Laboratory: Programming in Python (Conditionals and Repetition)
CSC 105 The Digital Age Spring 2009

Summary: In this laboratory, you continue the journey of learning to write computer programs in the Python programming language.

Contents

Exercise 0: Preparation

Open a terminal window, and move to the code directory you created in the first Python lab, and then open gedit as shown below. As before, you will move back and forth between these two windows, writing programs in gedit and running them in the terminal window. 

   cd 105/code
   gedit &

Next, please quickly review the previous lab so that the lessons you learned there will be fresh in your mind for you to use today.

Exercise 1: Making Decisions

So far, all the programs we have written work by executing each statement in the program in order, from top to bottom. To write programs that solve more complex problems, we need mechanisms that will allow statements to be executed in more complex ways. For example, we might like to execute some statements under some conditions but not others, or we might like to execute some statements multiple times.

In this exercise, we will explore programs that determine which statements should be executed based on the values of variables.

a. Please copy the following program into a file called sign.py and run it a few times, entering different numbers each time. You should find that if you enter a positive number, a message is printed to that regard, but if you enter a negative number or zero, no message prints. 

   number = input("Enter a number: ")
   if number > 0:
      print "That number is positive."

Note that the indentation of the print statement should be made by pressing the TAB key once.

Now add a second print statement (with any message you want) at the end of the program, being careful to indent it by the same amount the first print statement is indented. How does this affect the output of your program when you enter various values?

Now try removing the indentation from the second print statement. (In other words, leave the first print statement where it is, but move the second back to be aligned with if at the beginning of the line.) How does this affect the program's output?

If all went well, you should have discovered that when python encounters an if statement it checks whether the associated condition (in this case, number > 0) is true. If it is, python executes all statements that immediately follow the if clause and are indented beneath it. If the condition is false, these statements are skipped. In either case, execution will then continue with the next un-indented statement.

b. The example above uses one relational operator (greater than), but python has several more as shown below.
Operator In English
< less than
<= less than or equal to
> greater than
>= greater than or equal to
== equal to
!= equal not to

Notice in particular that to ask whether one value "equals" another value, you must use two equals signs, not one. For example, we could say: 

   number = input("Enter a number: ")
   if number == 0:
      print "You entered 0."

The if statement and relational operators work for comparing strings just like they do for comparing numbers.

Next, write a program called favorite.py that asks the user to enter a color name. If the color entered happens to be your favorite color, print a message to that effect. Otherwise, do not print any response. (Be very careful with punctuation in this program. The if clause must end with a colon, or python will complain.)

Notice that to check whether your program works correctly, you will now have to run it at least twice, entering colors that test both outcomes.

Now add one statement to your program that will cause it to print "Goodbye!" just before it ends, regardless of the color entered.

c. Let's return to the program sign.py. Suppose we want to print one message when the condition (number > 0) is true, and a different message when the condition is false. We can do that by adding an else clause as shown below. Please modify your copy of sign.py in this way, and do some experiments to convince yourself that it works. (Again, you must be careful to include the colon after else.) 

   number = input("Enter a number: ")
   if number > 0:
      print "That number is positive."
   else:
      print "Definitely NOT positive."
   print "This message prints every time."

d. Now return to favorite.py and modify your program so that it prints one message if the user enters your favorite color, and a different message if not. In addition, your program should print a final message for every user, regardless of what they entered.

Exercise 2: Repetition

Another way to modify the control flow of a program is to have it execute one or more statements repeatedly.

a. Please copy the following program into a file called loops.py, and then run it to see what it does. 

count = 1
while count <= 5:
   print "Grinnell"
   print "College"
   count = count + 1
print "Done"
How many timess are "Grinnell" and "College" printed? How many times is "Done" printed?

Now let's walk through the program to understand how it works. On the first line we create a variable named count and set it equal to one. The next statement contains a condition ( count <= 5 ). This condition will be evaluated, and if it is true, the statements indented beneath it will be executed. Once that has happened, execution returns to the top of the while loop, where the condition is checked again. This process will continue, with the indented statements being executed repeatedly as long as (i.e., while) the condition continues to be true. If the condition is ever false when it gets checked, execution will jump to the first statement after those associated with the loop (the first un-indented statement).

The careful reader will notice that it is important for something about the condition to change on each pass through the loop. What will happen if you remove the statement "count = count + 1"? Remove the statement to verify your prediction.

If all went "well", you should have discovered what we call an infinite loop. This means your program will continue running forever, unless you stop it from outside. You can do this by typing control-c in the terminal window where the program is running. (Go ahead and stop the program now.)

