This document discusses basic concepts in programming related to escape sequences, variables, constants, data types, and comments in the C language. It defines variables as memory locations for data, differentiates between constants and variables, and outlines rules for naming identifiers and variables. Additionally, it covers data types like int, float, and char, and emphasizes the importance of comments in code for clarity.

1. Lecture 2
Escape Sequences , Variables ,
Constants and Data Types

2. Escape Sequence
• Set of Characters to adjust the cursor(-)
location.
• Cursor: insertion point for text
n For New Line f Form Feed
t For tab ’ To Print ‘
b For back space ” To Print “
r For Carriage Return To Print

3. Use of Escape Sequences
#include<stdio.h>
#include<conio.h>
void main(void)
{
printf(“BUITMS nQuetta nPakistan”);
getch();
}
OUT PUT:

4. 4
20
2.5
A
A L I
Variable: a memory location where we store our data
Identifier: a name assigned to a variable is called identifier
age

5. Variables and Constants
• Constant
– A constant is a quantity that does not change. This
quantity can be stored at a location in the memory
of the computer.
• Variable
– A variable can be considered as a name given to
the location in memory where this constant is
stored. Naturally the Contents of the variable can
change.
• e.g. 3x+y=20
• 3 and 20 cannot change, they are called constants,
• X & Y can vary or changes, hence called variables.

6. Variables
• Variables are most fundamental aspects
of computer language.
• Variables are used to reserve some
space in computer’s memory for easy
reference to data stored in that space.

7. Identifiers
• An identifier is the name used for a
variable, function, data definition, etc.
• Rules for identifiers
– Legal characters are a-z, A-Z, 0-9, and _
(underscore).
– Case is significant.
– The first character must be a letter or _.
– Identifiers can be of any length (although
only the first 32 characters are guaranteed
to be significant).
– Aliiiiiiiiieiiiiiiii

8. What is a C identifier?
• Here are some examples of legal
identifiers:
– i
– count
– MAX_LENGTH

9. Keywords
• A keyword is a reserved word that
cannot be used as an identifier.

10. Reserved keywords
The following are reserved keywords:
auto double int struct
break else long switch
case enum register typedef
char extern return union
const float short unsigned
continue for signed void
default goto sizeof volatile
do if static while

11. Variables
• In English, we put words together to make up
sentences.
• Variables are the “words” for the C language.
– In a program, variables have a precise definition
and usage, but that definition and usage are
different for each program.
• Names of variables:
– Composed of letters, digits, and the underscore
(“_”)
• character. (NO spaces; use underscore
instead)
– The first character must be a letter
– Only the first 32 characters matter

12. Variables
• Names are case sensitive, so upper-
and lowercase letters are not
interchangeable.
– Uppercase is different from lowercase, so
the identifiers, PI, pi, and Pi specify different
variables.
– Use different names for variables and do
not depend on case differences.
• Variables beginning with an underscore
(_) are usually reserved for internal and
system names.

13. Variables
• The following are examples of valid
variable names in C:
x c14
area electron_mass
TEMPERATURE pi
radius average
growth_rate _setport

14. Variables
• The following are examples of invalid
variable names in C:
3rd_entry /*begins with a number */
the end /*contains a space */
int /* reserved word */

15. Variables
• Avoid variable names that are similar.
e.g. total totals
• Remember that only the first 32 characters
matter:
– So the following variable names are identical
thisisasuperduperlongname12345678
thisisasuperduperlongname12345679
• Multiple-word variables can help make a
program more readable.
e.g. growth_rate rather than growthrate

16. Some Rules about Variable Names
1. Use simple, descriptive variable names.
2. Never use l (lowercase L) or O (uppercase O)
as variable names.
3. Don’t use variable names that differ by only
one or two characters. Make every name
obviously different from every other name.
4. For variable names consisting of two words
joined together, separate the words by “_”:
e.g. growthrate → growth_rate

17. Some Rules about Variable Names
5. Use similar names for variables that perform
similar functions.
e.g. start_point, end_point, middle_point
6. When creating a two word variable name
where the words can be put in any order,
always put the more important word first.
e.g. max_elevation instead of elevation_max
7. Use standard prefixes:
e.g. n_ (“number of”) → n_points
8. Follow every variable declaration with a
comment that defines it.

18. Global vs. Local Variables
• Local variables are declared within the body of
a function, and can only be used within that
function.
• Alternatively, a variable can be declared
globally so it is available to all functions.
– A global variable declaration looks normal, but
is located outside any of the program's
functions. This is usually done at the beginning
of the program file, but after preprocessor
directives. The variable is not declared again in
the body of the functions which access it.

19. Initializing Variables
• C allows variables to be initialized in the
declaration statement.
• The following statement declares the
integer variable count and initializes it to 0.
int count=0;

20. Variable Declarations
• Variables must generally be declared before
use.
int lower;
int upper;
int step;
char a;
char b;
• Variables with the same type can be grouped
together:
int lower, upper, step;
char a, b;

21. Data Types
• In C each variable has a type as well as
a value:
– the type determines the meaning of the
operations that can be applied to the value
– For example, declarations like
int i, j;
double d;
float x;
determine the operations and space
requirements of the variables.

22. Data Types
• The data type of every variable in a C
program must be declared before that
variable can appear in a statement.

23. Data types
• There are four basic data types in C:
char – a single byte, capable of
holding one character
int – an integer
float – single-precision floating point
double – double precision floating point
• There is NO boolean (true/false) type in C –
the conditions true and false are represented
by the integer values 1 and 0, respectively.

24. Sizes of data types
On a 16-bit System
C Type Size
(bits)
Size
(Byte)
Format Pacifiers
char 8 1 %c
unsigned char 8 1 %u
int 16 2 %d
short int 16 2 %d
long int 32 4 %ld
float 32 4 %f
double 64 8 %lf
long double 80 10 %Lf

25. Comments
• Comments are an important part of any
program. They help person writing a
program and anyone else who must read
the source file, understand what’s going
on.
• The compiler ignores comments so they
can be read by humans but are invisible to
the compiler.

26. Comments Syntax
Comments start with a slash-asterisk “ / * “
and ends with an asterisk-slash “ */ “.
e.g.
/* My first Program */
#include<stdio.h>
void main (void)
{ /* Start of Main Function */
printf(“Hello World”);
} /* End of Main Function */

27. Some comments on comments
• Comments
– Comments are delimited by `/*' (slash-star)
and `*/ (star-slash)'.
– Any text between the first `/*' and the next
`*/' is ignored by the compiler, irrespective
of the position of line breaks.
– Comments can span multiple lines
– Any string of symbols placed between the
delimiters /* and */
– Comments can’t be nested (be careful!)

28. Some comments on comments
• Here are some examples of the valid use of
comments:
/* This is a comment */
/* here is another one
that spans two lines */
i = /* a big number */ 123456;

29. Some comments on comments
i = 123456; /* a comment starts
here
i = i + 2; this statement is
also part of the comment */

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