). Next are the keywords.
C Keywords
There are as many as 32 Keywords in C. Given below is the list of reserved words
which obviously you cannot use in your programs.
These are:
Auto
char
Break
Continue
Case
Do
Const
Else
Default
Extem
Double
Far
Enum
Goto
Float
Int
For
Near
If
Return
Long
Signed
Register
Struct
Short
Typedef
Static
Unsigned
Switch
while
Union
Void
Phase III: Instructions
Instruction are nothing but the combinations of the above mentioned constants,
variables and keywords. As we know from the definition of programming. Which is:
Programming is a set of instructions given to the computers to perform. In C we work
under 4 set of instructions. They can be defined as follows:
Input / Output Instructions
These are used to perform the function of supplying
input
data to a program and obtaining the output results
from it.
Arithmetic Instructions
These are used to perform arithmetic operations
between
constants and variables.
Type Declaration Instructions
These are used to declare the type of variables
in C Program.
Control Instructions
These are used to control the sequence of executions
of
various statements in C Program.
Let us talk about one of them Arithmetic Instructions.
Arithmetic Instructions
Those of you who have worked in BASIC would remember that a computer instruction
has two sides connected in between by a equal to sign ( = ).
The left hand side has a variable name and the right hand side has variables name and
constants these variables can be connected by arithmetic operators like + - * and / .
An arithmetic statement in C could be 3 types.
Integer Mode Arithmetic Statement
Here all the operands are either integer variables or integer constants.
For Example,
X=y+2
Real Mode Arithmetic Statement
In this case all operands are either real constants or real variables. For example,
z=x*y
Mixed Mode Arithmetic Statement
Here some of the operands are integers and some of the operands are real. For
example
I = prin * rat * y /100
Operations Hierarchy
Since we are dealing here with the various arithmetic operators. We must have the
order of their execution.
If you remember that we had the following preferences in our maths class which was
defined by BODMAS, as explained below:
B
Braces
O
Overhead
D
Divide
M
Multiply
A
Addition
S
Subtraction
In the case of C this is slightly different. Here we have the 3 levels of priorities.
They are:
1. *
Multiplication
2. /
Division
3. %
Modular Division
4. +
Addition
5. -
Subtraction
6. =
Assignment
To show the importance of setting this order of execution, I give here an
example. I remember that we in collage use to test the knowledge of our
colleagues by the following example,
2+3*4+5
If you use an ordinary calculator to get the result of this.You will get 25. Some
guys even gave the answer of 45. But, if you apply the BODMAS method and
operate the multiplication first then the answer would be 19, which is the correct
answer. I have seen some scientific calculators giving this result.