Variables in C

1
Variables in C

• Topics
• Naming Variables
• Declaring Variables
• Using Variables
• The Assignment Statement
• Reading
• Sections 2.3 - 2.4

2
Preprocessor

• All preprocessor directives (preprocessor
  commands) begin with .
• The preprocessor reads and may change source in
  RAM before a program is compiled. But the source
  code as stored on disk is not modified. Possible
  actions
• Include other files to be included in your C
  code.
• Performs various text replacements.
• Strips comments and whitespace from the code.
• For example
• include ltstdio.hgt The preprocessor inserts the
  contents of the stdio.h at this place in your
  code. ltgt means the file is located in the
  standard include file directory.
• define PI 3.14159 The preprocessor replace
  every occurrence of PI with 3.14159 before it
  passes the file to the compiler.

3
Output Data on Screen

• Use printf statement a function call
• To prompt user that the program is waiting for
  input data. Printf(Enter number of exams
  \n)
• Arguments to the function (format string and
  variables) are listed inside parentheses ().
• Multiple arguments are separated by commas ,.
• \n denotes a newline.
• To output results to user on the screen.
• Printf(Your grade is d\n, grade)

4
Input Data into Variables

• The scanf function can be used to obtain input
  data from a user (usually from keyboard) or input
  file.
• scanf ignores leading whitespace, i.e.
• Tabs
• Spaces
• ltEntergt or ltReturngt key
• scanf (d, numExams)
• Take users input and put it in the memory
  location of numExams.
• First argument Format control string type of
  input data
• d decimal integer
• f floating number
• s string
• The second argument the memory location
  (address) of numExams. The symbol is the
  address operator.

5
Naming Variables

• Variable names in C
• May only consist of letters, digits, and
  underscores.
• May be as long as you like, but only the first 31
  characters are significant
• May not begin with a number.
• May only begin with letters or underscores.
• May not be a C reserved word (keyword).

6
Reserved Words (Keywords) in C

• auto break
• case char
• const continue
• default do
• double else
• enum extern
• float for
• goto if

• int long
• register return
• short signed
• sizeof static
• struct switch
• typedef union
• unsigned void
• volatile while

7
Naming Conventions

• C programmers generally agree on the following
  conventions for naming variables.
• Begin variable names with lowercase letters
• Use meaningful identifiers
• Separate words within identifiers with
  underscores or mixed upper and lower case.
• Examples surfaceArea surface_Area
  surface_area
• Be consistent!

8
Naming Conventions (cont)

• Use all uppercase for symbolic constants (used
  in define preprocessor directives).
• Examples
• define PI 3.14159
• define AGE 18

9
Case Sensitivity

• C is case sensitive
• It matters whether an identifier, such as a
  variable name, is uppercase or lowercase.
• Example
• area
• Area
• AREA
• ArEa
• are all seen as different variables by the
  compiler.

10
Which Are Not Legal Identifiers?

• AREA area_under_the_curve
• 3D num45
• Last-Chance values
• x_yt3 pi
• num done
• lucky

11
Declaring Variables

• Before using a variable, you must give the
  compiler some information about the variable
  i.e., you must declare it.
• The declaration statement includes the data type
  of the variable followed by variable name.
• Examples of variable declarations
• int width
• float area
• Declarations must be placed at the beginning of a
  function before any executable statements.

12
Declaring Variables (cont)

• Variables in C correspond to locations (address)
  in the computers memory.
• When we declare a variable
• Space is set aside in memory to hold a value of
  the specified data type.
• That space is associated with the variable name.
• That space is associated with a unique address.
• Every variable has a name, type, and value.
• For example,
• int numExams
• numExams 2

13
More About Variables

• C has three basic predefined data types
• Integers (whole numbers)
• int, long int, short int, unsigned int
• Floating point (real numbers)
• float, double
• Characters
• char
• At this point, you need only be concerned with
  the data types that are bolded.

