CSC111 Lab 2

This lab deals with loops in python. Feel free to work in pairs, share insights and help your neighbors during the lab. This is true of all labs. While some homework assignments will have to be done as individual work, the labs are a good time to cooperate with neighbors or work in pairs.

Printing programs from Linux

If you want to print your programs directly from your account on Beowulf to the printer in FH243 (next door to the lab), you need to use the cprint command. Here are a few examples of how you would print a file called programname.py.

• Get the list of the printers

    cprint -l             (that's minus-ell)

    print 	lp queue	location
    ----- 	--------	--------
    1    	mc104          	McConnell 104                 
    2    	fh243          	Ford Hall 243                 
    3    	eng_203        	Engineering 203               
    4    	eng_203_color  	Engineering 203 Color         

• Note that the printer in FH243 is on the line starting with 2

• Print to Printer #2

  cprint -2 programname.py

and your listing will available at the black and white printer in FH243. Use different numbers to print to different printers, depending on which one is closest to you.

Single for-loops

For this section of the lab, use Python in interactive mode . Once you are logged in, at the Linux prompt, just type python by itself:

python

Python 2.6 (r26:66714, Nov  3 2009, 17:33:38) 
[GCC 4.4.1 20090725 (Red Hat 4.4.1-2)] on linux2
Type "help", "copyright", "credits" or "license" for more information. 
>>>

List of numbers

• Write a for-loop that prints all the numbers between -3 and 7, 7 included, using the range function, as follows (the user input is underlined):

 >>> for i in range( 3, 8 ):
 ...     print i             (Do not forget to indent this line)
 ...                         (Press the ENTER key to mark the end)
                             (of the loop)
 3
 4
 5
 6
 7 
 >>> 

• Now, your turn!

List of numbers, Version 2

Same question, but this time write a loop that does not use the range function. (Hint: you have to list all the values)

List of even numbers

write a loop that prints all the even numbers between 4 and 20, 4, and 20 included.

List of odd numbers

write a loop that prints all the odd numbers between -3 and 19, -3, and 19 included.

User-controlled loops

Exit python by typing Ctrl-D. Use emacs to write a Python program that prints a series of numbers, after having asked the user where the series should start, and where it should end. Here is an example of how it should run (the user input is in boldface):

     Python userLoop.py
 
     I will print a list of numbers for you.
 
     Where should I start? 3

     What is the last number of the series? 10

     3 4 5 6 7 8 9 10

Go ahead and write it. What happens if you ask the program to count from -5 to 5? Does it work correctly?

Does it work if you ask your program to go from 1 to 1?

Can you predict what will be printed if you ask your program to count from 10 to 1?

For-Loops ready for modification

• Create the following python program (we're not working in interactive mode at this point). Don't hesitate to cut and paste to go faster!

     # lab2seq.py
     # 
     # initialize the farm with animals
     farm = [ "dog", "cat", "horse" ]

     # print the contents of the farm
     for animal in farm:
        print animal

Farm and animal are variables. One contains a list, the other one contains a string.

• Add a pig to your farm, and make sure it gets listed by the program.

• When you are done with the addition of the pig, observe another way for the program to do the same thing:

      # lab2seq2.py
      #
      farm = []              # create an empty farm
      farm.append( "dog" )   # add a dog to it
      farm.append( "cat" )   # then a cat
      farm.append( "horse" ) # then a horse
      farm.append( "pig" )   # and a pig

      # print the contents of the farm
      for animal in farm:
          print animal

Modification #1

Edit this second program so that instead of always adding the same animals to the farm, it will get four names of animals from the user, and add them to the farm. It will then print the contents of the farm on the screen. (Hint: use raw_input)

Modification #2

Change the for-loop of your program so that it reads:

      counter = 0
      for animal in farm
          print counter, animal
          counter = counter + 1
          

• Run the program. Observe its output. From this observation, modify the program so that its output now reads something like this (user input is in boldface):

       python lab2seq3.py

       animal? sheep
       animal? dog
       animal? bear
       animal? mouse

       Your farm now contains
       animal #1 : a sheep
       animal #2 : a dog
       animal #3 : a bear
       animal #4 : a mouse

Old MacDonald's Farm

Old MacDonald's farm is a song (almost) all American kids learn at one point in their life. I certainly never heard it when I grew up in France, but I think most of you will be familiar with it. If you can think of other lyrics that would be good for this exercise, let me know! You can easily find the lyrics on the Web.

The goal of this problem is for you to add a section to the program below so that it prints parts of the lyrics of the famous song using a for-loop.

Here's the beginning program which you have to modify:

1. start with four animals in the farm

farm = [ "horse", "pig", "dog", "cat" ]

1. display the names of the animals

for animal in farm:

   print animal

First modification

Modify this program so that the output looks like the lyrics of the song:

Old MacDonald had a farm, E-I-E-I-O And on his farm he had a horse, E-I-E-I-O Here a horse, there a horse, everywhere a horse!

Old MacDonald had a farm, E-I-E-I-O And on his farm he had a pig, E-I-E-I-O Here a pig, there a pig, everywhere a pig!

Old MacDonald had a farm, E-I-E-I-O And on his farm he had a dog, E-I-E-I-O Here a dog, there a dog, everywhere a dog!

Old MacDonald had a farm, E-I-E-I-O And on his farm he had a cat, E-I-E-I-O Here a cat, there a cat, everywhere a cat! (don't worry if you get extra spaces in your output) Second modification

Modify your program some more, and make it prompt the user for the names of four animals first, and then make it display the lyrics as shown above with the user's selected animals.

Hard problem of the day

This problem is a bit harder, and requires some thinking... Write a loop that prints 20 characters on the screen. Always 20. But it is made up of stars (*), which are printed first, and exclamation marks (!) that complete the line. Your program will first ask the user to enter a number. It will represent the number of stars that should be present in the line. The program will then print exclamation marks until the total number of characters is 20.

Examples of how your program should work (user input underlined):

      How many stars? 10
      * * * * * * * * * * ! ! ! ! ! ! ! ! ! ! 

      How many stars? 20
      * * * * * * * * * * * * * * * * * * * *

      How many stars? 1
      * ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !