Python

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Python

• November 28, Unit 9

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Local and Global Variables

• There are two main types of variables in Python
  local and global
• The explanation of local and global is given here
  only in terms of what weve covered
• Global variables can be accessed by any of the
  functions in your program
• Local variables are local to the function they
  were created in

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Global Variables

• Variables declared outside of a function are
  usually global
• x 5
• def someFunction(a,b)
• z abx
• return z
• x is a global variable here
• The function, someFunction(a,b) can access the
  value stored in x
• If it couldnt an error would result

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Local Variables

• Local variables are those variables which only
  exist inside of a particular function
• They are local to the function
• x 5
• def someFunction(a,b)
• z abx
• return z
• In this function a, b, and z are all local
  variables

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Local and Global Variables, cont.

• When looking up a variable, Python checks the
  local variables first, then the global variables
• What does this mean?
• You can have global and local variables with the
  same name
• But, they dont have to have the same value
• Local variables are good because they protect you
  from changing values on variables you may want to
  reuse by accident.
• Come with more complicated programming mostly

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Local and Global Example

• x 5
• def someFunction(a,b)
• x 6
• z abx
• return z
• In this example, we have a global variable x
• And, a local variable x (along with z, a, and b)

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Example, cont.

• x 5
• def someFunction(a,b)
• x 6
• z abx
• return z
• Num someFunction(2,1)
• print num is , num, and x is , x
• This program would print num is 9, and x is
  5
• The local variable x is used for the calculation
  of z, but the print statement doesnt see that
  local x
• It uses the global x instead

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Another Example of Local and Global Variables

• a 2
• b 3
• x 5
• def someFunction(a,b)
• x 6
• z a b c
• Num someFunction(2,1)
• print a, b, x
• This would print 2, 3, 5
• In someFunction(), a is given the value of 2 and
  b is given the value of 1
• But these are local variables
• Does not affect the values of the global
  variables a and b

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Local Variables and Multiple Functions

• If you declare a variable inside of a function,
  it can only be used inside of that function
• Outside of that function it either
• Wont exist
• Or will actually be a different variable
• If you declare a variable inside of one function,
  it cannot be used by another function

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Multiple Function Example

• def firstFunction()
• x 2
• print x
• def secondFunction()
• z x2
• print z
• This would cause an error
• x is not defined for secondFunction()

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Issues with Global Variables

• Before a function can use a global variable, it
  must be declared
• You can put your function definitions at the top
  of your Python program even if they use global
  variables
• But, before you call your function, the global
  variables must be declared
• Remember declaring a variable involves assigning
  it a value

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Example with Global Variables

• def someFunction(a)
• z xa
• return z
• x 3
• num someFunction(2)
• print num
• This program executes fine because x is delcared
  before someFunction() is called

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Program with Error

• def someFunction(a)
• z xa
• return z
• num someFunction(2)
• x 3
• print num
• This program will not execute because Python runs
  sequentially
• You cant use the value of x before it is
  declared
• It is declared after the function which uses x
  has been called

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Changing the Value of a Global Variable In a
Function

• If we try to assign a new value to a global
  variable inside of a function, all it does is
  create a new local variable of the same name
• def someFunc(a)
• x a
• return x
• x 2
• print someFunc(3)
• The x inside of the function is the local x

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Changing Globals, cont.

• Python does, however, provide a way to change the
  value of global variables
• We can use the global keyword
• To use it we apply it to the variable we want
  before we assign it a value
• Basically says the x Im going to use is the
  global x

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Value of Globals, cont.

