Programming Logic and
Design
Fourth Edition, Comprehensive
Chapter 4
Designing and Writing
a Complete Program
Objectives
• Plan the mainline logic for a complete program
• Describe typical housekeeping tasks
• Describe tasks typically performed in the main loop
of a program
• Describe tasks performed in the end-of-job module
• Understand the need for good program design
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Objectives (continued)
• Appreciate the advantages of storing program
components in separate files
• Select superior variable and module names
• Design clear module statements
• Understand the need for maintaining good
programming habits
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Understanding the Mainline Logical
Flow Through a Program
• Understand what the goals are
– Ask the user to clarify if necessary
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Understanding the Mainline Logical
Flow Through a Program (continued)
• Ensure you have all the data required to produce
the desired output
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Understanding the Mainline Logical
Flow Through a Program (continued)
• Understand the big picture first
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Understanding the Mainline Logical
Flow Through a Program (continued)
• Procedural program: one procedure follows
another from beginning to end
• Mainline logic has three distinct parts:
– Housekeeping: steps to get ready
– Main loop: instructions executed for every input record
– End-of-job: steps taken at end of program
• Break the logic down into at least three modules
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Understanding the Mainline Logical
Flow Through a Program (continued)
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Understanding the Mainline Logical
Flow Through a Program (continued)
• Modularization of the program:
– Keeps the job manageable
– Allows multiple programmers to work simultaneously
– Keeps the program structured
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Housekeeping Tasks
• Housekeeping tasks: include all steps that occur
at the beginning of the program
– Declare variables
• Assumption in this book is the variables will be global
variables!
– Open files
– Perform one-time-only tasks such as printing
headings
– Read the first input record
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Declaring Variables
•
•
•
•
•
Assign identifiers to memory locations
Specify the name and data type
Use meaningful names and follow standards
Prefixes may be used to group related variables
Declare a variable for each field in a data file
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Declaring Variables (continued)
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Declaring Variables (continued)
• Group name:
– Name for a group of associated variables
– Can handle the entire group with a single instruction
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Declaring Variables (continued)
• Initializing (or defining) the variable: providing an
initial value
• Some languages provide default initial values
• Other languages leave variables with an unknown or
garbage value
• Variables representing data fields in files do not need
to be initialized
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Declaring Variables (continued)
• Can use variables for report headings
• Embed any required spaces
• Heading can be printed using these variables
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Declaring Variables (continued)
• Every language provides methods for: [ NOT TRUE! ]
– Advancing the paper to top of page
– Printing single, double, or triple spaced lines
• Use annotation symbol to show variables
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Opening Files
• Specify file name and path (location)
• Issue a file open command
• If no input file is opened, input may be accepted from the
standard input device (e.g., keyboard)
• You must open both input and output files to be used,
including printer output device
• If no output file is opened, standard output device (e.g.,
monitor) may be used
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A One-Time-Only Task
Printing Headings
• Printing headings for reports usually is done at
beginning of the program
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Reading the First Input Record
• Reading the first input record is the last housekeeping task
• Interactive application:
– Interacts with users via keyboard or mouse input
– Program pauses when the read command is executed until the
user enters data
• Delimiter: a character designated as a separator between
data values
• Prompt: an output statement that asks the user to enter
specific data
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Reading the First Input Record
(continued)
• Interactive input:
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Reading the First Input Record
(continued)
• Input from a data file:
• Input from a data file using a group name:
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Checking for the End of File ( EOF )
• First task after housekeeping
• For an interactive program, EOF may be determined when:
– User enters a predetermined sentinel value
– User selects a screen option using a mouse
• For input from a file, the input device recognizes EOF
• If no data in the file, EOF occurs on the first read
• If there is data, each record is processed before the next read
occurs
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• Handling the report headings in a separate module:
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Writing the Main Loop
•
Each data record passes through the main loop
once
•
Inventory program main loop steps:
1. Calculate the profit for an item
2. Print the item’s information on the report
3. Read the next inventory record
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Writing the Main Loop (continued)
• Must declare additional variables for calculation
results
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Writing the Main Loop (continued)
• Detail lines are printed one line at a time:
• Calculations can be done within the print statement:
• Work variable (or work field): a variable used to
temporarily hold a calculation ( aka calculated field )
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Performing End-of-Job Tasks
• End-of-job tasks may include:
– Printing summaries or grand totals
– Printing “End of Report” message
– Closing any open files
• Footer line (or footer): end-of-job message line
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Understanding the Need for Good
Program Design
• Good design is:
– Critical for very large programs
– Needed to guarantee that components work together
properly
• Well-designed program modules should work:
– As stand-alone modules
– As part of larger systems
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Storing Program Components in
Separate Files
• Large programs may contain hundreds of variables and
thousands of lines of code
• Manage lengthy programs by breaking into modules
• Many languages allow program components to be stored in
separate files
• Storing components separately simplifies reuse
• Accessing modules from separate files is done with a statement
like include, import, or copy
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Storing Program Components in
Separate Files (continued)
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Storing Program Components in
Separate Files (continued)
• Advantages of storing components separately:
– Simplifies reuse
– Can be provided in compiled form only, to hide details
• Implementation hiding: hiding details of how a
program or module works
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Selecting Variable and Module Names
• Using meaningful names:
– Improves code readability
– Is a form of self-documenting the program
• Use pronounceable names
• Commonly used abbreviations are ok (e.g., SSN)
• Avoid digits in a name to avoid confusing:
– Zeros and O’s
– Ones and lowercase L’s
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Designing Clear Module Statements
• Follow these rules:
– Select good identifier names
– Avoid confusing line breaks
– Use temporary variables to clarify long statements
– Use constants where appropriate
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Avoiding Confusing Line Breaks
• Free-form coding allows programmer to decide
where to break lines of code
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Using Temporary Variables to Clarify
Long Statements
• Use temporary variables to store intermediate
results
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Using Constants Where Appropriate
• Named constant: a constant whose value never
changes during execution
• Advantages
– Improves code readability
– If the value changes later, there is only one place in
the code to make the change
• Usually written with all uppercase letters
– ATHLETIC_FEE
– TUITION_PER_CREDIT_HOUR
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Using Constants Where Appropriate
(continued)
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Maintaining Good Programming Habits
• Program will be better written if you plan before you
code
• Walk through program logic on paper before coding
(desk-checking)
• Select good variable and module names for
readability
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Summary
• Three steps to designing a good program:
– Understand the output that is required
– Ensure you have the necessary input data
– Plan the mainline logic
• Housekeeping tasks done at the beginning of the program:
declaring variables, opening files, printing headings
• Main loop is controlled by EOF decision
• Each data record passes through the main loop once
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Summary (continued)
• End-of-job steps done at end of the program: printing
summaries, closing files
• Good design becomes more critical as programs get larger
• Program components can be stored in separate files
• Select meaningful, pronounceable names
• Avoid confusing line breaks, use temporary variables, and
use constants where appropriate
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