Free Book Chapter 19 - Programming Lists of Data
from http://www.appinventor.org/bookChapters/chapter19.pdf for translation
(picture of hands holding paper with printed list)
Programming Lists of Data
As you’ve already seen, apps handle events and
make decisions; such processing is fundamental to
computing. But, the other fundamental part of an
app is its data—the information it processes. An
app’s data is rarely restricted to single memory slots
such as the score of a game. More often, it consists
of lists of information and complex, interrelated
items that must be organized just as carefully as
the app’s functionality.
In this chapter, we’ll examine the way App Inventor handles data. You’ll learn the
fundamentals of programming both static information, in which the data doesn’t change,
and dynamic information, in which data is entered by the end user. You’ll learn how to
work with lists, and then you’ll explore a more complex data structure involving lists of
lists and a multiple-choice quiz app.
Many apps process lists of data. For example, Facebook processes your list of
friends and lists of status reports. A quiz app works with a list of questions and
answers. A game might have a list of characters or all-time high scores.
You specify list data in App Inventor with a variable, but instead of naming a single
memory cell with the variable, you name a set of related memory cells. You specify
that a variable is multi-item by using either the make a list or create empty list
blocks. For instance, the variable phoneNumbers in Figure 19-2 defines a list of three
Figure 19-2. phoneNumbers names three memory cells initialized with the numbers
Creating a List Variable
You create a list variable in the Blocks Editor by using an initialize global variable
block and then plugging in a make a list block. You can fnd the make a list block
in the Lists drawer, and it has only two sockets. But you can specify the number of
sockets you want in the list by clicking on the blue icon and adding items, as depicted
in Figure 19-3.
Figure 19-3. Click the blue icon on make a list to change the number of items
You can plug any type of data into the “item” sockets of make a list. For the
phoneNumbers example, the items should be text objects, not numbers, because
phone numbers have dashes and other formatting symbols that you can’t put in a
number object, and you won’t be performing any calculations on the numbers (in
which case, you would want number objects, instead).
Selecting an Item in a List
As your app runs, you’ll need to select items from the list; for example, a particular
question as the user traverses a quiz or a particular phone number chosen from a list.
You access items within a list by using an index; that is, by specifying a position in the
list. If a list has three items, you can access the items by using indices 1, 2, and 3. You
can use the select list item block to grab a particular item, as shown in Figure 19-5.
Figure 19-5. Selecting the second item of a list
With select list item, you plug in the list you want in the frst socket, and the
index you want in the second socket. For this phoneNumber sample, the result of
selecting the second item is “333–4444.”
Using an Index to Traverse a List
In many apps, you’ll define a list of data and then allow the user to step through (or
traverse) it. The Presidents Quiz in Chapter 8 provides a good example of this: in that
app, when the user clicks a Next button, the next item is selected from a list of
questions and displayed.
The previous section showed how to select the second item of a list, but how do
you select the next item? When you traverse a list, the item number you’re selecting
changes each time; it’s your current position in the list. Therefore, you need to define a
variable to represent that current position. “index” is the common name for such a
variable, and it is usually initialized to 1 (the frst position in the list), as demonstrated
in Figure 19-6.
Figure 19-6. Initializing the variable index to 1
When the user does something to move to the next item, you increment the index
variable by adding a value of 1 to it, and then select from the list by using that
incremented value. Figure 19-6 shows the blocks for doing this.
Figure 19-6. Incrementing the index value and using the incremented value to select
the next list item
Example: Traversing a List of Paint Colors
Let’s consider an example app with which the user can peruse each potential paint
color for his house by tapping a “ColorButton.” Each time the user taps, the button’s
color changes. When the user makes it through all of the possible colors, the app goes
back to the frst one.
For this example, we’ll use some basic colors. However, you could customize the
code blocks to iterate through any set of colors.
Your first step is to define a list variable for the colors list and initialize it with some
paint colors as items, as depicted in Figure 19-7.
Figure 19-7. Initializing the list colors with a list of paint colors
Next, define an index variable that tracks the current position in the list. It should
start at 1. You could give the variable a descriptive name such as currentColorIndex,
but if you aren’t dealing with multiple indexes in your app, you can just name it
“index”, as in Figure 19-6.
