CMPT 120

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CMPT 120

• Flow of Control

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Objectives

• Write simple programs that use if statements
• Understand and use boolean expressions
• Write simple programs that use for loops
• Write simple programs that use while loops

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Flow of Control

• A basic Python program starts, executes each line
  once and then ends
• This is not very interesting!
• Does not allow the user to repeat the process or
• Change the way the process works
• Generally, programs are more useful if they allow
  for sections of code to be repeated and for
  choices to be made

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Making Choices

• Before you stand two doors, behind one are the
  secrets of love, wealth and life everlasting,
  behind the other only a swift, but painful, death
  awaits. These doors are guarded by two djinns,
  brothers that are both cursed by the gods, one to
  utter only the truth, the other to speak naught
  but lies. Before choosing a door you may ask one
  of the djinns one question, and one only, mark
  you. What is your question?

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My Answer Is

• If I asked your brother which door leads to the
  secrets of love, wealth and life everlasting what
  would he answer?
• I would then take the other door

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Decisions

• It is often useful to change the way a program
  executes based on the result of a choice
• if statements allow a program to branch
• if a given condition is true then one set of
  statements is executed
• otherwise (else) a different set of statements is
  executed

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if Statement Anatomy
if age lt 19 print "No beer for
you!" else beer_consumed beer_consumed 1
keywords
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if Statement Anatomy
if age lt 19 print "No beer for
you!" else beer_consumed beer_consumed 1
condition must evaluate to true or false
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if Statement Anatomy
if age lt 19 print "No beer for
you!" else beer_consumed beer_consumed 1
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Conditions

• An if statement requires a condition
• A condition is a boolean expression that must
  evaluate to either true or false
• Typically, conditions compare two values
• These values can be constants, variables or
  methods that return values
• The comparisons are made with a comparison
  operator

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Comparison Operators
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Assignment versus Equality

• The operator is the assignment operator and not
  the test for equality
• x 12 assigns 12 to the variable x
• The operator is the test for equality
• x 12 tests to see if 12 is stored in x
• Python will not allow you to put an assignment in
  a condition in an if or while statement
• So if x 12 will cause an error in Python
• In other programming languages assignment is
  permitted in if statements so be careful

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Logical Operators

• an and expression is true if, and only if, both
  operands are true
• 3 lt 5 and 7 gt 8
• is False
• an or expression is true if either of the
  operands are true
• 3 lt 5 or 7 gt 8
• is True
• not is the negation operator, not x is true if x
  is false, and false if x is true
• not(3 lt 5 and 7 gt 8)
• is True

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A Bit of Python Shorthand

• It is often necessary to see if a value falls
  within a particular range
• e.g. is the variable age between 19 and 65
• How can we write this condition in Python?
• if age gt 18 and age lt 66 or
• if 18 lt age lt 66
• Warning! This shorthand may not work reliably in
  other programming languages.

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Truth Tables

• Boolean expressions can be evaluated in truth
  tables
• The symbol ? represents and
• The symbol ? represents or
• The symbol ? represents not

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Boolean Expressions Examples

• x lt y (x 5 and y 15)
• True
• x gt y or x gt z (x 7, y 8 and z 3)
• True
• x gt y and x lt len(s) (x 7, y 4 and s
  'cheese')
• False
• not(x 13 ! 7) (x 19)
• False
• (12 gt 12 or not(33 lt 22)) and 11 13 24
• True

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Rules of Precedence

• Operators are shown highest precedence first
• Operators with the same precedence are evaluated
  from left to right
• Note that comparisons are chained together with
  implicit ands

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Code Blocks in Python

• The code that is executed when a condition is
  true is indicated by indenting
• After an if statement indent the code that should
  execute if the condition is true
• Stop indenting to indicate that the if statement
  is ended
• The indented code is referred to as the body of
  the if statement

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Using else Statements

• An if can be followed by an else
• The (indented) code following the else statement
  is executed if the condition in the if statement
  is false
• What if you want more options?
• e.g. if you want to see if the user entered yes
  or no you should probably check to see if they
  entered an invalid response (e.g. maybe)
• Use an elif statement

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The elif Statement

• elif is an abbreviation of else if. It allows
  conditions to be chained together.
• You can have as many elif statements as you like
  but they must be preceded by an if statement
• There can only be one else statement and it must
  be the last statement in the chain
• In a set of if, elif, , elif, else statements
  exactly one branch will be executed

