3-2: Structures

Objectives:

Learn how to manipulate lists (arrays) Learn how to automate repetitive actions (loops)

Learn how to make decisions in code (condit ionals)

Set up the Pi kits. Start the X Windows System (startx) and launch IDLE.

Briefly review the syntax from the last session. We will make use of variables and strings, especially.

Most of the t ime we don't deal with just one thing. Consider a trip to the grocery

store: does the shopping list contain only one item? Sometimes, but most of the

t ime it is rather longer than that. Python lets us represent lists of things easily. We put them between square brackets [].

>>> groceries = ['milk', 'french fries', 'TACOS', 'ketchup', "lettuce", 'penguins',

'MORE TACOS']

>>> groceries ['milk', 'french fries', 'TACOS', 'ketchup', 'lettuce', 'penguins', 'MORE TACOS']

>>>

Notice that they are in the same order as they were when you entered them.

How many are there? In order to find out, we can use a function of Python called len (for length).

Functions are repeatable actions. If you hand a box of stuff to somebody and ask them to count what's inside, they can do that, and tell you the number. The

len function follows exactly the same principles. We give the len function a list –

or a variable that contains a list – and it tells us how many things are inside it . We do the giving by putt ing it inside parentheses ().

>>> len(groceries)

7

>>>

Okay, so we have seven items on the list . Can we get the second item on the list? Yes! But Python does a weird counting thing. The first item is number zero,

the second item is number one, and so on. The last item on this grocery list is

number six. Many programming languages start counting at zero, so as strange as this might be, it is a useful thing to remember.

In order to get an item from the list , we enclose the item we want after the

variable name. For the third item, we do the following:

>>> groceries[2]

'TACOS' >>>

Remember that we start counting from zero, so the third item is at posit ion number two. This is commonly pronounced “groceries sub two” because we

are finding an item with a part icular subscript in the array.

Now, what if we want to alphabetize the grocery list? For this task, we can ask

the list to go off and sort itself. We do so with a dot, and since it already knows what to do, we don't have to give the sort function anything in part icular, so we

give it just an empty parameter list – that is, nothing between the parentheses.

>>> groceries.sort()

>>> groceries ['MORE TACOS', 'TACOS', 'french fries', 'ketchup', 'lettuce', 'milk', 'penguins']

>>>

This idea of asking a thing to do something is very, very powerful: it allows us to

model the world around us. What if we were to write a dog, and then we could

ask it to bark(quiet ly) and fetch(newspaper)? After receiving the newspaper, could we ask it if it isCoveredInSlobber() or if it isCurrent()?

At any rate, what if we want to print each item in the grocery list on a separate

line? First of all, we can print something using the print() function:

>>> print("hello")

hello

>>>

This is important because it can be used in virtually any program, not just the interactive IDLE environment. So now we can print each item in turn.

>>> groceries[0] 'MORE TACOS'

>>> groceries[1] 'TACOS'

>>> groceries[2]

'french fries' >>> groceries[3]

'ketchup'

I don't know about you, but I 'm gett ing tired of typing “groceries” over and over

again. Let 's take a short cut and tell the computer to do that for us. The way we go about this is using a loop:

for item in groceries:

print(item)

This is a fairly readable syntax. Python will go through the list of groceries, and for

each thing on the list it will call it an “item” and then it will print it .

At this stage it is crit ical to point out a couple of syntactic elements. First of all,

the colon (:) indicates that we are not done with what we are doing. Second, indent ation matters. Although it is simply whitespace, many of the most

common errors result from misaligned or missing indentation within program

structures.

Let 's do the same thing another way. Instead of iterat ing over some items, let 's iterate over some numbers. The following syntax lets us do this:

>>> for index in range(0, 5): print(index)

0 1

2 3

4

>>>

Notice that the first number of the range (0) is included, but the last number of

the range (5) is excluded. Remember how len(groceries) told us there are 7 items in the list? This works out nicely for us, because the zero-based counting

scheme means that item 6 is the last one in the list , and a range from 0 to 7 will include 6 but not 7. Let 's put the pieces together.

>>> for index in range(0, len(groceries)): print(groceries[index])

MORE TACOS

TACOS french fries

ketchup

lettuce

milk

penguins >>>

Now let 's number the items.

>>> for index in range(0, len(groceries)): print(`index` + ": " + groceries[index])

0: MORE TACOS

1: TACOS 2: french fries

3: ketchup

4: lettuce 5: milk

6: penguins

That's great, but wouldn't it be nice (for humans) to start the list at 1? We could

do the following:

>>> for index in range(0, len(groceries)):

print(`index + 1` + ": " + groceries[index])

1: MORE TACOS

2: TACOS

3: french fries 4: ketchup

5: lettuce

6: milk 7: penguins

>>>

This is start ing to get complicated. Let 's simplify a lit t le bit by split t ing up the print

statement.

>>> for index in range(0, len(groceries)): num = index + 1

print(`num` + ": " + groceries[index])

1: MORE TACOS

2: TACOS

3: french fries

4: ketchup

5: lettuce 6: milk

7: penguins >>>

We can take the “index + 1” from the print and put that into num, then use num in the print instead. Notice that both lines in the loop are indented. If the

indentation does not match, Python will complain.

Decision making:

There are lots of t imes when we need to make a decision in a program. If you're

playing t ic-tac-toe and get three in a row, then the game is over. If the

damage your character receives is less than your current hit points, then your character lives. If the user enters invalid input, print a message that indicates

what was wrong. If the train is carrying carrots, then give it top priority; otherwise, give the train priority just behind Amtrak. (Nobody likes spoiled

carrots, after all.)

Of course, in order to make a decision, we have to be able to test things. We

do this using Boolean operators. Here is a brief list of the most common

operators:

== t rue if two things are equal != t rue if two things are not equal

> t rue if the thing on the left is greater than the thing on the right

< true if the thing on the left is less than the thing on the right >= t rue if the thing on the left is greater than or equal t o the thing on

the right

<= true if the thing on the left is less than or equal to the thing on the right

&& true if the thing on the left AND the thing on the right are both true || true if either the thing on the left OR the thing on the right is t rue

In Python, we can make make decisions using the if/elif/else construct.

if <condit ion>:

do something important

if <condit ion>:

do something important else:

do something less important

if <condit ion>:

do something important elif <different condit ion>:

do something cool else

do something normal

For example, let 's consider the numbers from 0 to 9, and determine whether they

are even or odd.

>>> for x in range(0, 10):

if x == 0: print(`x` + ": found zero!")

elif x % 2 == 0:

print(`x` + ": even!") else:

print(`x` + ": odd.")

0: found zero! 1: odd.

2: even!

3: odd. 4: even!

5: odd. 6: even!

7: odd.

8: even! 9: odd.

>>>

Feel free to play with loops, lists, condit ionals – these basic constructs can represent the decision making process for a lot of problems. It 's also fun to find

the limits of what Python can do. How many things can you put in a list? How

long does it take to count to a million? How long does it take if you don't print each number?

When you're sat isfied, or your brains hurt, or you're out of t ime, pack up the Pi

kits, as normal.