== Slicing lists ==

 - we saw that you can slice strings.  Works with lists too.

subjects = ["Computer Science", "Biology", "French", "History"]
print subjects[0:2]
print subjects[:1]
print subjects[:2]
print subjects[2:]


== Aliasing ==

 - last week we saw examples where two variables referred to the same object.
   This is called "aliasing"
 - Recap:
 - If the object is mutable, changing what one variable refers to changes
   what the other one refers to.
 - If it's not, you can't change it; you can only make a new object that
   has the changes.  So you can't effect what one variable references by
   changing the other.

 - New fact: Slicing creates a new object, even if you slice the whole list.

subjects = ["Computer Science", "Biology", "French", "History"]
subjects_copy = subjects		# Both refer to the same list object.
subjects
subjects_copy
subjects[0] = "Commerce"		# So changing one changes both.
subjects
subjects_copy

subjects_clone = subjects[:]	# A brand new object.
subjects						# Since they are two different objects,
subjects_clone
subjects[0] = "Philosophy"		# changing one can't change the other.
subjects
subjects_clone

 - Copy and paste into the text window and use the debugger to step through.
   Show how the values are different when slicing is used.


== More list practise ===

 - Try this out in a while loop:

def remove_i(text)
    '''Remove all instances of the letter i from the string text.'''

    text.count("i")
    while text.count("i") > 0:        # while there are more i's to remove
        position = text.find("i")     # find the next i
        text = text[0:position] + text[position + 1:]

 - Exercise with the person next to you:  rewrite the function above 
   so that it returns a new, modified string and doesn't change the original. 
   Hint:  You want to use an accumulator.  


== Nested Lists == 

 - Let's create a list of student and grade info:

student_grades = [['999888777', 78], ['111222333', 90], ['444555666', 83]]

 - We can refer to elements of the lists within the list as follows:

student_grades[0]
student_grades[0][0]
student_grades[0][1]

 - Write the functions below.
   Step through each function in the debugger, emphasizing 
   the order in which the values are taken in the for loops.

def average_grade(grade_list):
    '''Return the average grade (a float) for all of the students in the 
    list of lists grade_list, where the inner lists contain a student ID 
	(a str) and a grade (a number).'''
    
    total_grade = 0.0
    
    for student in grade_list:
        total_grade += student[1]

    return total_grade / len(grade_list)

def get_student_IDs(grade_list):
    '''Return a list of strs containing the student IDs from the list of lists
    grade_list, where the inner lists contain a student ID (a str) and a grade
    (a number).'''
    
    ID_list = []

    for student in grade_list:
        ID_list.append(student[0])

    return ID_list


== Nested Loops ==

# Print contents of a nested list
for student in student_grades:
   print student
   for x in student:
       print x

measurements = [[33, 34, 30], [29, 31, 34], [28, 27, 30]]

# Print the average of each inner list
for dataset in measurements:
    sum = 0.0
    for datapoint in dataset:
        sum += datapoint
    print sum / len(dataset)

# We can also use the function "sum"
for dataset in measurements:
    print sum(dataset) / float(len(dataset))
... but sum contains a loop.  I promise!

# Create a list of averages
... add "averages = []" before the loop
... change the print to an append