• def someFunction(a)
• global x
• x xa
• x 3
• someFunction(2)
• print x
• This would print the value of 6 for x
• Without the global it would print the value of 3
• The global keyword allows us to modify a global
  variable
• Should be used sparingly

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In-Class Runs of Examples

• Simple local and global example
• Multiple function example
• Using global keyword

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Lists

• Lists in Python look and act a lot like arrays in
  other languages
• What is a list?
• It is an ordered collection of Python values
• Basically, it allows us to group a bunch of
  values under one variable name
• The values can be any python objects
• Unlike arrays in many languages which all must be
  of the same type

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Example of a List

• We can create a list by using square brackets and
  separating the values with commas in the
  following manner
• breadList flour, oil, yeast, butter
• In this case a list of ingredients for a loaf of
  bread
• If I print the list
• flour, oil, yeast, butter

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Accessing Individual Values

• Having a list of a bunch of values does us little
  good unless we can access the individual elements
  of the list
• We can access individual elements by their index
• Index is their number in the order of the list
• breadList2 has the value yeast
• breadList0 has the value oil

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Indexes in Lists

• When counting the index in a list, it starts at 0
  and not 1
• Seems a bit counter-intuitive at first
• Fairly standard for many programming languages
• A list of 8 items has indices from 0-7
• A list of 200 items has indices from 0-199

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Lists, cont.

• Many people have difficulty grasping the concept
  of a list at first
• One useful metaphor is a series of bins
• Each bin holds a value

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Using a List

• We can access individual elements by their index
  number
• Name listOfNames2
• i 3
• Name listOfNamesi
• Another way of accessing the elements in a list
  is by using a for loop

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For Loop

• For loops are more restricted than while loops
• While loops check to see if a condition is true
  and execute code based on that
• For loops execute code a specific number of times
• The basic syntax of a for loops is
• for i in someList
• code to execute

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Simple For Loop Example

• print To make bread you need
• for ingred in breadList
• print ingred
• This would print
• flour
• oil
• yeast
• butter

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Another Simple Example

• for ingred in breadList
• print I love baking
• This would print
• I love baking
• I love baking
• I love baking
• I love baking

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For Loops, cont.

• You can think of a for loop as reading
• For each item in my list of items
• Execute this code
• So, if our list has 4 items with indices ranging
  from 0-3
• Our loop executes 4 times

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Adding Values to a List

• We do not have to declare every item in our list
  at the start
• We can add values to our list by using the append
  function
• Given our previous breadList, if we wanted to add
  an item
• breadList.append(sugar)
• breadList.append(salt)
• Now, breadList flour, oil, yeast,
  butter, sugar, salt

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Deleting Values

• We can remove a value from our list by using the
  del keyword
• We must know the index of the item we want to
  delete
• del breadList0 would remove flour
• del breadList2 would remove yeast

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Other Simple List Operations

• is the concatenation operator
• If we use it on two lists it adds them together
• aList 1,2,3
• bList 4,5,6
• cList aListbList
• cList would now be 1,2,3,4,5,6
• The or multiplication operator repeats a list a
  given number of times
• cList aList2
• cList would now be 1,2,3,1,2,3
• You cannot multiply two lists together
• cList aListbList will give you an error

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range(someNumber) Function

• range() is a useful function for for loops
• range returns a list of values from 0 to
  someNumber -1
• Example
• print range(5)
• would print 0,1,2,3,4
• This is great for for loops where you simply want
  to count and arent using a separate list

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For Loop with range() Example

• for i in range(10)
• print i
• This would print the numbers from 0 to 9
• for i in range(10)
• print i1
• This would print the numbers from 1 to 10

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Range(), cont.

• We can specify start and stop values for range as
  well, where the first number is the start value
  and the second number is the stop value
• range(2,10)
• Prints 2, 3, 4,5, 6, 7,8,9
• And we can specify the step
• Step is how much we count by each time (last
  number is the step)
• range(2,10,2)
• Prints 2,4,6,8
• The step can be backwards as well
• range(10,2, -2)
• Prints 10, 8, 6, 4
• Great for counting downwards
• To countdown by 1, specify the step to be -1

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While Loops and For Loops

• Any for loop can be written as a while loop
• for i in range(1,11)
• print i
• i 1
• while ilt10
• print I
• So why use for loops?
• They nicer when looping through regular lists
  (not produced by range())
• They are better when we know exactly how many
  times we want the loop to execute
• They dont require a separate counter variable
• Can use the range function to loop a certain
  number of times

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Questions?