The user traverses to the next color in the list by clicking the ColorButton. Upon
each tap, the index should be incremented and the BackgroundColor of the button
should change to the currently selected item, as shown in Figure 19-8.
Figure 19-8. Each tap of the button changes its color
Let’s assume the button’s background is initially set to Red in the Component
Designer. The frst time the user taps the button, index changes from its initial value
of 1 to 2, and the button’s background color changes to the second item in the list,
green. The second time the user taps it, the index changes from 2 to 3, and the
background color switches to Blue.
But what do you think will happen the next time the user taps it?
If you said there would be an error, you’re right! index will become 4 and the app
will try to select the fourth item in the list, but the list only has three items. The app
will force close, or quit, and the user will see an error message like the one in
Figure 19-9. The error message displayed when the app tries to select a fourth item
from a three-item list
Obviously, that message is not something you want your app’s users to see. To
avoid that problem, add an if block to check whether the last color in the list has
been reached. If it has, the index can be changed back to 1 so that the first color is
again displayed, as illustrated in Figure 19-10.
Figure 19-10. Using an if to check whether the index value is larger than the length of
When the user taps the button, the index is incremented and then checked to see
if its value is too large. The index is compared to length of list, not 3; this way your
app will work even if you add items to the list. By checking if the index is greater than
your list length (versus checking if it is greater than the specifc number 3), you’ve
eliminated a code dependency in your app. A code dependency is a programming
term that describes code that is defined too specifically and lacks flexibility. Thus, if
you change something in one place—in our example here, you add items to your list
—you’ll need to search for every instance where you use that list and change it
As you can imagine, these kinds of dependencies can get messy very quickly, and
they generally lead to many more bugs for you to chase down, as well. In fact, the
design for our Color app contains another code dependency as it is currently
programmed. Can you fgure out what it is?
If you changed the first color in your list from red to some other color, the app
won’t work correctly unless you also remembered to change the initial
Button.BackgroundColor you set in the Component Designer. The way to eliminate
this code dependency is to set the initial ColorButton.BackgroundColor to the first
color in the list rather than to a specific color. Because this change involves behavior
that happens when your app first opens, you do this in the Screen.Initialize event
handler that is invoked when an app is launched, as illustrated in Figure 19-11.
Figure 19-11. Setting the BackgroundColor of the button to the first color in the list
when the app launches
Creating Input Forms and Dynamic Data
The previous Color app involved a static list: one whose elements are defined by the
programmer (you) and whose items don’t change unless you change the blocks
themselves. More often, however, apps deal with dynamic data: information that
changes based on the end user entering new items, or new items being loaded in
from a database or web information source. In this section, we discuss an example
Note Taker app, in which the user enters notes in a form and can view all of her
Defning a Dynamic List
Apps such as a Note Taker begin with an empty list. When you want a list that begins
empty, you defne it with the create empty list block, as depicted in Figure 19-12.
Figure 19-12. The blocks to defne a dynamic list don’t contain any predefined items
Adding an Item
The first time someone launches the app, the notes list is empty. But when the user
types some data in a form and taps Submit, new notes will be added to the list. The
form might be as simple as the one shown in Figure 19-13.
Figure 19-13. Using a form to add new items to the notes list
When the user types a note and taps the Submit button, the app calls the add
items to list function to append the new item to the list, as illustrated in
Figure 19-14. Calling add items to list to add the new note when the user taps the
You can fnd the add items to list block in the List drawer. Be careful: there is
also an append to list block, but that one is a fairly rare block used to append one
entire list to another.
Displaying a List
The contents of list variables, like all variables, are not visible to the user. The blocks in
Figure 19-13 add items to the list each time SubmitButton.Click is invoked, but the
user will not receive feedback that the list is growing until you program more blocks
to actually display the content of the list.
The simplest way to display a list in your app’s user interface is to use the same
method you use for displaying numbers and text: put the list in the Text property of a
Label component, as illustrated in Figure 19-15.
Figure 19-15. Displaying the list to the user by placing it in a label.
