16.16: Exercises

Exercise \(\PageIndex{1}\)

Python provides a built-in function called len that returns the length of a string, so the value of len('allen') is 5.

Write a function named right_justify that takes a string named s as a parameter and prints the string with enough leading spaces so that the last letter of the string is in column 70 of the display.

>>> right_justify('allen')
                                                                 allen

Exercise \(\PageIndex{2}\)

A function object is a value you can assign to a variable or pass as an argument. For example, do_twice is a function that takes a function object as an argument and calls it twice:

def do_twice(f):
    f()
    f()

Here’s an example that uses do_twice to call a function named print_spam twice.

def print_spam():
    print 'spam'

do_twice(print_spam)

1. Type this example into a script and test it.
2. Modify do_twice so that it takes two arguments, a function object and a value, and calls the function twice, passing the value as an argument.
3. Write a more general version of print_spam, called print_twice, that takes a string as a parameter and prints it twice.
4. Use the modified version of do_twice to call print_twice twice, passing 'spam' as an argument.
5. Define a new function called do_four that takes a function object and a value and calls the function four times, passing the value as a parameter. There should be only two statements in the body of this function, not four.

Solution:

http://thinkpython.com/code/do_four.py.

Exercise \(\PageIndex{3}\)

This exercise can be done using only the statements and other features we have learned so far.

1. Write a function that draws a grid like the following:

   + - - - - + - - - - +
   |         |         |
   |         |         |
   |         |         |
   |         |         |
   + - - - - + - - - - +
   |         |         |
   |         |         |
   |         |         |
   |         |         |
   + - - - - + - - - - +
   

   Hint: to print more than one value on a line, you can print a comma-separated sequence:

   print '+', '-'
   

   If the sequence ends with a comma, Python leaves the line unfinished, so the value printed next appears on the same line.

   print '+', 
   print '-'
   

   The output of these statements is '+ -'.

   A print statement all by itself ends the current line and goes to the next line.

2. Write a function that draws a similar grid with four rows and four columns.

Solution:

http://thinkpython.com/code/grid.py.

Credit:

Credit: This exercise is based on an exercise in Oualline, Practical C Programming, Third Edition, O’Reilly Media, 1997.