l 08 Variables Inc

7/27/2019 l 08 Variables Inc

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CMSC 104, Version 9/01 1

Variables in C

Topics

Naming Variables

Declaring Variables

Using Variables

The Assignment Statement

Reading

Sections 2.3 - 2.4


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What Are Variables in C?

Variables in C have the same meaning asvariables in algebra. That is, they representsome unknown, or variable, value.

x = a + b

z + 2 = 3(y - 5)

Remember that variables in algebra arerepresented by a single alphabeticcharacter.


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Naming Variables

Variables in C may be given representationscontaining multiple characters. But there arerules for these representations.

Variable names in Co May only consist of letters, digits, and

underscores

o May be as long as you like, but only the first 31characters are significant

o May not begin with a number

o May not be a C reserved word (keyword)


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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 signedsizeof static

struct switch

typedef unionunsigned void

volatile while


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Naming Conventions

C programmers generally agree on thefollowing conventions for naming variables.

o Begin variable names with lowercase letters

o Use meaningful identifiers

o Separate words within identifiers with

underscores or mixed upper and lower case.

o Examples: surfaceArea surface_Area

surface_areao Be consistent!


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Naming Conventions (cont)

Use all uppercase forsymbolic constants(used in #define preprocessor directives).

Examples:

#define PI 3.14159

#define AGE 52


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Case Sensitivity

C is case sensitiveo It matters whether an identifier, such as a

variable name, is uppercase or lowercase.

o Example:area

Area

AREAArEa

are all seen as different variables by thecompiler.


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Which Are Legal Identifiers?

AREA area_under_the_curve3D num45

Last-Chance #values

x_yt3 pi

num$ %done

lucky***


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Declaring Variables

Before using a variable, you must give thecompiler some information about thevariable; i.e., you must declare it.

The declaration statement includes thedata type of the variable.

Examples of variable declarations:

int meatballs ;float area ;


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Declaring Variables (cont)

When we declare a variableo Space is set aside in memory to hold a value of

the specified data type

o That space is associated with the variable name

o That space is associated with a unique address

Visualization of the declaration

int meatballs ;meatballs

FE07

garbage


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More About Variables

C has three basic predefined data types:

Integers (whole numbers)

o int, long int, short int, unsigned int

Floating point (real numbers)

o float, double

Characters

o char

At this point, you need only be concernedwith the data types that are bolded.


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Using Variables: Initialization

Variables may be be given initial values, orinitialized, when declared. Examples:

int length = 7 ;

float diameter = 5.9 ;

char initial = A ;

7

5.9

A

length

diameter

initial


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Using Variables: Initialization (cont)

Do not hide the initializationo put initialized variables on a separate line

o a comment is always a good idea

o Example:int height ; /* rectangle height */

int width = 6 ; /* rectangle width */

int area ; /* rectangle area */

NOT int height, width = 6, area ;


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Using Variables: Assignment

Variables may have values assigned to them throughthe 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 (theexpression) to the variable on the lefthand side.

Examples:

diameter = 5.9 ;area = length * width ;

Note that only single variables may appear on thelefthand side of the assignment operator.


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Example: Declarations and Assignments (contd)

printf (Its depth at sea: \n) ;printf ( %d fathoms \n, fathoms) ;printf ( %d feet \n, feet) ;printf ( %d inches \n, inches) ;

return 0 ;}


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Enhancing Our Example

What if the depth were really 5.75 fathoms?Our program, as it is, couldnt handle it.

Unlike integers, floating point numbers can

contain decimal portions. So, lets usefloating point, rather than integer.

Lets also ask the user to enter the number

of fathoms, rather than hard-coding it in.


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Enhanced Program

#include

int main ( ){

float inches, feet, fathoms ;

printf (Enter the depth in fathoms : ) ;

scanf (%f, &fathoms) ;

feet = 6 * fathoms ;

inches = 12 * feet ;

printf (Its depth at sea:\n) ;

printf ( %f fathoms\n, fathoms) ;printf ( %f feet\n, feet) ;

printf ( %f inches\n, inches) ;

return 0 ;

}


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Final Clean Program

#include

int main( ){

float inches ; /* number of inches deep */float feet ; /* number of feet deep */

float fathoms ; /* number of fathoms deep */

/* Get the depth in fathoms from the user */

printf (Enter the depth in fathoms : ) ;

scanf (%f, &fathoms) ;/* Convert the depth to inches */

feet = 6 * fathoms ;

inches = 12 * feet ;


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Final Clean Program (cont)

/* Display the results */

printf (Its depth at sea:\n) ;

printf ( %f fathoms\n, fathoms) ;

printf ( %f feet\n, feet);printf ( %f inches\n, inches);

return 0 ;

}


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Good Programming Practices

Place each variable declaration on its ownline with a descriptive comment.

Place a comment before each logicalchunk of code describing what it does.

Do not place a comment on the same line ascode (with the exception of variabledeclarations).

Use spaces around all arithmetic andassignment operators.

Use blank lines to enhance readability.


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CMSC 104 Version 9/01 22

Good Programming Practices (cont)

Place a blank line between the last variabledeclaration and the first executablestatement of the program.

Indent the body of the program 3 to 4 tabstops -- be consistent!

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