Question

Deletion of List Elements

• The del statement removes an element or slice from a list by position.
• The list method list.remove(item) removes an element from a list by it value (deletes the first occurance of item)
• For example:

      
  a = ['one', 'two', 'three']
  del a[1]
  for s in a:
      print(s)
  
  b = ['a', 'b', 'c', 'd', 'e', 'f', 'a', 'b']
  del b[1:5]
  print(b)
  
  b.remove('a')
  print(b)
      
  # Output:
  '''
  one
  three
  ['a', 'f', 'a', 'b']
  ['f', 'a', 'b']
  '''
  

• Your textbook documents other List Methods in section 10.14. You can also find the more complete documentation of List methods here in the Python documenation.

Reference Variables

• A reference variable stores the memory address of an object. It's similar to the C++ concept of a pointer. So when we access a reference variable, it indicates where to find the actual data we want, and we get it from there automatically. This allows two different reference variables to refer to the same actual object (the same memory address).
• In Python, all variables are objects, so they all use reference variables. But in many other languages, such as C++ and Java, simple data (such as ints and floats) are stored directly, without reference variables.
• To avoid confusion and mistakes, the common data types are all immutable in Python, so that we don't have to worry about modifying shared data. This includes strings, ints, floats, and bools.
• Since lists are mutable, we have to be more careful about references to shared data, and knowing whether or not we are changing it.
• None - keyword indicating an empty reference. This is similar to null in Java. A variable whose value is None is one that doesn't refer to an object.
• For example, if you assign a variable to store the return value of a function that doesn't have a return statement, the variable will store None.
• The is operator returns True if two reference variables refer to the same object (the same memory location). Otherwise it returns False.
• The == operator compares the contents of its arguments (list elements), to see if they have the same value.
• To copy a list, so that you have two separate objects storing the same data as each other, you can use a slice like this:

  list1 = [1, 2, 3, 4]
  list2 = list1[:]
  print(list1 is list2)  # False
  print(list1 == list2)  # True
     

• The CodeLens example on this page from the book demonstrates "is" and "==" with reference variables well.

Lists and Functions

• An entire list can be passed as an argument to a function.
• The variable you use to refer to the list is really just a reference to the array's contents. Since a list is mutable, its contents may be changed by the function.
• Example: ArrayParam.py.txt - Demonstrates how array parameters work
• In-class exercise: write the method inputList to make this program work as shown.

# CS 110A - Starting poing for in-class exercise on List References and Functions


def main():
    a = []
    inputList(a)
    print("Here are the names you entered (excluding Craig):")
    for item in a:
        print(item)


# Insert your code here 
# Write the inputList function so that the program produces the output below.


main()

# Sample output:
'''
Please input one name on each line, hitting Enter on a blank line when finished:
 Craig
 Indika
 Deepa
 Man
 
Here are the names you entered (excluding Craig):
Indika
Deepa
Man
'''
'''
Please input one name on each line, hitting Enter on a blank line when finished:
 Erez
 Dan
 Craig
 Anita
 Henry
 
Here are the names you entered (excluding Craig):
Erez
Dan
Anita
Henry
'''

Answer 1

def main():
a = [] # creating empty list
inputList(a) # calling method below
print("Here are the names you entered (excluding Craig):")   
for item in a:
print(item) # printing elements of modified list a
# Insert your code here
# inputlist function that takes empty list, asks user to enter names, store them in list and removes Craig from it
def inputList(a):
print('Please input one name on each line, hitting Enter on a blank line when finished:') # printing instruction
name = raw_input(); # taking input name
while ( name !=''): # looping until user presses Enter (name = '' )
a.append(name) # this functions adds new element (name) to list a
name = raw_input(); # taking input again
a.remove('Craig') # removing first occurance of name 'Craig'
main()

1 def main(): 2 3 inputlist (a) 4 print( "Here are the names you entered (excluding Craig):") 5 for item in a: creating empty List calling method below print(item) printing elements of modified List a 7# Insert your code here 8# input List function that takes empty list, asks user to enter names, store them in list and removes Craig from it 9 def inputList (a): 10 print(' Please input one name on each line, hitting Enter on a blank line when finished,') # printing instruction 11 name-raw input) 12 while name): 13 14 15 a.remove'Craig' 16 mainOl # taking input name # Looping until user presses Enter (name = '' ) # this functions adds new element (name) to list a # taking input again # removing first occurance of name 'Craig a.append (name) nameraw input():

Sample outputs:

Please input one name on each line, hitting Enter on a blank line when finished: Craig Indika Deepa Man Here are the names you entered (excluding Craig): Indika Deepa Man

Please input one name on each 1line, hitting Enter on a blank line when finished: Erez Dan Cra1g Anita Henry Here are the names you entered (excluding Craig): Erez Dan Anita Henry