Lecture 3 Style and Variables

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Lecture 3Style and Variables

• ENGR17 Engineering Programming
• Section 1
• Fall 2001

9/13/01
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Outline

• Style
• Variables
• Literal Constants
• Select an appropriate name, data type, and
  initial value for a variable
• Create and initialize a variable
• Perform explicit type conversion

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Style
There is no programming language, no matter how
structured, that will prevent programmers from
writing bad programs. -- L. Flon

• Style is the most important part of programming!
• It separates the Software Engineer from the
  programmer
• Engineers have a reputation of writing awful
  programs
• Mainly due to lack of style
• Programs serve two purposes
• Set of instructions for the computer
• Clear description of what the program does for
  the programmer
• Make your programs easy to understand

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Comments

• More important than the code itself
• Makes the program readable for you and others
• Two basic flavors
• Multi-Line / comment // this is a
  multi-line comment /
• Single-Line // comment// single-line comment
• Which is better
• Working program without comments
• Broken program with comments
• Comments should explain everything the programmer
  needs to know about the program

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Where to Put Comments

• Header Comment at top of each file
  (multi-line)/ Your Name Your USD ID
  number ENGR 17 HW1 September 7, 2001
  Program ltyour initialsgt_HW1.cpp
  ltprogram descriptiongt /
• Comments before each function or class//
  Function name Main// This function starts the
  program ...int main()

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Where to Put Comments (cont.)

• Comments next to each variable (single-line)float
  currentPay // the users current pay
• Comments above each significant section of
  code/ The next section of code gathers input
  from the user and stores it in variables ///
  Input current pay from consolecin gtgt
  currentPay// Input raise rate from consolecin
  gtgt raiseRate
• Comments above each significant line of code//
  output the new pay to the consolecout ltlt "New
  Pay is " ltlt newPay

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Variable Names

• Badfloat xfloat y
• Betterfloat currentPayfloat raiseRate
• Goodfloat currentPay // current yearly
  payfloat raiseRate // percentage raise
• Three Styles
• All Lowercase (Variables) current_pay
• All Uppercase (Constants) CURRENT_PAY
• Mixed-Case (Local/Global Variables)currentPay
  CurrentPay

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Complexity

• Do not be clever
• Cleverness makes for unreadable/unmaintainable
  code
• Badwhile (\n ! p q)
• Goodwhile (1) destination_ptr
  source_ptr destination_ptr
  source_ptr if ((destination_ptr-1)
  \n) break // exit loop if done
• Good compiler will generate same machine code for
  both

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Indentation

• Indent for each new block or condition
• Development Environment will take care of this
  for you
• Two Styles
• Short formif (totallt 0) cout ltlt "Total is
  less than zero\n" else cout ltlt "Total is
  greater than or equal to zero\n"
• Long formif (totallt 0) cout ltlt "Total is
  less than zero\n"else cout ltlt "Total is
  greater than or equal to zero\n"

Use the short form!
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Other Styles Issues

• Single function should be no longer than 1 or 2
  pages
• Avoid complex logic and nested ifs
• Avoid long statements
• Split single code lines into multiple small ones
• Make sure the program is simple and easy to
  understand
• ReminderStyle is the most important part of
  programming!(thats why we talked about it
  first)

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Program Components

• Variables
• Memory locations for temporary data storage
• Constants
• Named
• Literal
• Arithmetic operators
• Used in equations
• Functions

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Using Variables to Store Information

• Data may be
• Entered by user at keyboard
• Read from a file
• The result of a calculation
• Contents can change as program runs
• Select an appropriate name, data type, and value
  for it
• Variable Names
• Use a descriptive name
• Be consistent
• Follow specific rules

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Rules for Names
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Example Names
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Key Words
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Selecting an Appropriate Data Type
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Special Characters
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Guidelines for Selecting Data Type

• Consider the memory requirement
• Short Integer, Integer, or Long Integer
• Use when a variable will always contain whole
  numbers
• Float or Double
• Use to store numbers with a decimal fraction
• Character
• Assign if the variable will always contain one
  character of data
• General guidelines
• Almost never use a short
• Usually use long and double

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Using Literal Constants

• An item of data whose value does not change while
  the program is running
• Types of literal constants
• Numeric
• Character
• String

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Using Literal Constants
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Declaring a Variable

• Must declare/create a variable before it can be
  used
• Enter a C statement that specifies
• Data type and name of the variable
• Beginning, or initial value of the variable
• Syntax
• datatype variablename initialvalue
• Declare variables at the beginning of the main
  function
• Initialize the variables that you create

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Declaring a Variable
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Initializing Variables

• Initialize a variable in the statement that
  creates the variable
• Assign a literal constant to it of the same data
  type
• If you dont initialize the variable, it will
  contain garbage until a value is assigned to it
• Examples// declare variableschar response
  short hours 0int sales 0long
  total_sales 0

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Explicit Type Conversion

• Precede the item with the C keyword that
  represents the desired data type enclose the
  keyword in parentheses
• Also known as type casting
• Example// declare variablesdouble salesAmt
  0.0float taxRate (float) 0.0

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Summary of Key Points Variables

• Memory location for temporary data storage
• Contents can change as program runs
• Assign both a data type and a name
• Initialize each variable in its declaration
  statement
• Short Integer, Integer, and Long Integer to the
  integer 0
• Double variables to the Double type number 0.0
• Character variables to a space enclosed in single
  quotes
• Use a type cast to initialize a Float variable

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Questions

• What is the most important part of programming?
• Style
• What two purposes does a program serve?
• Set of instructions for the computer
• Clear description of what the program does for
  the programmer
• Give an example of the two types of comments.
• Multi-line
• Single line
• What type of variable to store letters?
• char
• What type of variable to store integers between 0
  64?
• short

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Questions

• What type of variable to store integers between
  -64,000 and 64,000?
• int or long
• What type of variable to store the number
  1.37e100?
• double
• What type of variable to store the number 57.3?
• float
• Whats the new line character?
• \n
• Whats the tab character?
• \t
• How do I print the message I said, Not me
• cout ltlt I said, \Not me\\n