CSC111 Lab 5 2015

--D. Thiebaut (talk) 12:17, 22 February 2015 (EST)

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Preparation

• Copy paste the program below in a new Idle window, and call it lab5Companion.py.
• Save your new program to your working directory, where you normally save your lab programs.
• Run the program. It should create 3 text files in your folder. You will be using them in this lab. The files are called:
  • chocoQuotes.txt
  • hargitay.txt
  • joanneHarris.txt

# lab5Companion.py
# D. Thiebaut
# This program will create several files in the directory
# where you will run it.
# The files are retrieved from a Web server, stored temporarily
# in a string, and stored in a local file on your disk.
# The 3 files are very, very short.
# Their names are:
# - chocoQuotes.txt
# - hargitay.txt
# - joanneHarris.txt

import urllib.request  # the lib that handles the url stuff

# main program
def main():

    # list of files to retrieve 
    fileNames = [ "chocoQuotes.txt", "hargitay.txt", "joanneHarris.txt" ]

    # get each file, one at a time
    for fileName in fileNames:

        # url where the files are located
        url = "http://cs.smith.edu/~dthiebaut/111/files/" + fileName

        # fetch the file and put its contents in the
        # strign text.
        response = urllib.request.urlopen( url )
        text     = response.read().decode( 'UTF-8' )
        
        # write text to file, in the same directory as this program.
        file     = open( fileName, "w" )
        file.write( text + "\n" )
        file.close()

        # give feedback to user.
        print( "File {0:1} created in your folder".format( fileName ) )


main()

Splitting Long Strings

In this section you will see how to take text represented as a multi-line string, and process it various ways.

• Long strings that span several lines can be created by enclosing them in triple quotes; either triple single-quotes (), or triple double-quotes (""").
• Copy/paste the program below in a new program. You may call it lab5.py

# lab5.py

text = """Chocolate
Chocolate is the first luxury.
It has so many things wrapped up in it:
Deliciusness in the moment,
childhood memories,
and that grin-inducing
feeling of getting a reward for being good.
--Mariska Hargitay"""

def main():
    # make sure main knows that this variable exists
    global text
    
    # split the long string into individual lines
    lines = text.split( "\n" )

    # print each line
    for line in lines:
        print( line )

main()

• Run the program. Verify that it outputs the entirety of the text.
• Just to make sure we understand what the variable lines contain, make your program print lines, with a simple print( lines ) statement.
• Observe that it's just a regular Python list.

Challenge #1

• Modify your program, and make it print the poem so that each line is centered in a field of 60 spaces. You must use the text variable, and print its line. Do not create a new version of the poem!
• Expected output

                         Chocolate                          
               Chocolate is the first luxury.               
          It has so many things wrapped up in it:           
                Deliciusness in the moment,                 
                    childhood memories,                     
                   and that grin-inducing                   
        feeling of getting a reward for being good.         
                     --Mariska Hargitay

Challenge #2

• Make your program print the first line of the poem, centered, but also in uppercase.
• Make it display the other lines centered, without changing their case.
• Expected output

                         CHOCOLATE                          
               Chocolate is the first luxury.               
          It has so many things wrapped up in it:           
                Deliciusness in the moment,                 
                    childhood memories,                     
                   and that grin-inducing                   
        feeling of getting a reward for being good.         
                     --Mariska Hargitay

Challenge #3

• One more modification. Now make your program print the last line right-justified in a field of 60 spaces.
• The first line is still in upper-case.
• Make it display the other lines centered, without changing their case.
• Expected output

                         CHOCOLATE                          
               Chocolate is the first luxury.               
          It has so many things wrapped up in it:           
                Deliciusness in the moment,                 
                    childhood memories,                     
                   and that grin-inducing                   
        feeling of getting a reward for being good.         
                                          --Mariska Hargitay

Challenge #4

• Make your program print the poem in a box. The top and bottom lines have 50 dashes in them.
• Expected output

+--------------------------------------------------+
|                    CHOCOLATE                     |
|          Chocolate is the first luxury.          |
|     It has so many things wrapped up in it:      |
|           Deliciusness in the moment,            |
|               childhood memories,                |
|              and that grin-inducing              |
|   feeling of getting a reward for being good.    |
|                                --Mariska Hargitay|
+--------------------------------------------------+

Reading Text From File

• Open Finder or Windows Explorer and go to the folder that contains your lab5.py program, as well as your lab5Companion.py programs. Make sure the folder contains the 3 text files you created earlier.
• Open the file name hargitay.txt. Verify that it contains the same poem we have been playing with.
• Either create a new program or add a new main() function to your existing program (rename the original main, main2(), maybe?), and put the code below in the new function:

def main():
    fileName = "hargitay.txt"
    file = open( fileName, "r" )
    lines = file.readlines()
    file.close()

    for line in lines:
        print( line )

• Run the program. Observe its output.
• The reason for the double-spacing is that each line read from the file contains a '\n' at the end of it. And when you print such a line, the print() function adds its own \n at the end. This is why you get a blank line in between lines.
• One way to remove the '\n' at the end of each line is to print the line as follows: print( line.rstrip() ). The rstrip() method right-strips the string of all white-space characters, which include spaces, tabs, and '\n'-characters.