Modify the program loops.py so that it prints some word or phrase nine times, and also prints a line number at the beginning of each line. For example, your output might look like the following: 

1 Python
2 Python
3 Python
4 Python
5 Python 
6 Python
7 Python
8 Python
9 Python
IS FUN!

b. Write a program called squares.py that prints a list of the first nine positive integers and their squares. (Once again, be careful to include the colon at the end of the while statement, or Python will complain.) Your output should look similar to the following: 

Number Square
1        1
2        4
3        9
4        16
5        25
6        36
7        49
8        64
9        81

c. Write a program called blastoff.py that counts from 10 down to 1, and then prints "Blastoff!" Your output should look like this: 

10
9
8
7
6
5
4
3
2
1
Blastoff!

d. Write a program that asks the user to enter three numbers: a starting value, an ending value, and an increment. Your program should then "count" based on these criteria, as shown in the example below. 

This program counts for you.
Enter the starting value: 3
Enter the ending value: 13
Enter the increment: 2
3
5
7
9
11
13

Exercise 3: Combining Decisions with Repetition

We can place any valid python statement inside either a while-loop or an if-statement. Among other things, this means that we can place loops within loops, or if-statements within if-statements. Similarly, we can place if-statements within loops, and vice-versa. This flexibility allows us to solve many complex problems.

In this exercise, we will solve problems that require us to make a given decision multiple times. We can do this by placing an if-statement within a loop.

a. Write a program that uses a while-loop to print the output shown below. As you can see, the program should print the numbers 1 through 10, except that between 7 and 8 it should print the word "happy". This can be accomplished by placing an if-statement within the while-loop to do something special for the lineNumber == 7 case. 

1
2
3
4
5
6
7
happy
8
9
10

Exercise 4: Nested Loops

As mentioned above, we can have one loop inside the body of another loop. The following Python program prints a (not very pretty) "multiplication table" to the terminal window. 
numOne=1

while numOne<10:
  numTwo=1
  while numTwo<10:
    print numOne*numTwo,
    numTwo=numTwo + 1
  numOne=numOne + 1
  print
You can copy this program to your directory with the following command in the terminal window: 
  cp ~weinman/courses/CSC105/labs/times.py ./
The comma at the end of the first print statement prevents Python from immediately moving to the next line. Let's try to make a few other observations about how this works.
  1. Why is numTwo reset to 1 at the beginning of the "outer" loop?
  2. What would happen if this line was removed from the program?
  3. Test your prediction by commenting out the line. (That is, add a # to the beginning of the line.)
  4. Why is numTwo incremented in the body of the "inner" loop?
  5. What would happen if this line was moved to the "outer" loop (e.g., by removing one of the tabs from its indentation)?
  6. Hopefully, you should see that since numTwo would never change, the condition of the second while loop would always remain true, and we'd have another infinite lop.
  7. Why are there two different "count" variables for the nested loops? In other words, why wouldn't the following work? 
    ...
while count<10
  ...
  while count<10
    ...
    count=count+1
  ...
  count=count+1
The answer to the last question is that there is only one count variable for both loops. Any time count is modified in the inner loop, the value is also modified for the outer loop, and vice-versa. Thus, this code does not have the effect we want, which is to have two loops with two independent counters. This will be important for the next lab!

Exercise 5: For Those with Extra Time

a. Write a program that plays a familiar guessing game: first your program will set some number to be the "correct answer", then your program will ask the user to guess this answer until they get it right. For each guess your program should give a hint based on whether the guess was too high or too low.

For example, a session with your program may look like the following: 

Enter your guess: 46
Your guess is too low

Enter your guess: 97
Your guess is too high

Enter your guess: 62
Your guess is too low

Enter your guess: 88
Your guess is too high

Enter your guess: 82
CORRECT! You Win!
Hints:
1. Begin your program with a line similar to: answer = 82. In a real game we would want to begin with a random number, but for today let's pick a single number that will always be the correct answer.

2. This program will require a loop with some if-statements inside it.

3. Each time through the loop, you must accept a new guess from the user. This can be done by adding an input statement as the last statement within the loop.

4. How will you know when to stop looping? When the guess is your answer.

b. Write a program that reads in three strings and prints them out in alphabetical order. For example, 

Enter a word: hello
Enter a word: goodbye
Enter a word: zebra
In alphabetical order: goodbye hello zebra
Hint:
1. String comparisons, such as word1 < word2, work by comparing the ASCII values of the characters in the two strings. This means that word1 will be considered "less than" word2 if it comes first alphabetically. (Well almost. The ordering can be surprising if we compare capital letters to lower-case letters because all ASCII codes for capitals precede all codes for lower-case letters. I suggest you only enter lower-case words when running your program.)

2. This problem only requires if-statements, but your condition will need to be more complex than we have used before. A compound condition is one that includes multiple parts. Here are two examples. You may find that something similar will be useful in this program. 

if number > 2 and number < 10:
  **do something interesting here**

if word1 > word2 and word2 > word 3:
  **do something interesting here**
Written: Marge M. Coahran, March 2008
Revised: Jerod Weinman, 2 Janury 2009
 Adapted from CSC105: Programming in Python (Decisions and Repetition)