More About Loops Program Design and Testing

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Chapter 8
More About Loops Program Design and Testing
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Outline

• Guidelines for constructing a loop structure
• Exit a loop using the break statement
• Nested loop
• Designing and writing programs

• Chapter 7
• Paid attention to fixed-step and data input loops
• Chapter 8
• Loops that exit on completion of the task
• Dont know ahead of time how many times to loop
• Have to recognize exit criterion when it comes up
• Middle-exit loops using break statement

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Steps in Designing the Loop

• If 500 is invested at a rate of 4.5
  compounded annually, after how many years will
  the balance first exceed 1,000? (Page 178)
• What variables are needed?
• Year counter (int)
• Balance in the account (double)
• What must happen before the loop?
• Initialize Year to 0
• Initialize Balance to 500.0
• What happens after the loop?
• Print number of years
• Print balance

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Example (contd)

• What is the loop test?
• Is Balance lt 1000.00 or gt 1000.00?
• Any reason to use a bottom-exit loop (do-while)?
  No.
• Test is while (Balance lt 1000.0)
• What is in the body of the loop?
• Increase Balance by (Balance 0.045)
• Increase Year by 1
• Pseudocode
• Initialize Year to 0 and Balance to 500.0
• while (Balance lt 1000.0)
• Increase Balance by (Balance 0.045)
• Increase Year by 1
• Print Year Balance

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Coding the Loop

• int main ()
• const double RATE 0.045 // 4.5
• double Balance 500.0
• int Year 0
• while (Balance lt 1000.0)
• Balance Balance RATE Balance
• Year
• cout ltlt Year ltlt " " ltlt Balance ltlt endl //
  DEBUG
• cout ltlt "500 invested at " ltlt RATE
• ltlt " takes " ltlt Year ltlt " years to grow
  to "
• ltlt Balance ltlt " (over 1,000)." ltlt endl
• return 0

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Testing the Loop

• How do you know whether its right?
• Do you have a compound interest calculator?
• If you have Excel, you do
• Heres where the debug code comes in handy
• Tells you where program got to if it doesnt come
  back
• Tells you the steps it took so you can validate
  its operation
• What Can Go Wrong?
• What if you had used lt rather than lt as a
  test?
• What if you had said Balance RATE Balance?
• What if you had initialized Year to 1, instead of
  0? (Off-by-One errors)
• How difficult would it be to discover this one?

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What to check?

• Initialization
• Check the initial value of loop control or other
  key variables
• Order
• Check where the loop control variables be updated
• Relational Operator
• Check the relational operator in the loop exit
  (condition) test
• lt or lt
• or !
• First and Last Execution of Loop Body

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Multiple Reasons for a Loop Exit

• When there are two or more possible reasons for
  exiting a loop, a compound condition is used in
  the loop exit test.
• Use an if else statement after the loop to
  determine which condition caused the exit

do cout ltlt "The residency status should be
R or N. \n" cout ltlt "Please enter the
residency status (R or N) " cin gtgt
chResStatus while (chResStatus ! 'R'
chResStatus ! 'N') if (chResStatus R)
cout ltlt The student is resident. ltlt
endl else cout ltlt The student is
nonresident. ltlt endl
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The Middle-Exit Loop

• Uses the break Statement to exits innermost loop
  construct
• int Number, Count 0
• int Sum 0
• while (true)
• cout ltlt "Enter a number "
• cin gtgt Number
• if (Number lt 0)
• break
• Sum Number
• Count
• cout ltlt Sum is ltlt Sum ltlt endl
• cout ltlt Count is ltlt Count ltlt endl
• Another Example capital2.cpp

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Nested Loops - Loop Within a Loop
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Nested Loops - Loop Within a Loop

• int Number, Sum
• while (true)
• cout ltlt "Enter a number "
• cin gtgt Number
• if (Number lt 0)
• break
• Sum 0
• for (int i0 iltNumber i)
• Sum i
• if (Sum gt 10)
• break
• cout ltlt Sum is ltlt Sum ltlt endl

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Nested Loops - Loop Within a Loop

• int row, col
• for (row1 rowlt4 row))
• cout ltlt Row ltlt row ltlt
• for (col1 colltrow col)
• cout ltlt
• cout ltlt endl

Row 1 Row 2 Row 3 Row 4
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Nested Loops - Loop Within a Loop
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Nested Loops - Loop Within a Loop
include ltcctypegt
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Designing WritingPrograms
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Four Steps in Program Development

• Requirements analysis
• Understand the requirements (what must be done)
• Program design
• Figure out the logical flow of the program (how
  to do it)
• Program coding
• Write the code for the program (do it)
• Program testing
• Figure out the best test cases for it run it on
  those cases (make sure its being done right)

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Example Assignment 3

• The delivery charge for the first pound (i.e., 16
  ounces) is 3.00 and 0.50 is added to the charge
  for each additional 4 ounces. For example,
• a package weighing more that 16 ounces but at
  most 20 ounces costs 3.50 to deliver
• a package weighing more than 20 but at most 24
  ounces costs 4.00 to deliver etc.
• An optional delivery confirmation at 0.45 per
  package.
• Input the weight and delivery confirmation
  choice, compute and output the delivery charge.

