This document discusses key concepts in C programming including variables, data types, constants, keywords, comments, and rules for writing C programs. It defines variables as containers for storing data in memory locations. It describes predefined data types like char, int, float, and double as well as derived and user-defined data types. It also covers identifiers, declarations, initialization, keywords, constants, comments, and general rules for writing C programs.

1. Variables, Datatype,
Constants, Keywords,
Comments, Rules for Writing
C program
Part 2
By: Er. Aman Kumar

2. Variables
• Variable is simply name given to memory
location which acts as placeholder or
container for storing data. It may help to think
of variables as a container for a value.

3. Data type
• We can store variables in our computer
memory. Since each type of data takes
different amount of memory, like integer takes
2 bytes, decimal numbers take 4 byte, and
SINGLE character takes 1 byte. So we have to
tell computer about the type of data we are
going to store in our variable

4. • Predefined/inbuilt :
These datatypes are inbuilt.
eg : char, int, float, double
• Drived Datatype
These datatypes are inherited from predefined
datatype. eg : arrays, pointer.
• Userdefined datatype : These are created by user
for his/her own purpose
• eg : typedef, enum, structure, union

6. Predefined Datatypes

8. Character set
• Character set: every character is assigned unique
number called character set. C/C++ follows ASCII
character set.
• ASCII: American standard code for information
interchange
• Here lowercase, uppercase characters (English),
special symbols, digits have been assigned unique
integer number.
• Total 256 characters are there.
• Range is: -128 to 127
• Here limitation is only English is used.

9. Identifiers
• Identifier is simply a name given to your
variable, class, union, structure, function, etc.
• Rules for creating valid identifiers:
• 1. An identifier can be any combination of
alphabets, digits, underscore.
• 2. Neither spaces nor special symbol other
then underscore can be used.
• 3. It won‘t begin with a digit

11. Declaration and initialization of
variable
• How to declare variable?
• Declaration of variable means specify data type
of variable and name assigned (identifier).
• A variable declaration provides assurance to the
compiler that there is one variable existing with
the given type and name so that compiler
proceed for further compilation without needing
complete detail about the variable.

12. • syntax :
datatype varible_name;
eg :
int a;
float b;
char c;
double first;
int i, j, k;

13. Initialization
• It means assigning the values to variable

14. Declaration and initialization in single
step
int a = 10;
float b = 30.05;
char c = 'a';
(Value to characters always assigned in single
quotes)

15. Keywords
• Are simply the reserved words whose
meaning has been explained in library of C
language.
• There are 32 keywords in c language.
• We can’t use keywords as identifiers or
variables because if we do so we are trying to
assign a new meaning to the keyword, which
is not allowed by the computer

16. List of Keywords

17. int float;
char if;
Int void;
Above will give error as we are using keywords
as identifier or variable name

18. Constants
• Is an identity which does not change its value.
Once the value is initialized, that can’t be
changed.
• Constants are just like variables only condition
is that their value doesn’t change.
• We will use const keyword for declaring
constants

19. Comments
• Comments are that part of program, which are
ignored by compiler. These are used for better
readability of user. Compiler doesn‘t compile the
text written inside comments.
• The comment describes the purpose of the code
in the file, and might include some license or
copyright information.
• It is not compulsory to have comments in our
program, but its good practice and what most C
programmers will expect to find.

20. Single Line Comment
// is used to make single line comment. All the
text in a line after // will become part of
comment and will be ignored by compiler.
• E.g.
int a; // this is ignored

21. Multiline Comment
This starts from /* and ends at */.
All the text in between these will be part of
comment.
E.g.
/* This Is
Multiline
Comment */

22. Rules for writing c program
• Every instruction in c program is written as separate
statement because C program is collection of statements
• There is no particular hard and fast rule for placing the
statements in our program. We can place it anywhere in
our program, that why C is known as free form language.
• Every statement will end with ; (semi-colon). So it is also
known as statement terminator.
• Proper comments and indentation should be given in c
program for better readability
• C is case sensitive language. It differs in lower and upper
case characters.
• We can‘t declare variable with same name multiple
times in single block.