Laboratory: Programming in Python
CSC 105 - The Digital Age
Summary: In this laboratory, you begin the journey of
learning to write computer programs in the Python programming language.
Contents
-
Open a terminal window. Create a directory in which to store your
programs, with the command:
mkdir 105/code
-
Move to that directory:
cd 105/code
-
Open the text editor gedit, as shown below. (Note that
the ampersand in the command is important. It allows you to
open gedit, yet retain access to your terminal window.)
gedit &
-
Arrange your windows so that you can see the terminal window and
gedit at the same time. During this lab and the next few,
you will move back and forth between these two windows as you
work. Your work cycle will be:
- modify your program in gedit
- save your work
- run your program in the terminal window
-
Your first program will contain a single Python statement. Enter
the following statement into a new document in gedit, noting
that the quotation marks are important.
print "Welcome to the wacky world of computer programming!"
-
Save your work, naming the file first.py. Note that it is
important to use the file extension .py. This specifies
that your file is a Python program. The dialog box that appears
should have a box for Name:. The Save in folder: box
should say
code. (If it does not, you may need to use
the "Browse for other folders" option in order to save your file
in your 105/code directory.)
Note that when you save the file, gedit will automatically
colorize the text for you. Much like it did with HTML, the text
editor recognizes (from the file name) the "langauge" you are
writing in, and uses color/font highlighting to visually help you
write and understand your programs.
-
Run your program in the terminal window, with the following
command.
python first.py
If all goes well, your program should print the greeting in the
terminal window. If that does not happen, try to figure out why, or
ask for help.
-
Add a second print statement to your program, as follows:
print "Are we having fun yet?"
Save your program, and run it in the terminal window.
-
When you have that working, modify your program to add a few more
lines of text to your output. Note that you can use any
punctuation or printable symbol on the keyboard.
-
Now open a new file in gedit to start writing a new
program named figure.py.
-
In your new file, write a program that draws a figure similar to
the one below, using a set of
print commands. This kind of drawing is called "ASCII
Art" since it is made solely with characters in the ASCII
character set.
* *
* *
* *
*
* * *
* * *
* * *
*
* *
* *
* *
* *
* *
Remember to save your work before running your program in the terminal window.
-
Write a program (create a new file) named ascii.py in
which you draw your own ASCII Art design using
Python print statements. (Please do give your program
this name, because we may come back to it later, and it would be
nice to be able to find it easily.)
Have fun with this exercise, but please only work on your drawing for
more more than 5 minutes. We have a lot of fun things left to do!
In the previoius exercises, you used character strings,
which are more commonly called just strings for short. A
string is a single data element, composed of zero or more
characters.
When typing a string directly into a program, as we have done, we
enclose the string in quotation marks. This tells Python that the
enclosed characters comprise a single string. (Note that the
quotation marks were never printed in the output. They are not part
of the string; rather, they are part of the program used to
define the string.)
We can also store a string in a variable. To do so, we use an
assignment statement such as the following. This is an
instruction that tells Python to store the string "Sue" into a
variable named name. We say that the value "Sue"
is assigned to the variable name.
Please note carefully, that (although it seems strange) the
"equals sign" here does not denote equality. Rather, it is an
instruction telling Python to assign the value on its right into the
variable on its left.
-
Try this out by entering the following program into a new file
called name.py. Be careful to include all the punctuation
exactly as specified. The commas in the print statement allow us
to give the print command three
different arguments: a string, a variable, and another
string. If all goes well, each of them will be printed in turn.
name = "Sue"
print "Hello " + name + "! how are you?"
-
Now save your program and run it. You should also find that
the print statement prints the value stored in the
variable name, rather than printing the
word name itself.
-
We can also use string concatenation to create new strings and
assign them to variables. For example,
name = "Sue"
greeting = "Hello, " + name + "! How are you?"
print greeting
-
We can also ask the "user" (i.e., the person running the
program) to input a string for our program to use. To do this,
we use statements like the following. Here, raw_input
is a subroutine, one of our key algorithm
ingredients. We may also refer to a subroutine as
a function. Note that it is followed immediately by a
pair of parenthesis with a string inside them. This string is
an parameter (another of our algorithm ingredients),
and we say that we pass it to the
subroutine raw_input. We may also call a parameter
an argument.The purpose of raw_input is to
print its argument, wait for the user to input something, and
then
return that input as a string. The string will then get
assigned to the variable str.
strn = raw_input("Please enter a string: ")
print "You typed " + strn
Try out the code above to see exactly what it does. When you run
the program, it should print a prompt asking you for
data. It should then wait until you type something and
press Enter. Only when you have done that will the
program move on to process the print statement.
