CSC111 Lab 3 2015

--D. Thiebaut (talk) 21:22, 9 February 2015 (EST)

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Understanding how Floats take over Expressions

In this section I am asking you to play and get an understanding of important functions, and also how floating point numbers (numbers with a decimal point) take over expressions, when they are present in them.

Open the Python shell and type the expressions below. Try to predict the output of the interpreter when you are about to press the ENTER key.

>>> a = 3
>>> x = 1.5
>>> type( a )

>>> type( x )

>>> type( a * x )

>>> type( a * 100 )

>>> type( 1 )

>>> type( 1.0 )

>>> type( "1" )

>>> a / 3

>>> a // 3

>>> type( a/3 )

>>> type( 5/4 )

>>> type( 5//4 )

>>> type( int( 1.5 ) )

>>> int( 1.5 )

>>> round( 1.6 )

>>> round( 1.4 )

>>> round( -1.8 )

>>> type( round( 1.4 ) )

>>> round( -1.8 )

>>> abs( -4 )

>>> abs( 10 )

>>> type( abs( -19.3 ) )

>>> type( abs( -4 ) )

• Note how once something is a float, if forces whatever it gets combined with to yield a result that is float, except for the int() function, of course.

Challenge #0: (we always start at 0 in Computer Science!)

• Write a small program (either in the Python Shell, or in the Edit window), that asks the user to enter a number between 0 and 100, and that prints out 0 if the number is between 0 and 25 (25 not included), 1 if the number is between 25 and 50 (50 not included), 2 if the number is between 50 and 75 (75 not inclulded), and 3 if the number is between 75 and 100 (100 not included).

• Here is an example of the type of exchange I had with

>>> 
>>> x = eval( input( "Enter a number: " ) )
Enter a number: 75.7
>>> result =  ...  write an expression here that takes x and computes the expected result
>>> result
3
>>> x = 1
>>> result = ...  use the same expression you wrote above...
>>> result
0
>>> x = 24.99999
>>> result = ...  use the same expression you wrote above...
>>> result
0
>>> x = 25.00000001
>>> result = ...  use the same expression you wrote above...
>>> result
1
>>>

Teller Machine Program

This section will get you to write a program similar (though not necessarily the same) to the program we wrote in class on Monday. The program takes an integer (without a decimal part) amount of dollars and figures out how to break it down into the least number of 20-, 10-, 5-, and 1-bills.

Reviewing the Division Operators, // and %

Use the Python shell, and try to predict the result of the following operations.

>>> 21 // 5

>>> 21 % 5

>>> 9 // 2

>>> 9 % 2

>>> 13 // 3

>>> 13 % 3

>>> 139 // 20

>>> 139 % 20 

Beginning Program

• Write a program that contains
  • a short header with the program name (lab3.py, for example),
  • your name,
  • the date, and
  • A short description of the program.
• Create 3 different comment lines that will create an outline of your program.

# lab3.py
# yourName
# date
# blah blah blah blah blah blah...
#

# get the initial amount

# compute number of bills

# output number of bills to give out

• Save and Run the program, just to make sure you do not have a syntax error.

Define original amount

• Under the #get the initial amount comment, create a variable called amount and initialize it with a value of your choice. Pick a value that is not a multiple of 5.
• Add a print() statement under the #output number of bills comment, and make it print the amount the user wants to withdraw.
• Verify that your program works.

Compute the number of $20s to give out

• Using the right operator (//, /, or %), make your program compute the number of $20 and store that value in a new variable, called no20s. Add this code under the # compute number of bills.
• Make your program output the no20s variable in the output section.
• Verify that your program works.

Computing the Left-Over Amount

• Go back to the computation of the number of $20s, and compute the amount of money left over once the $20s are taken out of the amount. You have several ways of doing this. You can do it using the % modulo operator, or using multiplication and subtraction. Whichever method you use is fine for today.
• Make your program output the left-over amount, just to make sure that value is computed correctly.
• Verify that your program works fine.

Computing the Remaining Quantities

• Now that you have the structure for your program, add enough Python code to make your code display
  • The total amount
  • The number of $20-bills
  • The number of $10-bills
  • The number of $5-bills
  • The number of $1-bills

• Verify that your program works. Below is a typical output you should try to emulate:

Amount to withdraw =  97

Please lift keyboard and find: 
4 $20-bill(s)
1 $10-bill(s)
1 $5-bill(s)
2 $1-bill(s)

Flexibility and Adaptability

Imagine that your program will be used in an area where the bills do not come in 20, 10, 5 or 1 denominations, but in 100, 50, 10, and 1.

Figure out a way to make the least amount of change to your program so that it now outputs the correct break down for any amount, but in $100-, $50-, $10- and $1-bills.

Make sure your program works! Below is the output of the program if the amount is set to $97:

Amount to withdraw = $ 97

Please lift keyboard and find: 
0 $ 100 bill(s)
1 $ 50 bill(s)
4 $ 10 bill(s)
7 $ 1 bill(s)

Challenge #1: A Change Machine

• Using a similar approach, write a program that, given some number of pennies, will output the correct number of quarters, dimes, nickels, and pennies. You should initialize the amount of pennies with an integer, like this:

 pennies = 84

• Here is an example of what your program should output for 84 pennies:

Amount to withdraw = 84 cent(s)

Please lift keyboard and find: 
3 quarter(s)
0 dime(s)
1 nickel(s)
4 cent(s)

Formatted Output

Strings

In this section you will work on formatting strings, ints, and floats using the {...} formatting command.

• Try this statement out:

    name = "Snow White"
    print( "Hello {0:1}! How are you?".format( name ) )

• Then modify the {0:1} part and try these different combinations: {0:5}, {0:10}, {0:15}, {0:20}. For each one, make sure you understand why the output changes (or not).
• Then try these different formats: {0:>1}, {0:>5}, {0:>10}, {0:>15}, {0:>20}. Notice the right-justification of the string Snow White in this second set of outputs.

• Now try this piece of code:

    print( )
    for friend in [ "Bashful", "Doc", "Dopey",  "Happy", "Sleepy", "Sneezy", "Grumpy"]:
        print( "Hello {0:10}!".format( friend ) )

Printing 2 strings at once

• Change the print() statement so that you now print two different strings, each with its own {...} format:

    print( )
    for friend in [ "Bashful", "Doc", "Dopey",  "Happy", "Sleepy", "Sneezy", "Grumpy"]:
        print( "{0:1} wants to take {1:1} to the Valentine's ball." . format( friend, "Snow White" ) )

• Now switch the 0 and the 1 in the two {...} expressions, as illustrated in the line below:

        print( "{1:1} wants to take {0:1} to the Valentine's ball." . format( friend, "Snow White" ) )

• Notice how the first number in the {...} expressions is used to figure out which string to pick in the format(...) block.

Printing a String and an Int

• Try this formatting command:

    name = "Valentine's Day" 
    day = 15
    print( "{0:>20} is on the {1:1}th of the month" . format( name, day ) )
    name = "My birthday"
    day  = 6
    print( "{0:>20} is on the {1:1}th of the month" . format( name, day ) )

• Here's another statement to try:

    name = "Snow White"
    length = len( name )
    print( "the string {0:1} contains {1:1} characters" . format( name, length ) ) )

Challenge #2:

• Modify the for-loop you used before so that your program now outputs friend and len( friend ), as shown below.

Bashful 7
Doc 3
Dopey 5
Happy 5
Sleepy 6
Sneezy 6
Grumpy 6

• Once you get this output, format it into a nice looking table using the {...} formatting command.

+------------+-----+
| Bashful    |   7 |
| Doc        |   3 |
| Dopey      |   5 |
| Happy      |   5 |
| Sleepy     |   6 |
| Sneezy     |   6 |
| Grumpy     |   6 |
+------------+-----+

Printing Floats

• Copy/paste this block of code below in your program. Then run it. Verify that the output makes sense.

    print( "Printing PI" )
    pi = 3.14159
    print( "-------------------------" )
    print( "Pi={0:1.0f}!".format( pi ) )
    print( "Pi={0:2.0f}!".format( pi ) )
    print( "Pi={0:3.0f}!".format( pi ) )
    print( "Pi={0:4.0f}!".format( pi ) )
    print( "Pi={0:5.0f}!".format( pi ) )
    print( "Pi={0:6.0f}!".format( pi ) )
    print( "Pi={0:7.0f}!".format( pi ) )
    print( "Pi={0:8.0f}!".format( pi ) )
    print( "Pi={0:9.0f}!".format( pi ) )
    print( "-------------------------" )
    print( "Pi={0:9.1f}!".format( pi ) )
    print( "Pi={0:9.2f}!".format( pi ) )
    print( "Pi={0:9.3f}!".format( pi ) )
    print( "Pi={0:9.4f}!".format( pi ) )
    print( "Pi={0:9.5f}!".format( pi ) )
    print( "-------------------------" )
    print( "Pi={0:<9.1f}!".format( pi ) )
    print( "Pi={0:<9.2f}!".format( pi ) )
    print( "Pi={0:<9.3f}!".format( pi ) )
    print( "Pi={0:<9.4f}!".format( pi ) )
    print( "Pi={0:<9.5f}!".format( pi ) )

Temperature Conversion

• Remember that we saw that a temperature in Celsius is computed as the temperature in Fahrenheit - 32, the difference multiplied by 5/9?
• Write a for-loop that displays two columns of numbers, the column on the left showing degrees Fahrenheit, and the column on the right showing Celsius.

