(Aim to complete this before your lab in week 5.)

Learning Objectives

·      to gain practice in the use of while loops

·      to gain practice in the use of functions

·      to gain practice in the writing of functions

Make a new folder called Lab 3 in your Labs folder. All the example programs that you develop during this lab should be kept in this folder.                                                                         

Loops and Functions

Processing Exam Marks

Open an IDLE editor window and copy and paste the following program into it.

"""
average.py
program to calculate average of a series of exam marks
created by sands 23/10/10
"""

# initialise total and counter
total = 0.0
count = 0

print("Enter marks one per line")
print("Use a negative number to end")

# get first mark
mark = float(input("Mark: "))

# process mark and get next one
# continue until negative number entered
while mark >= 0 :
    total = total + mark
    count = count + 1
    mark = float(input("Mark: "))

# output average
if count == 0 :
   print("No marks entered")
else :
   print("The average mark is", round(total/count, 2))

This file is a slightly modified version of the program from slide 13 of part 3 to calculate the average of a series of exam marks.  Save the file as  average.py in your Lab 3 folder.

Observe that the displayed average has been rounded to 2 decimal places.

Run and test the program.

Modify the program so that it also calculates the minimum and maximum marks. You will need to use two new variables, mini and maxi, to keep track of the smallest and largest values entered so far. These should be initialised to the first mark entered, so this needs to be done after the initial input but before the loop. Inside the loop you will need to check whether the newly-entered mark is smaller than mini or larger than maxi, and if so, update the value of the appropriate variable - use if statements to do this. Add another print statement after the output of the average to output a message such as

  The largest mark was 82.5 and the smallest mark was 17.3

The output values should this time be rounded to one decimal place. Run and test.

Displaying Patterns

Copy and paste the following program into a new editor window.

"""
rows1.py
program to display rows of characters
created by sands 23/10/10
"""

def printRow(c, length) :
    """
    print a row of identical characters
    arguments:
    c: string: character to be used
    length: int: length of row
    """
    line = c * length
    print(line)

printRow('=', 10)
printRow('*', 7)
myChar = '+'
myLen = 9
printRow(myChar, myLen)

Save it as rows1.py.

This file contains a function to display a row of characters and some sample calls to the function.

The function contains the code to print a row; each time we want to print a row we need to make a call to the function.

Observe that the function definition uses the keyword def, followed by the name of the function, and in parentheses the names of the arguments to the function. This is followed by documentation for the function and the body of the function (i.e. the code to print a row) - these must be indented.

When the function is called the arguments are given the values supplied in the call and then the code in the function body is run: hence the first call gives the value '=' to the argument c and the value 10 to the argument length and then generates the output string and prints it.

Modify the program so that the user is asked to supply values for the variables myChar and myLen.

Test and run. What happens if the user types more than one character when asked to supply a value for myChar?

To ensure that only the first character is used if the user types more than one, you should modify the call to the printRow on the last line of the program so that the argument is myChar[0] instead of myChar.

Test and run, and again check what happens if the user types more than one character.

We now wish to display a triangular pattern of the form

   +++++
   ++++
   +++
   ++
   +

Modify the documentation at the top of the program so that the filename is rows2.py and save the program as rows2.py.

The code already written can be used to allow the user to select the character to be used and the length of the first row; just modify the prompt so that it now asks the user for the length of the first row.

You will need to use a while loop to draw the rows. This should take the form

   while myLen>0 :
       ......

Inside the loop body you need to display a row (using a call to the printRow function) and then decrement myLen.

Run and test the program.

Add after the while loop, another while loop so that the program will generate output of the form

   +++++
   ++++
   +++
   ++
   +
   ++
   +++
   ++++
   +++++

In the second while loop we need to increment the length and continue until it reaches to its initial value - hence you will need to save the initial value of myLen in an extra variable; this will of course need to be done before the first loop. Note that at the end of the first loop myLen will have reached 0 so its value will need to be adjusted before entering the second loop.

Run and test. Check that there is just one row containing one character. (If there is more than one row you will need to modify the value adjustment that was performed between the two loops.) When the program appears to be working correctly check what happens when the length of the first row is 1 or 0.

You will need to come back to the current version of the program in a later exercise so before proceeding use Save As... to change the program name to rows3.py to ensure that the next program does not get saved as rows2.py when you run it. Change the program name in the documentation string to rows3.py.

Copy and paste the following code into the program, immediately beneath the body of the printRow function

def printRec(c, rows, cols) :
    """
    print a rectangle of identical characters
    arguments :
    c : str : character to be used
    rows : int : number of rows
    cols : int : number of columns
    """
   

This function should use a loop to make several calls to printRow (c, cols)- the number of calls should be equal to the value of the argument rows. Add code to do this immediately beneath the documentation for the function, remembering that it must be indented by the same number of spaces as the documentation.

"""
pow1.py
Calculates powers without using **
created by sands 24/10/10
"""

def power(a, n) :
    """
    calculates powers
    arguments: a : int, n : int
    returns: a to the power n : int
    """
    result = 1

    if n<0 :
        print("Warning - negative powers not integers - returning 0")
        return 0
    else :
        # need to do some arithmetic in a loop here
        return result

# some calls to test the function
print("2 to the power 3 is ", power(2,3));
print("3 to the power 4 is ", power(3,4));
print("4 to the power 5 is ", power(4,5));

Optional Additional Exercises

Exercise 1

Make in your Lab 3 folder a copy of your euro.py program from lab 1 (or use my solution if you didn't get it working correctly).

Open the file in IDLE and modify it so that it performs the inputs in a loop, asking for Cost of item 1, Cost of item 2 etc. (the structure of the loop should be similar to that of the average-calculation program). The program should continue to accept inputs until the user enters a cost of 0, at which point it should display the total cost in both euros and pounds and pence.

Exercises 2-3

In each of the following exercises each function that you write should contain documentation in the style used in the examples already seen.

Exercise 2

Write, in a new program, a function that takes one integer argument and calculates and returns the factorial of that integer.  Beneath the function write a loop that allows the user to input numbers, calculates and displays the factorial of each number entered, and terminates when the user enters a negative number.

[ The factorial of n, written by mathematicians as n!, is the product of all the numbers from 1 to n, so 5! is 5×4×3×2×1. ]

Exercise 3

Write a program that contains a function that takes as an argument a string and returns the number of vowels contained in that string. The main body of the program should ask the user to supply a line of text, pass the input string as an argument to the function and display the result returned by the function.

[ Hints: You will need to loop through the characters in the string until you reach the end, incrementing the result whenever the character is a vowel. To control the loop you will need a variable to keep track of how many characters have been checked and stop when this is equal to the length of the string. The length of a string s is obtained using len(s) .To determine whether the character at location n in the string s is a vowel you can use the expression s[n] in "AEIOUaeiou" as the condition for an if statement - this has a boolean value which will be True if the character occurs in the string "AEIOUaeiou"; don't forget that the first character in the input string is at location 0. ]