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The Requirement Analysis

• Stay at the same level as the words of the
  problem statement until you understand what its
  asking for
• Inputs
• weight
• delivery confirmation choice
• compute the delivery charge
• need to handle the delivery confirmation choice
• use calcCharge function
• output the delivery charge
• use dispCharge function

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Program design and coding
Some things well always haveCompiler
directivesmain Function Declarations
initializations Main code for the function (in
this case, compute delivery charge) Other things
we may haveDeclarations outside
functions (e.g., constants)In main function
Outputs (at end of main body or in middle of
main body)Additional functions Perform needed
sub-tasks
Complier directives
Global constants declarations
Function declarations
main
Variable declarationsand Initializations
Input
Compute by function call
Output by function call
Function definitions
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Components before main

• Compiler directives
• include ltiostreamgt
• directives for the libraries well use
• using namespace std
• Namespaces used in very large programming
  projects
• For us, just put this line at the top of every
  program
• Optional global constants
• const double MIN_CHARGE 3.00
• Function Declarations
• double calcCharge(double dWeight, char
  chConfirm)

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main Function

• Variable Declarations
• double dPackageWeight
• char chDeliveryConfirm
• double dTotalCharge
• Youll see what you need to declare as you
  move through the design of the function
• Add declarations as you realize or decide
  that you need places to store things in the code

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main Function

• Initializations and Input
• For some variables, add initializations onto
  the declarations
• Things that start as 0 (integers) or "" (strings)
• For other variables, initialize explicitly in the
  code
• Later, when program has run for a while
• Example loop counter (Assignment 4)
• Values gotten from outside in this case,
  get the weight and delivery confirmation choice
  from the user
• Prompt user for data (tell user what you want)
• Get data from user
• Check input data for correctness

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main Function Call other Functions

• // call calcCharge()
• get the total charge
• // call dispCharge()
• output the total charge

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Implement Functions

• double calcCharge()
• what parameters are needed to computer
  totalcharge?
• weight double? or int?
• delivery confirmation choice char
• how to compute the totalcharge?
• two cases delivery confirmation / or not?
• math function ceil(double x)
• return the total charge
• void dispCharge()
• parameter the totalcharge
• cout ltlt

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Testing a Program

• Overall Process
• Give your program inputs whose output you know
• Look at the outputs from those inputs are they
  correct for the inputs you provided?
• But theres more to it than that
• Most important deciding which inputs test
  cases to give it
• Test a wide range of input values
• Normal (correct) input values
• Erroneous (incorrect) input values
• Boundary conditions values
• And any cases youre not sure about

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Example Assignment 4

• In main function, input the number of employees
  with input prompts.
• LOOP 1 Ask the user to reinput the value if the
  it is invalid, e.g., a negative number, until the
  input value is valid.
• LOOP 2 in main function
• (1) input the full name, hours worked, and base
  pay rate for each employee
• (2) calculate the gross wage for each employee by
  calling the function CalcGrossWage
• (3) calculate the net tax for each employee by
  calling the function CalcNetTax
• (4) calculate the net wage for each employee
• (5) output a table like the following based on
  the information of each employee

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Top-down Design

• Think of your program from the top down (or from
  the outside in)
• If you need a loop, figure out what the loop as
  a whole needs to do before you work on what goes
  inside it
• Write the skeleton for a loop as a whole first
• for ( )
• Then go back and fill in the blanks

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Controlling the loop

• What kind of controls are there in general?
• while? do-while? for?
• LOOP1
• controlled by the input of user,
• We do not know how many times the user have to
  try
• so we use
• LOOP2
• controlled by the number of employees, i.e., the
  input we get from LOOP1,
• We know how many times we should loop,
• so we use

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Contents of the Loop 2 (computation loop)

• For each employee
• Prompt user for the input record
• May want to put full prompt only once, above the
  loop
• Then use a minimal prompt each time user input is
  requested
• Get record from user
• Need an int variable for hours worked, a double
  variable for base pay rate, and a string for the
  full name
• Be careful when inputting the full name
• can we use cin gtgt ?
• Any validity checks needed? Not for this problem

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Contents of the Loop 2 (computation loop)

• Two function calls
• // call the function CalcGrossWage to get gross
  wage
• // call the function CalcNetTax to get net tax
• // calculate the net wage for each employee
• // Output the results as the required format

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Contents of the Loop 2 what else?

• (a) the total gross for all employees, (b) the
  total tax withheld from all employees, (c) the
  average net pay, and (d) the name and gross wage
  for the employee with the largest gross.
• Initialize variables of total gross and total
  tax
• LOOP2
• // calculate the net wage for each employee
• // calculate (a) and (b)
• // find (d)
• // Output the results as the required format
• calculate (c)

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Using if to Handle Error Cases

• Situation
• Use an if test to discover whether theres been
  a problem
• If there has been, issue a message and get out
• If not, do a (sometimes big) hunk of code
• Question which side of the if block to
  write first?
• The code to handle the success case
• The code to handle the fail case

• Answer write the shorter piece of code first
• Whether its the success or fail case
• That way, both sides are right there under your
  eyes as you look at the if statement
• Keeps you from having to search down the program
  page to find the other side