Python%20(yay!)

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Python (yay!)

• November 16, Unit 7

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Recap

• We can store values in variables using an
  assignment statement
• gtgtgtx 4 8
• We can get input from the user using raw_input()
• gtgtgtname raw_input(Enter your name)
• We can change the data from one type to another
• gtgtgtnum int(raw_input(Enter a number))

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Conditional Statements

• So far the programs weve written are pretty
  boring
• Like with XSL we can use conditional statements
  to do more
• Basically conditional statements check to see if
  something is true.
• If it is true, some code is executed
• Perhaps if its not true, different code is
  executed
• Simplest conditional statement is the if
  statement

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The If Statement

• The if statement in Python works just like the if
  statement in XSL
• The syntax is different
• Concept is the same
• If the number is less than 5, print that number
  is less than 5
• If the name is C.S.Lewis, print this book is
  by C.S.Lewis

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If Statement, cont.

• num int(raw_input(Enter a number less than
  10))
• if numgt10
• print That number is bigger than 10. You cant
  follow directions
• print Thanks for playing!

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If, cont.

• The syntax for the if statement is as follows
• if expression
• code
• The code you want executed if the expression is
  true must be indented
• This is the body of your if statement
• Spacing is important in python
• You should indent consistently
• Convention is to indent 4 spaces (use the tab)
• When you stop indenting, the body of the if
  statement is considered to be done
• The code after the indention will execute no
  matter what

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In-Class Example

• Using if statements

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Boolean Expressions

• When we use an if statement, the result of the
  expression is either true or false
• if 4lt3 (false)
• if 3lt4 (true)
• The result of the expression must be true or
  false
• Otherwise it doesnt make sense
• These are called boolean expressions
• The two boolean values are true and false

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Boolean Values, cont.

• Boolean values in python (like most languages)
  are stored as integers
• 0 represents false
• 4lt3 as an expression has the value 0
• Any other integer (usually 1) represents true
• 3lt4 has the value of 1
• You can actually write code like
• if 1
• if 5
• .

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Boolean Expressions

• Were pretty used to seeing lt, lt, gt, gt in
  conditional statements
• They make sense and read just like regular
  English
• What if we want to check if something is equal to
  something else?
• if 4-3 1
• This is an assignment statement
• Does not check to see if 4 minus 3 equals 1
• We need to use the operator
• if 4-3 1
• If we want to check if two items are not equal
  use the ! operator
• - if 4-2!1

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Else Clause

• In XSL we can use ltxslchoosegt, ltxslwhengt, and
  ltxslotherwisegt
• This provides us with a sort of if, else if, else
  if.else structure
• Python has something similar
• We can use the else clause to provide a sort of
  if not
• If 3lt4 then print woohoo
• else print darn. I dont know math
• Code inside the else clause will be executed only
  if the expression in the if statement evaluates
  to false

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If Else Example

• guess int(raw_input(Pick a number between 1
  and 10))
• if guesslt10
• print Good job!
• else
• print Cant you follow directions!?

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In-Class Example

• Example using if/else
• Example using strings

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elif block Else if

• The if/else structure is pretty useful
• There is one more component to this structure
  the else if block
• Basically we can now write code that reads
  something like
• if, else if, else if, ..,else
• To use an else if structure we need the word
  elif
• elif requires an expression (just like if)

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Elif Example

• if guess number
• print you guessed it!
• elif guesslt number
• print thats too low
• else
• print thats too big!

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In-Class Example with elif

• Changing the number guessing game to use elif
• Inventory with elif

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We Can Nest if Statements

• We can nest if statements inside other if
  statements
• Or inside elif statements
• Or inside else statements
• Basically anywhere
• Remember that nested ifs are only executed if the
  outer ifs are evaluated as true

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In-Class nested If example

• Checking to be sure the number is within range

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Random Number

• So far we have had to pre-select a number for the
  user to guess
• num 7
• Its the same every time
• Pretty boring once you guess the number
• It would be better if every time we ran the
  program it chose a random number for us to guess
• In order to use random numbers it requires two
  parts
• Importing the random module
• Using a function from the random module to
  generate our random number

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Random Numbers, cont.

• import random
• num random.randint(1,10)
• Import tells python to include the random module
• There are lots of modules
• More on this later
• random.randint(1,10)
• In the random module, use the randint function to
  select a random number between 1 and 10
• This is assigned to the variable num
• random.randint(5, 50)
• Select a random integer between 5 and 50

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In-Class Example

• Adding a random number to our guessing game

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Libraries

• It would stink if every time we needed a random
  number we had to write the code to produce one
• Or how about something like calculating the sin
  of a number
• Or the square root
• With most programming languages some functions
  are included with the language
• These are usually grouped into libraries
• We only have to import the libraries or modules
  we need
• If we imported every module it would take a long
  time to run
• In python, all of the prepackaged functions are
  considered to be part of the python library
• The individual parts we will import will be
  modules or libraries
• Terms are pretty much interchangeable
• Slightly different terminology from other
  languages

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Modules

• When we needed the random number we had to import
  the random module
• The random module contains a function called
  randint() which is what actually gives us the
  random number
• The syntax for importing a module is
• import moduleName
• We only import the modules we need

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Accessing Functions in Modules

• To use a function found in a module we have to
  use the following syntax
• moduleName.functionName()
• There are a lot of modules
• There is a link to the reference on the course
  page
• Some of the common ones are random, math, and
  cmath
• Well be covering many more modules

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In-Class Example

• Import the string module
• Use it to uppercase an entire string

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Questions?

• What you should get from this lecture
• If statements
• else clause
• elif clause
• Nesting if statements
• How to produce a random integer
• How to access functions in modules