Let us play with the concepts from chapter 3.
Exercise 3.3) p. 29
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
… print (70 – len(s)) * ” ” + s
…
>>> right_justify(‘allen’)
allen
>>>
I knew that:
70 = padding (blank characters) + s
Following the same logic, the padding would be equal to 70 – s.
****
Exercise 3.4) p. 29
A function object is a value you can assign to a variable or pass as an argument.
You can go back to the first part of Chapter 3 in order to recall the concept.
Exercise 3.4) p. 29
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): # The function 'do_twice' uses the parameter 'f'.
f() # If parameter 'f' happens to be a function, then it will be called twice.
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)
- Type this example into a script and test it:
>>> def do_twice(f):
… f()
… f()
…
>>> def print_spam():
… print ‘spam’
…
>>> do_twice(print_spam)
spam
spam
>>> - 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.
>>>def do_twice (f, bingo): # The function ‘do_twice’ has two parameters. ‘f’ is a function object and ‘bingo’ is a parameter referring to a particular value.
… f (bingo) # Here we will call function ‘f’ twice with the variable ‘bingo’ as an argument.
… f (bingo) - Write a more general version of print_spam, called print_twice, that takes a string as a parameter and prints it twice.
>>> def print_twice(banana):
… print banana
… print banana
…
>>> - Use the modified version of do_twice to call print_twice twice, passing ‘spam’ as an argument.
>>> def do_twice (f, bingo):
… f(bingo)
… f(bingo)
…
>>>
>>> def print_twice (banana):
… print banana
… print banana
…
>>>
>>> do_twice (print_twice,’spam’)
spam
spam
spam
spam
>>> - 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.
Working on what we already defined as a function, I will use:
>>> def print_twice(banana):
… print banana
… print banana
…
>>>
>>> def do_four (g, cat):
… g(cat)
… g(cat)
…
>>>
>>> do_four(print_twice, ‘Eureka’)
Eureka
Eureka
Eureka
Eureka
>>>
NOTE: My answer does not match the solution provided by the link below. So, I still have to double-check it.
Solution: http://thinkpython.com/code/do_four.py
Exercise 3.5) p.29-30
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:
Exercise 3-5 Think Python
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.
Write a function that draws a similar grid with four rows and four columns.
So, here are the early stages of me coding:
Step 1) Write the code in Sublime Text :
x = ‘+’
y = ‘ -‘
w = ‘ ‘
z = ‘|’
f = (x + 4*y + w) * 2 + x
g = (z + 9*w)*2 + z
def print_twice (banana):
print banana
print banana
def do_twice (f, apple):
f(apple)
f(apple)
def box ():
print f
do_twice(print_twice,g)
print f
do_twice(print_twice,g)
print f
box()
Step 2) Save the code under small_box.pyStep 3) Run the program in the Terminal :
Here is the grid:
2. Write a function that draws a similar grid with four rows and four columns.
Step 1) Write the code in Sublime Text :
x = ‘+’
y = ‘ -‘
w = ‘ ‘
z = ‘|’
f = (x + 4*y + w) * 4 + x
g = (z + 9*w)*4 + z
def print_twice (banana):
print banana
print banana
def do_twice (f, apple):
f(apple)
f(apple)
def box ():
print f
do_twice(print_twice,g)
print f
do_twice(print_twice,g)
def big_box():
box()
box()
print f
big_box()
Step 2) Save the code under big_box.py
Step 3) Run the program in the Terminal :
Here is the grid:
Solution: http: // thinkpython. com/ code/ grid. py.
Credit: This exercise is based on an exercise in Oualline, Practical C Programming, Third Edition, O’Reilly Media, 1997.
Problems and Solutions
Python Project 