Decision Structures: Selection Statements

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Decision Structures Selection Statements
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What Are Decision Structures?

• Code tools which allow the program to perform
  different tasks for different conditionsin
  effect, the code chooses a set of actions when
  certain requirements are met.
• Increases complexity and flexibility in the
  sequential execution of programs
• Part of a larger set of programming protocols
  known as Control Structures

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Temperature Example
Input Celsius Convert to Fahrenheit
Fahrenheit gt 90?
Condition
yes
Print heat warning
Fahrenheit lt 30?
Condition
yes
Print cold warning
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Convert2.py

• convert2.py
• A program to convert Celsius temps to
  Fahrenheit.
• This version issues heat and cold warnings.
• def main()
• celsius input("What is the Celsius temperature?
  ")
• fahrenheit 9.0 / 5.0 celsius 32
• print "The temperature is", fahrenheit, "degrees
  fahrenheit."
• if fahrenheit gt 90
• print "It's really hot out there, be
  careful!"
• if fahrenheit lt 30
• print "Brrrrr. Be sure to dress warmly"
• main()

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Relational Operators
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Boolean Data Types

• When an expression evaluates as true or false, we
  call it a Boolean statement.
• Python (like many languages) has a Boolean data
  type. Some languages interpret a true as 1 or -1
  and a false as 0. In Python, a Boolean data type
  is called bool and is represented simply as True
  or False.
• gtgtgt 3 4 lt 3 4False

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A one-way decision structure

• quadratic2.py
• A program that computes the real roots of a
  quadratic equation.
• Bad version using a simple if to avoid
  program crash
• import math
• def main()
• print "This program finds the real solutions
  to a quadratic\n"
• a, b, c input("Please enter the
  coefficients (a, b, c) ")
• discrim b b - 4 a c
• if discrim gt 0
• discRoot math.sqrt(discrim)
• root1 (-b discRoot) / (2 a)
• root2 (-b - discRoot) / (2 a)
• print "\nThe solutions are", root1,
  root2
• main()

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Two-way Decisions (If-Else)

• Using just the if keyword provides us with a
  single alternative and, implicitly, a second
  alternativewhat to do if the if condition is NOT
  met. But this is not very clear or satisfactory
  as good programming.
• Python provides us with a structure that covers
  all exceptions to the condition linked with the
  if keyword.
• if ltconditiongt (note the colons remember
  indentation) ltstatementsgtelse ltstatement
  sgt

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If..else with the quadratics program

• quadratic3.py
• A program that computes the real roots of a
  quadratic equation.
• Illustrates use of a two-way decision
• import math
• def main()
• print "This program finds the real solutions
  to a quadratic\n"
• a, b, c input("Please enter the
  coefficients (a, b, c) ")
• discrim b b - 4 a c
• if discrim lt 0
• print "\nThe equation has no real roots!"
• else
• discRoot math.sqrt(b b - 4 a c)
• root1 (-b discRoot) / (2 a)
• root2 (-b - discRoot) / (2 a)
• print "\nThe solutions are", root1,
  root2

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Another if else example

• gpa_award.pyimport math
• def main() print This program determines
  student awards\n GPA float(input(Enter the
  students GPA)) if GPA gt 3.8 print
  Congratulations! You won the Mitchell
  scholarship! else print Sorry. Your GPA
  does not qualify for an award.main()

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Multiple Decisions (if elif)

• Sometimes there are multiple choices, not just
  two. In these cases we want to nest if
  statements inside each other
• 3.8 GPA and above gets the Mitchell scholarship
• But if you dont qualify for that and still have
  a GPA of 3.5 or better, you get the Wilson award
• But if you have a GPA between 3.0 and 3.5, you
  may qualify for the Anderson Recognition Honorary
  Pocket Protector

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Example GPAs and awards

• gpa_award.pyimport math
• def main() print This program determines
  student awards\n GPA float(input(Enter the
  students GPA)) if GPA gt 3.8 print
  Congratulations! You won the Mitchell
  scholarship! elif GPA gt 3.5 and GPA lt
  3.8 print Congrats! You won the Wilson
  award! elif GPA gt 3.0 and GPA lt 3.5 print
  OK, so you won the Anderson Honorary Pocket
  Protector. else print Sorry. Your GPA does
  not qualify for an award.main()

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Back to Quadratics

• quadratic4.py
• A program that computes the real roots of a
  quadratic equation.
• Illustrates use of a multi-way decision
• import math
• def main()
• print "This program finds the real solutions
  to a quadratic\n"
• a, b, c input("Please enter the
  coefficients (a, b, c) ")
• discrim b b - 4 a c
• if discrim lt 0
• print "\nThe equation has no real roots!"
• elif discrim 0
• root -b / (2 a)
• print "\nThere is a double root at", root
• else
• discRoot math.sqrt(b b - 4 a c)

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Exception Handling

• A lot can go wrong with programs, mainly because
  either a) the programmer made a logical or syntax
  error b) the programmer didnt give proper
  instructions to the user or c) the user is
  difficult or just stupid.
• When programmers can predict the kinds of errors
  that might arise from user input, she/he can
  write in ways of dealing with itusually giving
  new instructions to the user. The program can
  handle exceptions or handle errors.
• Python provides a special syntax for exception
  handling. try ltstatementsgtexcept
  ltErrorTypegt lterror handling statementsgt

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Handling Exceptions in the Quadratic program

• quadratic5.py
• A program that computes the real roots of a
  quadratic equation.
• Illustrates exception handling to avoid
  crash on bad inputs
• import math
• def main()
• print "This program finds the real solutions
  to a quadratic\n"
• try
• a, b, c input("Please enter the
  coefficients (a, b, c) ")
• discRoot math.sqrt(b b - 4 a c)
• root1 (-b discRoot) / (2 a)
• root2 (-b - discRoot) / (2 a)
• print "\nThe solutions are", root1,
  root2
• except ValueError
• print "\nNo real roots"
• main()

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Exception Handling in the GPA example

• gpa_award.pyimport math
• def main() print This program determines
  student awards\ntry GPA float(input(Enter
  the students GPA)) if GPA gt 3.8 print
  Congratulations! You won the Mitchell
  scholarship! elif GPA gt 3.5 and GPA lt
  3.8 print Congrats! You won the Wilson
  award! elif GPA gt 3.0 and GPA lt 3.5 print
  OK, so you won the Anderson Honorary Pocket
  Protector. else print Sorry. Your GPA does
  not qualify for an award.except
  TypeError print You must enter a decimal point
  number.main()