Exam 2

Larger Programs --
Chapters 5 through 7

CS 1510
Introduction to Computing

Thursday, November 20, 2014 @ 12:45 PM

Instructions

• The exam consists of ten (10) questions. Be sure that you have all ten and that they are all legible.


• Read all questions and their instructions thoroughly before you begin. It is worth your time to plan ahead!


• You have also been given three blank pages. Use these sheets for your exam answers. Be sure to write your name on each sheet.


• The exam is worth sixty (60) points. Each question is worth six points.


• Points will be awarded based on your explicit answers. Partial credit will be given where possible, so show all of your work.


• Whenever you are asked to answer or describe "briefly", a one-sentence response should be enough to express the solution.


• Whenever you are asked to "# fill in the blank", you need only write the new code. You do not have to copy the other code from the problem.


• The exam lasts sixty (60) minutes. It is due at 1:45 PM.

Problems

1. When working with files, we use four new bits of Python. Answer briefly these questions about input files.
   1. Write the Python statement that opens a text file named grades.txt for input.
   2. Write the Python control statement that loops through the text file one line at a time.
   3. Write the Python statement that closes the text file when are done reading from it.


2. Write a Python function named list_to_file() that takes as arguments a list and a filename. The function prints the items of the list, one per line, to a file of the given name.

   For example,

        classes = ['CS 1510', 'CS 1520', 'CS 2530']
        list_to_file(classes, 'schedule.txt')
   

   will write three lines to a file named schedule.txt.



3. Consider this definition:

        def f(x, y, z):
           ...
           return total
   

   List the steps that occur when the statement   var = f(a, 2, int(arg))   is executed.



4. Write a Python function named sum_of_digits() that takes an integer as an argument and returns the sum of its digits. For example:

        >>> sum_of_digits(1984)
        22
   

   Recall that we can separate a number into its last digit and the rest of its digits using % and //, respectively.



5. What does this program print?

        def mystery(x, z, y):
            x = x + y
            y = y + z
            return y * z
   
        x = 1
        y = 3
        z = 2
        x = mystery(z, x, y)
        print(x, y, z)
   



6. Write a Python function named unique() that takes a list as an argument and returns a list that contains no duplicates. For example:

        >>> unique([1, 2, 1, 2, 3, 1, 4, 2, 3, 4])
        [1, 2, 3, 4]
   



7. Consider this code:

        def empty_grid(rows, cols):
            grid = []
            row  = []
            for i in range(cols):
                row.append(0)
            for i in range(rows):
                grid.append(row)
            return grid
   

   This function does create a grid of independent cells, as expected.

   1. What is wrong with this code?
   2. Why does it work this way?
   3. Suggest a way to fix the code.



8. Write a Python function named get_selection() that takes two numbers as arguments low and high. The function asks the user to enter a number between low and high, inclusive, and returns it. If the user enters an invalid choice, the function keeps asking. For example:

        >>> choice = get_selection(1, 4)
        Enter value between 1 and 4: 6
        Sorry, 6 is not in range.
        Enter value between 1 and 4: 0
        Sorry, 0 is not in range.
        Enter value between 1 and 4: 3
        >>> print(choice)
        3
   



9. Answer briefly each of these questions about functional decomposition:
   1. What is the relationship between a module at Level n and the module it is connected to on n-1?
   2. What is the difference between a concrete step and an abstract step?
   3. Why is functional cohesion as valuable property for a module to have?


10. Unix provides a handy little program named wc that prints the number of lines, words, and characters in a text file.

         > wc card-exercise.txt 
               13     448    2572 card-exercise.txt
    

    Write a Python function named counts that takes a file as an argument (not a filename) and returns a tuple containing the number of lines, words, and characters in the file.

    With your function, we can implement wc with this code:

         lines, words, characters = counts( text_file )
         print('{:8d}{:8d}{:8d} {:s}'.format(lines, words, characters, filename))