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elif Example
def get_grade(score) grade 'F' if score gt
85 grade 'A' elif score gt 70 grade
'B' elif score gt 55 grade 'C' return grade
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Nested if statements

• if statements can be nested within each other
• The structure of a nested if statement is
  indicated by the indentation
• So be careful!
• Sometimes nested if statements can be replaced by
  using conditions of greater complexity

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Loops

• We often want programs to repeat a process
• Either as part of an algorithm (like sorting)
• Or to give the user an opportunity to continue
  using the program
• There are two types of loops
• for loops (counting loops, or definite iteration)
• while loops (conditional loops, or indefinite
  iteration)

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Iteration

• A loop statement consists of the controlling
  statement and a body
• Like if statements the body is indicated by
  indentation
• When a loop is executed it repeats (or iterates)
  the body until the loop is finished
• The loop statement (either for or while)
  determines when the iteration stops

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Iteration with for Loops

• for loops can be used to repeat the code in the
  body of the loop a given number of times
• In Python for loops iterate over a range of
  values
• The syntax allows data structures with multiple
  elements to be easily iterated over
• For counting loops use the range function

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ook, ook, ook!

• Here's some code that gets the user to say how
  many times they want ook to be printed

n int(raw_input("How many 'ook's? ")) for i in
range(n) print 'ook'
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The for Loop Exposed

• The range function can be used to create for
  loops that iterate a fixed number of times
• In fact the process is a little more complicated
  than that
• Let's modify the ook loop a little
• It now prints 0 to n by each of the ooks, note
  that it is necessary to convert i (an integer) to
  a string (using the str function) so that it can
  be concatenated it with the ooks

for i in range(n) print str(i) ' ' 'ook'
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The range function

• So what does the range function actually do?
• It returns a list of items as identified by the
  arguments to the range
• range(10) returns 0,1,2,3,4,5,6,7,8,9
• range can take other arguments to return lists
  that dont start at 0 and where the values dont
  increase by 1
• range(-3,5) returns -3,-2,-1,0,1,2,3,4
• range(8,0,-1) returns 8,7,6,5,4,3,2,1
• The second argument specifies (one after) the end
  point of the list, the third argument specifies
  the step

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Re-visiting for Loops

• Consider for i in range(10,0,-1)
• The loop will repeat a number of times equal to
  the number of items in the list returned by the
  range function (10 in this case)
• Each time through the loop, the variable i is
  assigned the value of the next item in the list
  returned by the range function (10,9,8,7,6,5,4,3,
  2,1 in this case)
• In fact, Python for loops can iterate through the
  items in any sequence (for example, strings)
• We'll cover this in more detail later in the
  course

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while loops

• for loops are very useful for repeating a certain
  number of times, or (particularly in Python) for
  iterating through a sequence
• But the user (or programmer) needs to know how
  many times the loop is going to repeat before it
  starts
• It would be useful to have another type of loop
  that repeats until we want it to stop
• when we are done or
• when some condition is no longer true

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How much have you spent?

• Let's write a loop to add up the value of a stack
  of DVDs
• Presumably we don't want to count the number of
  DVDs there are first (there's no guarantee the
  count will be correct!)
• Instead it makes more sense to just keep entering
  amounts until we run out of DVDs

msg "What is the cost (type 'done' to quit)?
" total 0.0 cost raw_input(msg) while cost !
'done' total float(cost) cost
raw_input(msg) print 'the total cost of the DVDs
equals', total
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while Loop Example Notes

• Note that, in the example, we needed input before
  entering the while loop
• The request for input is then repeated inside the
  loop to ensure that it terminates
• Note this line total float(cost)
• The operator assigns the value of the
  (original) contents of total added to float(cost)
• It is synonymous to total total float(cost)
• It is necessary to convert cost to a float
  because raw_input returns a string
• Adding a string to a float would cause an error

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Iteration with while Loops

• while loops are used to repeat the body of the
  loop until the while condition is false
• The while condition must be a boolean expression
  that evaluates to true or false
• This is just like the condition in the if
  statement
• Make sure that the condition becomes false
  eventually or the loop will never end!
• type control-c to escape from an infinite loop

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Using while loops

• The while loop condition should change (or at
  least have the possibility of changing) in the
  loop body or the loop will repeat infinitely
• Using the break command in a (for or while) loop
  will quit the loop regardless of how many time it
  has repeated
• If the while condition is false the first time
  through the loop the body will not be executed at
  all
• Python does not implement do while loops