Computer Science 111, Spring 2012

Lab #3

February 15 and 16, 2012

The purpose of today's lab is to work with data structures (strings and lists) and sequence controllers (definite loops and the decision structure if-statement).
Work with a partner as before.

python3 statements/functions used:

  1.  a definite loop (for loop)
      
      for i in list:
          <statements>
      We will use a list written explicitly with square brackets,
         or a list that is stored in a variable.
  2.  string functions.
      examples 
      x = "Computer Science is great"
      words = x.split()  makes a list ["Computer","Science","is","great"]
       We can use this list in a definite loop 
          for i in words:
              print(i)
  3.  random numbers
      x = random.randrange(4,9)
      x will have a value that is either 4,5,6,7, or 8
  4.  equality operator ==
          "brown" == "brown" is True
          "brown" == "white" is False
  5.  if-statement
        if <True or False condition>:
	     <statements>
      if word=="You":
          print("Hey you!")

Get a copy of the file lab3judy.pyc from my handout directory (with the command getcopy lab3judy.pyc)
- and then run this program by invoking the python interpreter, and typing import lab3judy. The program will run right away because I placed a call to main() at the end of the file lab3judy.py.
Note that you only get to have the .pyc file, which has already been interpreted, and is not the source file, full of python statements that would tell you how to write the program!

This is the program that you will write for the this lab

Use emacs to write your own version of the program. This is not a deep, meaningful program. It's just a way to try out some of python's functions and statements.
I have broken it into three parts. Get each part to work first before moving on the the next part.
Here is what your program should do:

Part 1.

Set up a list of 8 strings at the beginning of the main() function definition, that are stored in a variable called canned.
Print a brief introduction on the screen.
Get the user's input: using the input() function, print out a conversational message to the user and read in what the user types.
Use the split() function from the string module and split the user input into a list of words. Assign the result to the variable user_words.
print out each string from the list assigned to user_words on a separate line, using a definite (for) loop. This kind of loop is useful to separately examine each item in a list.
Make a new empty string and assign it to a variable called computer, by using the statement
computer = ""
For now, just concatenate, using the string concatenate operator (+), each string from user_words onto variable computer. You will again use a definite (for) loop. Put a space character between each string.
After the loop, print the string stored in the variable computer to verify that the original sentence is reproduced.
Now use a for loop to print every string in the list of strings stored in variable canned, one string per line, and then print 2 blank lines after that.
Finally for this part, print the following string:
```
We just demonstrated some string manipulations
```
Get all of this part to work before moving to Part 2. Place a call to the function main() after its definition, in the file. The call, main(), will not be an indented statement. It will be aligned with the reserved word def.

Part 2.

Next, the program will get one of the canned responses fron the list. The program will use the random.randrange() function to choose one randomly. To use python's randrange() function, type
import random before the beginning of the def main() at the top.

If you want a random number between n and m-1, type
random.randrange(n,m). One integer value will be generated, with a value between n and (m-1), inclusive. Use
```
len(canned)
```
as the second parameterm, m. What is the first parameter, n? Think of the possible index values of a list.
You can set this up as a set of several statements
```
m = len(canned)
n = ??
index = random.randrange(n,m)
```
and use the variable called index with the variable canned to get one of the strings from the list.
Next use the random canned response as a prompt with the input() function to get a user message. Store this in a variable called message.
Now set up a definite (for) loop, that goes through the list [1,2,3,4].
After the definite loop is over, the program prints another random canned response (done the same way as before).

Part 3. Now try out the if statment
Now use the input() function to ask the user to say something with lots of "Is" and "Yous" in it. Assign the user string to the varialbe message.

Type the following statements at the end of your lab3.py main() function definition:

    message = message.split()
    computer = ""
    for word in message:
        old_word = word
        word = word.replace("I","You")
        if old_word==word:
            word = word.replace("You", "I")
        computer = computer + word + " " 
    print(computer)

This code takes each word from the sequence message, and first places the word in a variable called old_word, for safe-keeping.
The statement

        word = word.replace("I","You")

finds all occurrences of the string "I" in word, and replaces it with "You". The variable word will only contain one word so it is either replaced or not. It then assigns the resulting string to the variable word. The next line is the beginning of an if statement

        if old_word==word:

it checks if old_word and word are the same using the equality operator (==). This is a nice feature of python. If they are the same, then no replacement happened. In that case, we want to try

            word = word.replace("You", "I")

and if the word contains "You", we'll replace it with "I" and assign the new string to word.

After this statement, but still inside the for loop, concatenate the string in variable word to variable computer, with the string that contains one space, " ", as before. After the loop statement, print out the string contained in the variable computer.

Make sure all three parts are in just one function definition, for function main(). And make sure there is one call to main() at the end of your file, without indentation. All three of these parts are useful for the homework assignment.