-
Write a program called emphasize.py that prompts the
user to enter a word, and then prints that word surrounded by a
pair of asterisks on each side. For example, a run of your
program might look like the following:
Please enter a word: Hello
** Hello **
-
Write a program called greeting.py that asks the user to
enter his or her name and favorite color, and then writes a message
that includes that information. For example, a run of your program
might look something like this:
What is your name? Sue
What is your favorite color? turqoise
Hi, Sue! I like turquoise too!
-
Variables are called variables because we can change the values
that they store. Consider the following program. What do you
expect to be printed when the program is run?
color = "red"
print color
color = "blue"
print color
Give this program a try to see if it does what you expect. You
should see that the original value stored in color
("red") is overwritten by the new value "blue". A variable can
hold one only value at a time, so when you assign a new value to
a variable, the old value is overwritten.
-
Add the following statements to the bottom of the previous
program. What do you expect will be printed when the program is
run?
hue = "green"
print hue
color = hue
print color
print hue
-
Run the program to see if you are right. This example should show
that we can also assign the value stored in one variable
(hue) into another variable (color). Just
remember that an assignment statement always assigns the value on the
right to the variable on the left.
-
Write a program, in a file named swap.py, that begins as
follows.
color1 = "red"
color2 = "blue"
You can tell whether your program is successful by ending it with
the following statement:
print color1 + " " + color2
We can also extract (or modify) individual characters within a string. To do
so, we need a way to specify which character we mean. This is done by giving
each position in the string an index number, which is determined by
simply counting off (starting at 0) from left to right. We then use the
index number as a subscript into the string. The syntax for
doing this is shown in the example below.
For example, if the variable color holds the string "turquoise",
then
- color[0] holds the letter t
- color[1] holds the letter u
- color[2] holds the letter r
etc.
-
Write a program that prompts the user to enter a 6-letter word,
and then prints the first, third, and fifth letters of that word. A
sample run of your program might look like this:
Please enter a 6-letter word: sadist
The first, third, and fifth letters are: s d s
-
We can also instruct the computer to search for a given character
within a string, using a method called index. (A
method is very similar to a function or subroutine, but the syntax
is a bit different as shown in the example below.)
For example, suppose again that the variable color holds
the string turquoise. We can retrieve the index of the
letter q (which is 3) as follows:
color = "turquoise"
index = color.index("q")
print "The index number for the letter q within the word " + color + " is" , index
Try this program to convince yourself that it will, in fact, determine
the correct index number for the letter q.
Note: We have used a comma rather than a plus
before index above to concatenate the string and a
number. As you will learn on the next lab, the difference between
these two types of data is important. For now, please take our
word that the comma is the "right thing" to do, and a plus would
render the program incorrect.
-
Modify the program to have it search for other characters in the
string.
- Does it always return the index number you expect?
-
What index is returned if you ask for the index of the
letter u?
-
What happens when the desired character appears more than once
in the string)?
a.
We can also isolate substrings within a string based on the index
values of the characters we are interested in. Try out the following program
to see if you can discover how the syntax for substrings works.
color = "turquoise"
print "color = " + color
print "color[0:1] = " + color[0:1]
print "color[2:4] = " + color[2:4]
print "color[1:5] = " + color[1:5]
print "color[3:8] = " + color[3:8]
print "color[:4] = " + color[:4]
print "color[2:] = " + color[2:]
b.
Write a program called time.py that prompts the user to enter the
current time in HH:MM format, and then prints a message that
states the time in a sentence. (To begin with, assume that the user will
always enter two characters for the hour, even if one would do. Similarly,
your program may print two characters, even if one would do.)
For example, a sample run of your program might look something like this:
What is the time please (HH:MM)? 06:55
Thanks! It is now 55 minutes after 06 o'clock.
Now modify your program so that it will still work correctly if the user
enters one-digit hours using only one digit. For example:
What is the time please (HH:MM)? 2:55
Thanks! It is now 55 minutes after 2 o'clock.
c.
CHALLENGE PROBLEM:
From a Java lab written by Sam Rebelsky.
You may be familiar with Shirley's Ellis's Name Game. Ms. Ellis
describes her procedure for developing phrases based on her colleague's
names as follows:
Come on everybody!
I say now let's play a game
I betcha I can make a rhyme out of anybody's name
The first letter of the name, I treat it like it wasn't there
But a B or an F or an M will appear
And then I say bo add a B then I say the name and Bonana fanna and a fo
And then I say the name again with an F very plain
and a fee fy and a mo
And then I say the name again with an M this time
and there isn't any name that I can't rhyme.
She also gives us a number of examples:
Shirley!
Shirley, Shirley bo Birley Bonana fanna fo Firley
Fee fy mo Mirley, Shirley!
Lincoln!
Lincoln, Lincoln bo Bincoln Bonana fanna fo Fincoln
Fee fy mo Mincoln, Lincoln!
Write a program called NameGame.py that gets a name from the
user and prints out a verse of the form Ms. Ellis suggests. Your
program need not handle names that begin with vowels or with more
than one consonant.
