Python

1. Python November 28, Unit 9+

2. 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

3. Global Variables • Variables declared outside of a function are usually global x = 5 def someFunction(a,b) z = a+b+x return 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

4. 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 = a+b+x return z • In this function: a, b, and z are all local variables

5. 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

6. Local and Global Example x = 5 def 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)

7. Example, cont. x = 5 def someFunction(a,b) x = 6 z = a+b+x 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 doesn’t see that local x • It uses the global x instead

8. 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

9. 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

10. Multiple Function Example def firstFunction(): x = 2 print x def secondFunction(): z = x*2 print z • This would cause an error • x is not defined for secondFunction()

11. 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

12. Example with Global Variables def someFunction(a): z = x*a return z x = 3 num = someFunction(2) print num • This program executes fine because x is delcared before someFunction() is called

13. Program with Error def someFunction(a): z = x*a return z num = someFunction(2) x = 3 print 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

14. 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

15. 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”

16. Value of Globals, cont. def someFunction(a) global x x = x*a 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

17. In-Class Runs of Examples • Simple local and global example • Multiple function example • Using global keyword

18. 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

19. 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”]

20. 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”

21. 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

22. listOfNames=[“Sam”,”Jim”, “Amy”,”Art”,”May”,”Max”,”Jen”,”Ray] Sam Jim Amy Art May Max Jen Ray 0 1 2 3 4 5 6 7 Index 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

23. 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

24. 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

25. Simple For Loop Example print ”To make bread you need: for ingred in breadList: print ingred • This would print: flour oil yeast butter

26. 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”

27. 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

28. 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”]

29. 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”

30. 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

31. 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

32. 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

33. 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

34. 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 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

35. Questions?