Python

November 28, Unit 9+

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 we’ve 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

Global Variables

• Variables declared outside of a function are usually globalx = 5

def someFunction(a,b)z = a+b+xreturn z

• x is a global variable here• The function, someFunction(a,b) can access the

value stored in x– If it couldn’t an error would result

Local Variables

• Local variables are those variables which only exist inside of a particular function– They are “local” to the function

x = 5def someFunction(a,b)

z = a+b+xreturn z

• In this function: a, b, and z are all local variables

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 don’t 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

Local and Global Example

x = 5def someFunction(a,b)

x = 6 z = a+b+x

return z

• In this example, we have a global variable x

• And, a local variable x (along with z, a, and b)

Example, cont.

x = 5def someFunction(a,b)

x = 6 z = a+b+x

return zNum = 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 doesn’t see that local x– It uses the global x instead

Another Example of Local and Global Variables

a =2b = 3x = 5def someFunction(a,b):

x = 6z = 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

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:– Won’t exist– Or will actually be a different variable

• If you declare a variable inside of one function, it cannot be used by another function

Multiple Function Example

def firstFunction():x = 2print x

def secondFunction():z = x*2print z

• This would cause an error– x is not defined for secondFunction()

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

Example with Global Variables

def someFunction(a):z = x*areturn z

x = 3num = someFunction(2)print num• This program executes fine because x is

delcared before someFunction() is called

Program with Error

def someFunction(a):z = x*areturn z

num = someFunction(2)x = 3print num• This program will not execute because Python

runs sequentially– You can’t use the value of x before it is declared– It is declared after the function which uses x has been

called

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

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 I’m going to use is the

global x”

Value of Globals, cont.

def someFunction(a)global xx = x*a

x =3someFunction(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

In-Class Runs of Examples

• Simple local and global example

• Multiple function example

• Using global keyword

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

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”]

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

• breadList[2] has the value “yeast”

• breadList[0] has the value “oil”

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

• 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

Lists, cont.

listOfNames=[“Sam”,”Jim”, “Amy”,”Art”,”May”,”Max”,”Jen”,”Ray]

Sam Amy Art May Max Jen RayJim

0 1 2 3 4 5 6 7

Index

Using a List

• We can access individual elements by their index number– Name = listOfNames[2]– i =3

Name = listOfNames[i]

• Another way of accessing the elements in a list is by using a for loop

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

Simple For Loop Example

print ”To make bread you need:for ingred in breadList:

print ingred

• This would print:flouroilyeastbutter

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”

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

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”]

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 breadList[0] would remove “flour”• del breadList[2] would remove “yeast”

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 = aList+bList– cList would now be [1,2,3,4,5,6]

• The * or multiplication operator repeats a list a given number of times– cList = aList*2– cList would now be [1,2,3,1,2,3]– You cannot multiply two lists together

• cList = aList*bList will give you an error

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 aren’t using a separate list

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 i+1

• This would print the numbers from 1 to 10

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

While Loops and For Loops• Any for loop can be written as a while loop

for i in range(1,11):print i

i =1while i<=10

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 don’t require a separate counter variable• Can use the range function to loop a certain number of times

Questions?