CS414 Lab 3: Lists

Due: September 16th, 2017, before midnight

Late penalties: Fri -5%, Sat -10%, Sun 20%.

For this lab, you will work with single file, called lab3.py. I've prepared a starter file for you, available here on the course webpage.

Before you begin

1. Create a folder for your work.
2. Go to the lab's webpage, and save the file lab3.py into that folder.
3. Run IDLE, and open the file lab3.py .

Exercises

1. Contains: look at the code below, which determines if a list contains the value 3:

               list1 = [1, 2, 'apple', 3, 'banana']
               found_3 = False
               for value in list1:
                   if value == 3:
                       found_3 = True
               if found_3:
                    print 'list1 has a 3'
               else:
                    print 'list1 has no 3s'
             

   Modify the code, so that it determines whether the list contains a string. To check whether value is a string, compare its type to the type of a known string:

                  if type(value) == type('abc'):
                     # it's a string!
             

2. Counting values: Here is some code that counts how many times the list contains a 1:

               list2 = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]
               ones_count = 0
               for value in list2:
                   if value == 1:
                       ones_count += 1
               print 'list2 has', ones_count, 'ones'
             

   Modify the code, so that it counts many values in list2 are odd. To check if a value is odd, get its remainder when divided by 2:

               if value % 2 == 1:
                   # it's odd!
             

    

3. Repeats: Here is code that checks if a list has two consecutive equal values:

               list3 = [12, 32, 34, 34, 23, 23, 12, 12]
               previous_value = list3[0]
               two_in_a_row = False
               # Start at the SECOND value in the list:
               for value in list3[1:] :
                   if value == previous_value:
                       two_in_a_row = True
                   previous_value = value
               if two_in_a_row:
                   print 'list3 has 2 in a row'
               else:
                   print 'list3 does not have 2 in a row'
             

   Modify the code so that it counts how many times there are two consecutive equal values. In the above list, this occurs 3 times.

4. Stats: Here is a program that adds up all the values in a list (assuming they're all numbers):

               list4 = [12, 32, 33, 35, 23, 23, 12, 12]
               total = 0
               for value in list4:
                    total += value
               print 'sum:', total
             

   Modify to compute the average of all the odd values in the list. You will need two variables:
   • total will sum the odd values
   • n_odd will count the odd values
   Of course, these variables should be updated only if you find an odd value!

5. Palindromes: Here is another way to visit a list's values: use an index . We're looking for odd values:

               list5 = [1, 2, 3, 4, 3, 2, 1]
               for index in range(len(list5)):
                   if list5[index] % 2 == 1:
                       print 'odd value at index',index
             

   Modify this code so that it checks whether a list is palindromic (it reads the same forwards and backwards). list5 is palindromic if, for every index, the following is True:

               list5[index] == list5[-(index + 1)]
             

   As in exercise 1, you will need a boolean variable, which holds the result. Think: should it start True, and possibly become False inside the loop, or the other way around?

    

    

    

    

    

6. Transitions: This code finds all the runs of consecutive equal values:

               list6 = [1, 1, 1, 3, 3, 1, 1, 1, 0, 2, 2, 2]
               previous_value = list6[0]
               current_run_length = 1
               for value in list6[1:] :
                   if value == previous_value:
                       current_run_length += 1
                   else:
                       # a run just finished!  Report it, and start counting again
                       print 'A run of', current_run_length, previous_value
                       current_run_length = 1
                   previous_value = value
               # There will always be a run left over:
               print 'A run of', current_run_length, previous_value
             

   Modify the code, so that it reports the transitions from one value to a different one. For example, in the above list, there's a transition from 1 to 3, then from 3 to 1, then from 1 to 0, then from 0 to 2.

   For the above list, you should output

                 1 -> 3
                 3 -> 1
                 1 -> 0
                 0 -> 2
               

Turning in your work

You can re-submit your work until midnight tonight, with no lateness penalty.