Chap10

General Issues in Using Variables Chapter:10

Contents
• ■ 1 Data Literacy
1.1 Data Literacy Test
• ■ 2 Making Variable Declarations Easy
2.1 Implicit Declarations
2.2 Explicit Declarations
■ 3 Guidelines for Initializing Variables
3.1 Guidelines for avoiding initialization problems:
• ■ 4 Scope
• ■ 5 Persistence
• ■ 6 Binding Time
• ■ 7 Relationship Between Data Types and Control Structures
• ■ 8 Using Each Variable for Exactly One Purpose
• 8.1 General Considerations In Using Data
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Chp:10 General issues in using varibale

Data Literacy
Defination:
Data literacy is the ability to read,create, and communcate
data as infomation and has been formally described in varying
ways.
1.1
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Data literacy
Data literacy Terms
Abstract data type literal Array
local variable bitmap lookup table
boolean variable member data B-tree
pointer character variable private
container class retroactive synapse double precision
referential integrity elongated stream stack
enumerated type string Floating point
Structured Variable Heap Tree
Index Typedef integer
Union Linked List Value Chain
Named Constant Variant
1.2
These are some Common Terms that use in programming language
1/29/2016Chp:10 General issues in using varibale

Making Variable Declaration Easy
Two types of Declaration occur
1. Implicit Declaration
(use in visual Basic,Fortron,PHP)
2. Explicit Declaration
(use in java,C++)
2.1
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Implicit Declaration:
• Some languages compiler implicitly declare, which means that you can
use every variable without Declaration. You can remove this
requirement and permit explicit declaration.
• In implicit declaration variable automatically convert to that type
which type of value assign to that variable.
2.2
Explicit Declaration:
• Most of the language use explicit declaration, which means that
you can not use any variable without Declaration. You should declare
the type of variable before use it .After this compiler assign the
memory to that variable according to that type.
• In explicit declaration user should follow the type of variable that he
declare otherwise compiler give error Message.
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Explicit Declaration is Recomended.
2.3
Which one is the best implicit or explicit??

What do you do if you program in a language with implicit
declarations?
Some suggestions:
1. Turn off implicit declaration
2. Declare all variables
3. Use naming conventions
4. Check Variable names
2.4Making Variable Declaration Easy

Guidelines for Initializing Variables
Improper data initialization is one of the most fertile sources
of error in computer programming.
3.1
 Reasons:
 The variable has never been assigned a value.
 Part of the variable has been assigned a value and part has not.
 The value in the variable is outdated .

Following are guidelines for avoiding initialization
problems:
 Initialize each variable as it's declared.
 Use const/final when possible.
 Pay attention to counters and accumulators.
 Initialize a class's member data in its constructor.
 Check the need for re-initialization.
 Take advantage of your compiler's warning messages
 Check input parameters for validity .
 Use a memory-access checker to check for bad pointers.
 Initialize working memory at the beginning of your program.
3.2Guidelines for Initializing Variables

Scope
“Scope” is a way of thinking about a variable’s celebrity
status: how famous is it?
• It refers to the area to which your variables are known and
can be referenced throughout a program.
• A variable with limited or small scope is known in only a
small area of a program
• A variable with large scope is known in many places in a
program
4.1

Scope
 We can measure scope by:
 Span.
It is a method of measuring how close together the
references to a variable are.
 Live Time.
The total number of statements over which a variable is live
4.2

Scope 4.3
 Measuring span and life time
1. a = 0;
2. a++; Life time = (4-1 +1) = 4
3. a+= 10; Average span = (0+0+0)/3 = 0;
4. a = 3;
1. a = 0;
2. b= 1; Life time = (4-1 +1) = 4
3. c = 0; Average span = 2/1 = 2;
4. a = b + c;

Scope 4.4
 Why we try to restrict live time?
 You can concentrate on a smaller section of code.
 Code more readable.
 reduces the chance of initialization errors .
 Easy to modify.

Scope 4.5

Scope 4.6
 Initialize variables used in a loop immediately before the
loop .
 Don't assign a value to a variable until just before the
value is used.
 Break groups of related statements into separate routines .
 Begin with most restricted visibility, and expand the
scope if necessary .
 Keep both span and live time as short as you can.
Guideline for minimizing Scope:

Persistence 5.1
 Persistence is life span of a piece of data.
 Problem:
When you assume variable is persist but it's not .
 Solution:
 Set variables to "unreasonable values" when they doesn’t persist.
 Use assertions to check critical variables.
 Declare and Initialize all data right before it's used.

Binding Time 6.1
 The time at which the variable and its value
are bound.
The later you bind, the more flexibility you get
and the higher complexity you need .
titleBar.color = ReadTitleBarColor( )
titleBar.color = 0xFF;
titleBar.color = TITLE_BAR_COLOR;
Run Time
Compile Time
Code-Writing Time

Relationship Between Data Types
and Control Structures
• Data types and control structures relate to each other in well-
defined ways that were originally described by the British
computer scientist Michael Jackson
• Jackson draws connections between three types of data and
corresponding control structures:
1. Sequential data
2. Selective data
3. Iterative data
7.1

Using Each Variable for Exactly
One Purpose
• Use each variable for one purpose only
it’s sometimes tempting to use one variable in two different
places for two different activities. Usually, the variable is
named inappropriately for one of its uses or a “temporary”
variable is used in both cases (with the
8.1
// Compute roots of a quadratic equation
temp = Sqrt( b*b - 4*a*c );
root[0] = ( -b + temp ) / ( 2 * a );
root[1] = ( -b - temp ) / ( 2 * a );
...
// swap the roots
temp = root[0];
root[0] = root[1];
root[1] = temp ;

Using Each Variable for Exactly
One Purpose 8.2
 Avoid hybrid coupling.
Double use is clear to you but it won't be to someone
else.
 Make sure all declared variables are used.

Checklist: General Considerations
In Using Data : 8.3
Initializing Variables
 Does each routine check input parameters for validity?
 Does the code declare variables close to where they're first
used?
 Does the code initialize variables as they're declared, if
possible?
 Are counters and accumulators initialized properly and, if
necessary, reinitialized each time they are used?
 Does the code compile with no warnings from the compiler?

Checklist: General Considerations
In Using Data Cont.. 8.4
Other General Issues in Using Data
 Do all variables have the smallest lift time possible?
 Are references to variables as close together as possible?
 Are all the declared variables being used?
 Are all variables bound with balance between the flexibility
and the complexity associated?
 Does each variable have only one purpose?
 Is each variable's meaning explicit, with no hidden meanings?

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