14
Integer Variable

• int, long int, short int, unsigned int
• Int is machine-dependent, can be positive, 0, or
  negative integer number.
• Each compiler is free to choose appropriate size
  for its own hardware, provided size of short lt
  size of int lt size of long. Typically short is
  16 bits, long is 32 bits, so int is either 16 or
  32 bits.
• Unsigned int is either 0 or positive integer.

15
Using Variables Initialization

• Variables may be be given initial values, or
  initialized, when declared. Examples
• int length 7
• float diameter 5.9
• char initial A

length
7
diameter
5.9
initial
A
16
Using Variables Initialization (cont)

• Do not hide the initialization
• put initialized variables on a separate line
• a comment is always a good idea
• Example
• int height / rectangle height /
• int width 2 / rectangle width /
• int area / rectangle area /
• NOT int height, width 2, area

17
Using Variables Assignment

• Variables may have values assigned to them
  through the use of an assignment statement.
• Such a statement uses the assignment operator
  .
• This operator does not denote equality. It
  assigns the value of the righthand side of the
  statement (the expression) to the variable on the
  lefthand side.
• Examples
• width 2
• area length width
• Note that only single variables may appear on
  the lefthand side of the assignment operator.

18
Example Declarations and Assignments

• include ltstdio.hgt
• int main( )
• int length, width, area
• width 2
• length 4 width
• area length width

length
garbage
width
garbage
area
garbage
width
2
length
8
area
16
19
Example Declarations and Assignments (contd)

• printf (The dimension of a rectangle )
• printf ( width d , width)
• printf ( length d \n, length)
• printf (The area of a rectangle d. \n,
  area)
• return 0

20
Enhancing Our Example

• What if the width were really 2.5? Our program,
  as it is, couldnt handle it.
• Unlike integers, floating point numbers can
  contain decimal portions. So, lets use floating
  point, rather than integer.
• Lets also ask the user to enter the value of
  width, rather than hard-coding it in.

21
Enhanced Program

• include ltstdio.hgt
• int main ( )
• float length, width, area
• printf (Enter length of a rectangle )
• scanf (f, length)
• printf (Enter width of a rectangle )
• scanf (f, width)
• area length width
• printf (The dimension of a rectangle \n)
• printf ( width d \n, width)
• printf ( length d \n, length)
• printf (The area of a rectangle f. \n,
  area)
• return 0

22
Final Clean Program
include ltstdio.hgt int main( ) / Variable
Declarations / float length /
length of a rectangle in decimal / float
width / width of a rectangle in decimal
/ float area / area of a
rectangle in decimal / / Get the
dimension of a rectangle from the user /
printf (Enter width of a rectangle )
scanf (f, width) printf (Enter length
of a rectangle ) scanf (f, length)
/ Compute area of a rectangle / area
length width
23
Final Clean Program (cont)

• / Display the results /
• printf (The dimension of a rectangle is \n)
  
• printf ( width f \n, width)
• printf ( length f \n, length)
• printf (The area of a rectangle f \n,
  area)
• return 0

24
Sample Run (cont)

• Sample output run
• Create a source code product.c xemacs
  product.c
• Compile source code product.c gcc ansi Wall
  product.c
• Run and test product.c a.out
• Sample screen dump
• Enter width of a rectangle 3.2
• Enter length of a rectangle 5.4
• The dimension of a rectangle is
• width 3.200000
• length 5.400000
• The area of a rectangle 17.280001

25
Good Programming Practices

• Place each variable declaration on its own line
  with a descriptive comment.
• Place a comment before each logical chunk of
  code describing what it does.
• Do not place a comment on the same line as code
  (with the exception of variable declarations).
• Use spaces around all arithmetic and assignment
  operators.
• Use blank lines to enhance readability.

26
Good Programming Practices (cont)

• Place a blank line between the last variable
  declaration and the first executable statement of
  the program.
• Indent the body of the program 3 to 4 spaces --
  be consistent!

27
Assignment and Next

• Read 2.3 2.4, and 2.5 (optional)
• Next
• Arithmetic Operators