Learning Python - Week 4

Learn Python the Hard Way
Exercises 27 – 34
http://learnpythonthehardway.org/

New things in ex. 27–34
• Booleans: True, False, logic, operators
• if – elif – else: Note the relationship to True
and False
• loops: 2 kinds, for and while
• lists and how to
– create a new list
– add and delete things from lists
– find out what’s in a list

How the Boolean AND works
There is a very long bungee jump above a deep
river gorge in Africa. You say:
“If the bungee jump looks safe and it is cheap,
then I will do it.”

How the Boolean AND works
looks safe = False (you won’t do it)
is cheap = False (you won’t do it)
You’ll only do it if both conditions are TRUE.

How the Boolean OR works
There is a great party Tuesday night, but you are
in danger of failing your Wednesday test.
You say:
“If I finish studying in time or the test is
canceled, then I will go to the party.”

How the Boolean OR works
“If I finish studying in time or the test is
canceled, then I will go to the party.”
finish studying = True (can go)
test canceled = True (can go)
You can go if only one of the conditions is TRUE.
Or both. But one is enough.

and
True and True : True
True and False :
False
False and True :
False
False and False :
False
True or True : True
True or False : True
False or True : True
False or False : False
“If I finish studying in time or the test is canceled, then I
will go to the party.”
or

Boolean operators (symbols)
1 != 0
1 == (3 – 2)
5 >= 1
10 <= 100
1 != (3 – 2)
1 == 0
5 <= 1
10 >= 100
True
True
True
True
False
False
False
False

if – elif – else
(if statements)

Loops
• Every programming language has loops
• A loop typically runs over and over until
something makes it stop (different from a
usual function, which runs only once)
• The “for” loop is a standard kind of loop
• for-loops can appear inside a function
Exercise 32

Loops
• Standard syntax for starting a for-loop in
Python:
for fruit in fruits:
Exercise 32
Note that fruit could be any variable name, like x or a or cat.
In this case, there must already be a list named
fruits
(more about lists in a minute)

Loops
• Also standard syntax (but different) for
starting a for-loop in Python:
for i in range(0, 9):
Exercise 32
In this case, we don’t know the name of the list
yet. Or maybe this loop does not even use a list.

So, 2 different for-loops
print fruit
Exercise 32
for i in range(0, 9):
print "Hello!"
Like functions, loops must be indented. They can have many
lines. These are short just to make them simple.

while-loops
Exercise 33
• The “while” loop is another standard kind of
loop
• while-loops can appear inside a function
• If you can figure out how to do what you want
with a for-loop, then use a for-loop.
• If you must use a while-loop, be careful that it
will not run forever. If it does, it’s an infinite
loop (which is NOT good).

Lists
• In many languages, a “list” is called an “array”
(but Zed says we should say “list” for Python)
• Lists can be gigantic—there can be thousands
of items in one list, or none
• The standard format of a list in Python:
fruits = ['apples', 'oranges', 'pears',
'apricots']
Exercise 32
Note: If there are numbers in the list, they won’t have quotes around them.

Loops and Lists
These two for-loops do exactly the same thing:
fruits = ['apples', 'oranges', 'pears',
'apricots']
print "A fruit of type: %s" % fruit
for y in fruits:
print "A fruit of type: %s" % y
Exercise 32

Lists
• You actually already saw a list in Exercise 25,
when you did this:
words = stuff.split(' ')
• After that, words contained a list:
['All', 'good', 'things', 'come', 'to',
'those', 'who', 'wait.']
• You can use pop() and len() on any list
Exercise 32

Some things we do with lists
fruits.pop()
fruits.append("bananas")
fruits.extend(["plums", "mangoes"])
del fruits[2]
print fruits
With append() you can add only one item at a time to the list.

Items in lists
Exercise 34:
Items in lists can be counted.
Items in lists can be referenced by number, but the first
number is not 1 — it is 0.

Back to while-loops
• You might think the while-loops act a lot like
the for-loops that used range (and you would
be right)
• However, the while-loops are different
• The condition at the start of a while-loop
could test for something not numeric, such as
“while we are not yet at the end of the file”
• Note: for-loops are very common, and
while-loops are less so (as Zed says: “Usually a
for-loop is better”)
Exercise 33

Learning … while-loops
• You really need to play with a lot of the extra
credit or “study drills” in Exercise 33 to get this
into your brain
• I made four different .py files to test all the
comparisons that Zed recommends
• If you play with this, you can really understand
how for-loops and while-loops are different
Exercise 33

A word of advice
So Exercise 31 is long, but easy. You might feel
like now it’s all getting easy …
BUT WAIT! No, it’s not.
• Exercise 32 introduces the for-loop.
• Exercise 33 introduces the while-loop.
• Exercise 34 introduces how we navigate
through the elements in a list.
These last 3 exercises are QUITE challenging!

Indents in Python
• The usual way is to indent 4 spaces (not a tab
indent — actually type 4 spaces)
• If you accidentally type 3, or 5, or something
else, and all the others are 4, Python will
throw an error
• Most programming languages require multiple
levels of indents, and these levels (and the
format of indenting) are important, so take
care

An example of multiple
indent levels within a
Python program

Learn Python the Hard Way
Exercises 27 – 34
(now we know some stuff)

Learning Python - Week 4

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