Challenge #5

• Make your program get the name of the file from the user, with an input() statement.
• Then, make your program output the contents of the file in a box, as you did in Challenge 4.
• Test your program by providing it with different file names:
  • hargitay.txt
  • joanneHarris.txt
  • chocoQuotes.txt
• Adjust the width of the box if necessary.

Challenge #6

• Exact same challenge as Challenge 5, but this time start with this program, where I have replaced the .readlines() method with .read().

def main():
    fileName = input( "File name? " )
    file = open( fileName, "r" )
    text = file.read()
    file.close()

    print( "The type of text is", type( text ) )
    print( "text = ", text )

Note

when Python says that something is of type <class 'str'>, it means that it is a string.

Writing Text to File

• Try this new version of main:

def main():
    fileName = "hargitay.txt"
    file = open( fileName, "r" )
    text = file.read()
    file.close()
    
    text = text.replace( "chocolate", "carrot" )
    text = text.replace( "Chocolate", "Carrot" )

    file = open( fileName, "w" )
    file.write( text + "\n" )
    file.close()

• Open Finder or Windows Explorer, and take a look at the file "hargitay.txt", either with TextEdit or with Notepad. See anything different?
• BTW, You can recreate the original file by running the lab5Companion.py program again.

Challenge #7

• Modify the program so that it prompts the user for
  • The name of the file to read from and write to, and
  • a word

and make the program read the file, replace the word chocolate in it with the word the user picked, and save the resulting text back to the file.

• Verify that your program works correctly and modifies the file you select and replaces chocolate with the word you choose.

Moodle Submission

Submit your solution program for Challenge #7 to Moodle, in the Lab 5 PB 7 section.

Functions

Preparation

• Try the following statements in the Python console:

>>> def test():
	print( "hello" )
	print( "there" )

	
>>> test()


>>> def test():
	print( "hello", end="" )
	print( "there" )

	
>>> test()

>>> def test():
	print( "hello", end="-" )
	print( "there" )

	
>>> test()
>>>

Notice what happens when you add the end="" construct in the print statement. It changes the way the print() function ends printing what it has to print. By default, if you do not have an end="" construct, print() brings the cursor to the next line. But this behavior can be changed. We are going to use this, in the new program below.

Functions and Stick Figure

• This section will be cleaner if you start a new program, rather than add to an existing program.
• Add this code section to your new program:

def head():
    print( "  o" )

def leftArm():
    print( " /", end="" )

def torso():
    print( "O", end="" )

def rightArm():
    print( "\\" )

def leftLeg():
    print( " /", end="" )

def rightLeg():
    print( " \\" )

def stickFigure():
    head()
    leftArm()
    torso()
    rightArm()
    leftLeg()
    rightLeg()

def main():
    stickFigure()

main()

• Take a close look at the program. Locate the end="" construct, which prevents the print from going to the next line when it's done.
• Notice also the "\\" string. Because the '\' character is used to print special characters, like '\n' (new line), or '\t' (tab), if we want to print a regular backslash character, we have to use 2 backslashes next to each other.

• Run the program. Notice how it displays a stick figure.

Functions and Stick Figure, Version 2

• Modify the stick figure program so that it looks like the code below:

def head():
    print( "  o" )

def leftArm():
    print( " /", end="" )

def torso():
    print( "O", end="" )

def rightArm():
    print( "\\" )

def leftLeg():
    print( " /", end="" )

def rightLeg():
    print( " \\" )

def body():
    leftArm()
    torso()
    rightArm()

def newStickFigure():
    head()
    body()
    leftLeg()
    rightLeg()

def main():
    newStickFigure()

main()

• Observe how the code is different, and the new function body() is used to display the body of the stick figure (torso plus arms).
• Using the example of body(), add a new function called legs() that will call leftLeg() and rightLeg() to draw both legs. Modify newStickFigure() by removing the calls to leftLeg() and rightLeg(), and adding instead a call to legs(). Your new function should look something like this:

def newStickFigure():
    head()
    body()
    legs()

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