CS 240 Section 3

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CS 240 Section 3
Keep Cool Under Pressure

• Computer Science I
• Dr. Dale E. Nelson

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Chapter 4 Basic Flow of Control(Conditional
decisions and Iteration)
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Chapter Goals

• To be able to implement decisions using if
  statements
• To understand statement blocks
• To learn how to compare integers, floating-point
  numbers, and strings
• To develop strategies for processing input and
  handling errors
• To understand the Boolean data type
• To avoid infinite loops and off-by-one errors

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The if Statement

• The if statement is used to implement a decision.
• It has two parts a test and a body.
• Example
• if (area lt 0)
• cout ltlt "Error Negative area.\n"
• Multiple statements can be grouped together in a
  block statement by enclosing them in braces
• if (area lt 0)
• cout ltlt "Error Negative area.\n"
• return 1

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The if Statement (Syntax 4.1 if Statement)
Syntax 4.1 if Statement if (condition)
statement Example if (x gt 0) y
sqrt(x) Purpose Execute the statement if the
condition is true.
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The if Statement (Syntax 4.2 Block Statement)
Syntax 4.2 Block Statement statement1
statement2 ... statementn Example
double length sqrt(area) cout ltlt area ltlt
"\n" Purpose Group several statements into a
block that can be controlled by another statement.
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The if Statement (area1.cpp)
include ltiostreamgt include ltstringgt
include ltcmathgt using namespace std int
main() //Variables double
area //area of square in square inches
double length //length of side of square in
inches cout ltlt "Please enter the area of
a square " cin gtgt area if
(area lt 0) cout ltlt
"Error Negative area.\n" return
1 / now we know that area
is gt 0 / length sqrt(area)
cout ltlt "The side length of the square is "
ltlt length ltlt "\n" return 0

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The if/else Statement

• The if/else consists of a condition of two
  alternatives.
• The first condition is performed is the condition
  is true.
• The second condition is performed if the
  condition is false.
• if (area gt 0)
• cout ltlt "The side length is " ltlt
  sqrt(area) ltlt "\n"
• else
• cout ltlt "Error Negative
  area.\n"
• The if/else statement is a better choice than a
  pair of if statements with complementary
  conditions.
• if (area gt 0) / complementary conditions
  /
• cout ltlt "The side length is " ltlt
  sqrt(area) ltlt "\n"
• if (area lt 0) / complementary conditions /
• cout ltlt "Error Negative area.\n"

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The if/else Statement(Syntax 4.3 if/else
Statement)
Syntax 4.3 if Statement if (condition)
statement1 else statement2 Example if (x gt 0)
y sqrt(x) else cout ltlt "Bad
input\n" Purpose Execute the first statement if
the condition is true, the second statement if
the condition is false.
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The if/else Statement(Syntax 4.3 if/else
Statement)
include ltiostreamgt include ltstringgt
include ltcmathgt using namespace std int
main() //Variables double area
//Area of a square in square inches cout ltlt
"Please enter the area of a square " cin
gtgt area if (area gt 0) cout ltlt
"The side length is " ltlt sqrt(area) ltlt "\n"
else cout ltlt "Error Negative area.\n"
return 0
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Relational Operators
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Relational Operators

• C has six relational operators to implement
  conditions

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Operator Precedence

• Highest
• ( )
• ! -- -
• /
• -
• lt lt gt gt
• !
• - /
• Lowest

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Relational Operators (comparing strings)

• The relational operators listed above can also be
  used to compare strings using lexicological
  comparison.
• "car" is less than "cargo"
• "cargo" is less than "cathode"

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Input Validation

• Input validation is an application of the if
  statement that verifies the user has given
  reasonable input.
• Example
• double area
• cin gtgt area
• user types "five" and hits return! (Causing cin
  to fail).
• After every input a good program will test
• if (cin.fail())
• The actual input should also be validated, even
  if the type is correct.
• if (area lt 0)
• Other strategies
• if (cin)
• / the stream did not fail /
• else
• / the stream failed /
• if (cin gtgt x) ...

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Input Validation (area3.cpp)
include ltiostreamgt include ltstringgt
include ltcmathgt using namespace std int
main() //Variables double area
//Area of a square in square inches cout ltlt
"Please enter the area of a square " cin
gtgt area //Check for valid input if
(cin.fail()) cout ltlt "Error Bad
input\n" return 1 if
(area lt 0) cout ltlt "Error
Negative area.\n" return 1
cout ltlt "The side length is " ltlt sqrt(area) ltlt
"\n" return 0
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Simple Loops

• A loop is a block of code that can be performed
  repeatedly.
• A loop is controlled by a condition that is
  checked each time through the loop.
• The while statement makes a check before each
  execution of the code.
• / how long does it take an investment to double?
  /
• while( balance lt 2 initial_balance)
• balance balance ( 1 rate / 100)
• year

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Simple Loops (Syntax 4.4 while Statement)
Syntax 4.4 while Statement while (condition)
statement Example while (x gt 10) x
sqrt(x) Purpose Execute the statement while the
condition remains true.
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Processing a Sequence of Inputs (Sentinels)

• Whenever you read a sequence of input values, you
  need to have some method of terminating the
  input.
• A number used to signal termination is called a
  sentinel.
• Sentinels only work if there is some restriction
  on the input.
• Common sentinel values are 0 or -1.

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Processing a Sequence of Inputs (sentinel.cpp)
include ltiostreamgt using namespace std
int main() //Variables double sum
0 //Total of all salaries in dollars int
count 0 //Number of salaries read
double salary 0 //Employee salary in dollars
while (salary ! -1) cout ltlt
"Enter a salary, -1 to finish " cin gtgt
salary if (salary ! -1)
sum sum salary count
if (count gt 0) cout ltlt
"Average salary " ltlt sum / count ltlt "\n"
return 0
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Processing a Sequence of Inputs (Causing the
Stream to Fail)

• When reading input from the console, you can
  close the stream manually.
• Ctrl Z in Windows
• Ctrl D in UNIX
• Reading from a closed stream causes the stream to
  enter the failed state.

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Processing a Sequence of Inputs (maxtemp.cpp)
include ltiostreamgt using namespace std
int main() //Variables double next
//Temperature in degrees F double
highest //Highest temperature in degree F
cout ltlt "Please enter the temperature
values\n" if (cin gtgt next) highest
next else cout ltlt "No
data!\n" return 1 while
(cin gtgt next) if (next gt highest)
highest next cout ltlt
"The highest temperature is " ltlt highest ltlt
"\n" return 0
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Using Boolean Variables

• The bool type can hold exactly two values,
  denoted false and true.
• Boolean variables are named after George Boole
  (1815-1864), a pioneer in the study of logic.
• Example
• bool more true
• while (more)
• cin gtgt next
• if (cin.fail())
• more false
• else
• // process next
• Don't
• while(more false) / don't /
• while(more ! false) / don't /

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Case Study

• Crazy Als Computer Emporium Case Study
• Crazy Als Computer Emporium is a retail seller
  of home computers. The sales staff at Crazy Als
• work strictly on commission. At the end of the
  month, each salespersons commission is
  calculated
• according to Table 4A.
• For example, a salesperson with 16,000 in
  monthly sales will earn a 12 commission
  (1,920.00).
• Another salesperson with 20,000 in monthly sales
  will earn a 14 commission (2,800.00).
• Because the staff only gets paid once per month,
  Crazy Als allows each employee to take up
• to 1,500 per month in advance. When sales
  commissions are calculated, the amount of each
• employees advanced pay is subtracted from the
  commission. If any salespersons commissions are
• less than the amount of their advance, they must
  reimburse Crazy Als for the difference.
• Here are two examples Beverly and John have
  21,400 and 12,600 in sales, respectively.
• Beverlys commission is 2,996 and Johns
  commission is 1,260. Both Beverly and John took
• 1,500 in advance pay. At the end of the month,
  Beverly gets a check for 1,496, but John must
• pay 240 back to Crazy Als.
• Youve been asked to write a program that eases
  the task of calculating the end-of-month

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Program Design

• Select variable required by the program
• Create general pseudocode
• Create detailed pseudocode
• Expand to final program
• First comments, includes, namespace
• Second begin program and define variables
• Third get input
• Fourth determine commission rate
• Fifth calculate commission and remaining pay
• Sixth display results and return

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Case Study

• Week3CaseStudy.ppt