20 -6.666666666666667
22 -5.555555555555555
24 -4.444444444444445
26 -3.3333333333333335
28 -2.2222222222222223
30 -1.1111111111111112
32 0.0
34 1.1111111111111112
36 2.2222222222222223
38 3.3333333333333335
40 4.444444444444445

Challenge #3:

• Use the {...} formatting command to make your output look nice:

     20.00F =      -6.67C
     22.00F =      -5.56C
     24.00F =      -4.44C
     26.00F =      -3.33C
     28.00F =      -2.22C
     30.00F =      -1.11C
     32.00F =       0.00C
     34.00F =       1.11C
     36.00F =       2.22C
     38.00F =       3.33C
     40.00F =       4.44C

Challenge #4:

• Add some bars and a legend, too!

+-------------+-------------+
| Fahrenheit  |    Celsius  |
+-------------+-------------+
|      20.00F |      -6.67C |
|      22.00F |      -5.56C |
|      24.00F |      -4.44C |
|      26.00F |      -3.33C |
|      28.00F |      -2.22C |
|      30.00F |      -1.11C |
|      32.00F |       0.00C |
|      34.00F |       1.11C |
|      36.00F |       2.22C |
|      38.00F |       3.33C |
|      40.00F |       4.44C |
+-------------+-------------+

Accumulating Quantities

The idea for this section is that some quantities can be computed not all at once, but instead, one piece at a time. Computers are extremely fast and ready for this type of accumulation of data. In this section we look at several ways to do this with various types of data. We will see later on in the semester, that some of the operations we are performing here can be done more easily some other way. That's ok. Programming is also the art of solving problems, and not necessarily to find the most elegant or fastest solutions (although it is good to be aware of them!)

Sum of a list of numbers

• Add this piece of code to your program (or create a new one):

    sumAll = 0
    for n in [100, 10, -30, -2, 20, 5 ]:
        sumAll = sumAll + n

    print( "sum = ", sumAll )

• Run the code. Add the numbers by hand to make sure your program outputs the correct information.
• Add this print statement inside the for-loop, right after the "sumAll = ..." statement:

         print( "n = {0:3} sum = {1:3}" . format( n, sumAll ) )

• Run your code. Observe how the loop allows the variable sumAll to grow with the addition of every new number in the list.

Note that the rule for accumulating quantities with a loop is always the same:

1. first you initialize a variable to represent some null quantity
2. then you loop, and in the loop you increase the variable with a new quantity given by the loop.
3. at the end of the loop, your variable will contain an accumulation of all the values your loop touched, or created.

Challenge #5:

• Replace the list of numbers [100, 10, -30, -2, 20, 5 ] by the range() function and make your program compute the sum of all the even numbers between 0 and 100, included.
• Verify that your program outputs 2550 as the result! You do not need to make your program output all the intermediate values of sumAll.

Count the Items in a List

• Try this piece of code now:

count = 0
for n in [ 100, 10, -30, -2, 20, 5 ]:
        count = count + 1

print( "there are", count, "items in the list" )

• See how this code is different from the loop in the previous section. Here we add 1 to the counter every time we go through the loop. In other words, we add 1 for every item we find in the list. So, in effect, we are counting the items in the list.
• Run your code and see what it outputs.
• Add print statement inside the loop, below the count = count + 1 line, and make the print statement print n and count. Don't worry about the formatting.
• Run your code. Do you see the program counting the items it finds in the list of numbers?

Challenge #6:

• Write a program that uses a for-loop that scans a list of numbers: [1, 2, 2, 1, 2, 3, 3, 2, 1, 0]
• Make your program compute the average of the numbers in the list. The average is the sum of all numbers divided by the number of items.
• Your program should output the average at the end, as a floating-point number.
• Run your program. Verify that you get the correct result (sum = 17 average = 1.70).