Conditional Statements

1
Conditional Statements

• Introduction to Computing Science and Programming
  I

2
Conditional Statements

• So far we have only talked about writing
  statements that are executed one after another
  from top to bottom
• This simply isnt sufficient to write very many
  useful programs

3
Simple Guessing Game

• Here is an algorithm for a very simple guessing
  game.
• write Think of a number between 1 and 10.
• set guess to 6.
• write Is your number equal to guess?
• read answer
• if answer is yes, then
• write I got it right!
• if answer isnt yes, then
• write Nuts.
• write Thats the end of the game.

4
Simple Guessing Game

• Looking at this algorithm we see that there are
  statements that will be run under certain
  conditions, but not others.
• if answer is yes, then
• write I got it right!
• if answer isnt yes, then
• write Nuts.

5
Simple Guessing Game

• Depending on the value of answer we will do one
  of two different things.
• If you want to write code like this that will
  execute if a certain condition is true, you use
  an if structure

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If Structure

• if condition
• statement1
• statement2
• statement3
• If the condition is satisfied when execution
  reaches the if statement, then all statements
  that are indented after it will be executed. If
  the condition is not satisfied then they will be
  skipped and execution will continue after the
  indented statements

7
Blocks of Code

• Python uses indentation to indicate blocks of
  code.
• For an if statement we indent every statement in
  the block of code that will be executed based on
  the result of the if
• if x1
• print first line of code block
• print last line of code block
• print first line outside of code block

8
Blocks of Code

• Blocks of Code can be within other blocks of code
• if x gt 5
• print x is greater than 5
• if x gt 10
• print x is also greater than 10
• Lines 2, 3, and 4 are the block of code for the
  first if statement. Line 4 is the block of code
  for the second if statement

9
Guessing Game

• Here is how the guessing game is translated into
  Python
• print "Think of a number between 1 and 10.
• guess 6
• answer raw_input("Is your number equal to \
  str(guess) "? ")
• if answer "yes"
• print "I got it right!"
• if answer ! "yes"
• print "Nuts."
• print "That's the end of the game."

10
The bool Data Type

• Before this point all of the expressions we have
  used evaluate to either a string, integer, or
  float.
• For the if statement we have a condition that
  Python will evaluate to be either True or False.
  These conditions are boolean expressions for
  which we have the bool data type

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The bool Data Type

• For the guessing game we have two conditional
  statements that contain simple boolean
  expressions
• answer yes
• Evaluates to True if answer is equal to yes
• answer ! yes
• Evaluates to True if answer is not equal to yes

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Boolean Operators

• Comparison Operators
• , is equal to
• This is different from
• x5, assigns x the value 5
• x5, a boolean expression that evaluates to True
  if x is equal to 5, False if is not
• !, is not equal to
• Returns the opposite of
• x!5 is true if x does not equal 5
• lt, gt, lt, gt all act as youd expect

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Boolean Operators

• Logical Operators
• and,
• A and B is true if both A and B are True
• x5
• (x gt 1 and x lt 4) is False
• or,
• A or B is true if either A or B is True
• x5
• (x gt 1 or x lt 4) is True
• not, !
• not A is true if A is False
• x5
• not x lt 4 is True

14
Boolean Variables

• Since bool is a data type, you can store a bool
  value into a variable as with any other data type
• xint(raw_input(Enter an integer between 0 and
  10 ))
• xIsInCorrectRange (x gt 0 and x lt 10)
• if not xIsInCorrectRange
• print You didnt enter an integer between 0
  and 10
• print Im giving you the number 5
• x5
• print Your number is str(x) .

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else Clause

• if condition
• code block1
• else
• code block2
• If the condition is satisfied when execution
  reaches the if statement, then code block1 will
  be executed. Otherwise, code block2 will be
  executed

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else Clause

• In our guessing game example we can simplify the
  code using an else clause instead of a second if
  statement
• We have this, but we know that if the first
  condition is false the second is true
• if answer yes
• print I got it right.
• if answer ! yes
• print Nuts.
• We can use an else clause. If the condition in
  the if statement is false execution will move to
  the associated else clause.
• if answer yes
• print I got it right.
• else
• print Nuts.

17
elif Clauses

• elif is how Python shortens else-if
• elif clauses can be used if there are multiple
  related conditions you want to check for and
  execute different code depending on which one is
  true

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elif Clauses

• if condition1
• code block1
• elif condition2
• code block2
• .
• .
• else
• code block
• If condition1 is true, code block1 will be
  executed and the other clauses will be skipped,
  otherwise if condition2 is true, code block2 will
  be executed, and so on. If none of the
  conditions are true and there is an else clause,
  the code block for that clause will be executed.
  You can use as many elif clauses as needed.

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elif Clauses

• One more extension of the guessing game using an
  elif clause.
• if answer "yes"
• print "I got it right!"
• elif answer "no"
• print "Nuts."
• else
• print "You must answer yes or no.

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elif Clauses

• Remember that only the first elif clause whose
  condition is satisfied will have its block of
  code executed.
• weight int(raw_input("How many grams does the
  object weight? "))
• if weight gt1000
• print "It weighs " str(weight/1000.0) "
  kilograms."
• elif weight gt 100
• print "It weighs " str(weight/100.0) "
  hectograms."
• elif weight gt 10
• print "It weighs " str(weight/10.0) "
  dekagrams."
• elif weight gt 0
• print "It weighs " str(weight) " grams."
• else
• print "You entered an invalid number."

21
elif Clauses

• Easy way to set up a simple menu.
• print 1. Create a new file
• print 2. Open a file
• print 3. Exit
• x int(raw_input(Select an option ))
• if x1
• .
• .
• elif x2
• .
• .
• elif x3
• .
• .
• else
• print You didnt enter a valid option.