Unfortunately, this simple method of displaying a list isn’t very elegant; it puts the
list within parentheses, with each item separated by a space and not necessarily on
the same line. For instance, if the user were to type, “Will I ever finish this book?” as
the first note, and “I forget what my son looks like!” as the second, the app would
display the notes list similar to what we see in Figure 19-16.
Figure 19-16. These entries are listed using default formatting
In Chapter 20, you can see a more sophisticated way to display a list.
Removing an Item from a List
You can remove an item from a list by using the remove list item block, as shown in
Figure 19-17. Removing an item from a list
The blocks in Figure 19-17 remove the second item from the list named notes.
Generally, however, you won’t want to remove a fixed item (e.g., 2), but instead will
provide a mechanism for the user to choose the item to remove.
You can use the ListPicker component to provide the user with a way to select an
item. ListPicker comes with an associated button. When the button is tapped, the
ListPicker displays the items of a list from which the user can choose one. When the
user chooses an item, the app can remove it.
ListPicker is easy to program if you understand its key events, BeforePicking and
AfterPicking, and its key properties, Elements, Selection, and SelectionIndex (see
|BeforePicking: Triggered when button is clicked.
||Elements: The list of choices.
|AfterPicking: Triggered when user makes a choice.
||Selection: The user’s choice.
||SelectionIndex: Position of choice.
Table 19-1. The key events and properties of the ListPicker component
BeforePicking: Triggered when button is clicked. Elements: The list of choices.
AfterPicking: Triggered when user makes a choice. Selection: The user’s choice.
SelectionIndex: Position of choice.
The user triggers the ListPicker.BeforePicking event by tapping the ListPicker’s
associated button. In the ListPicker.BeforePicking event handler, you’ll set the
ListPicker.Elements property to a list variable so that the data in the list displays. For
the Note Taker app, you’d set Elements to the notes variable that contains your list of
notes, as shown in Figure 19-18.
Figure 19-18. The Elements property of ListPicker1 is set to the notes list
With these blocks, the items of the list notes will appear in the ListPicker. If there
were two notes, it would appear as shown in Figure 19-19.
Figure 19-19. The list of notes appears in the ListPicker
When the user chooses an item in the list, it triggers the ListPicker.AfterPicking
event. In this event handler, you can access the user’s selection in the
However, your goal in this example is to remove an item from the list, and the
remove item from list block expects an index, not an item. The Selection property
of the ListPicker is the actual data (the note), not the index. Therefore, you need to
use the SelectionIndex property instead because it provides you with the index of
the chosen item. It should be set as the index of the remove list item block, as
demonstrated in Figure 19-20.
Figure 19-20. Removing an item by using ListPicker.SelectionIndex
Lists of Lists
The items of a list can be of any type, including numbers, text, colors, or Boolean
values (true/false). But, the items of a list can also, themselves, be lists. Such complex
data structures are common. For example, a list of lists could be used to convert the
Presidents Quiz (Chapter 8) into a multiple-choice quiz. Let’s look again at the basic
structure of the Presidents Quiz, which is a list of questions and a list of answers, as
shown in Figure 19-21.
Figure 19-21. A list of questions and a list of answers
Each time the user answers a question, the app checks to see if it is correct by
comparing the answer to the current item in the AnswerList.
To make the quiz multiple choice, you need to keep an additional list, one which
stores the choices for each answer to each question. You specify such data by placing
three make a list blocks within an inner make a list block, as demonstrated in
Figure 19-22. A list of lists is formed by inserting make a list blocks as items within an
inner make a list block
Each item in the variable answerChoices is itself a list containing three items. If you
select an item from answerChoices, the result is a list. Now that you’ve populated your
multiple-choice answers, how would you display that to the user?
As with the Note Taker app, you could use a ListPicker to present the choices to
the user. If the index were named currentQuestionIndex, the
ListPicker.BeforePicking event would appear as shown in Figure 19-23.
Figure 19-23. Using the List Picker to present one of the list of answer choices to the
These blocks would take the current sublist of answerChoices and let the user
choose from it. So, if currentQuestionIndex were 1, the ListPicker would show a list
like the one in Figure 19-24.
Figure 19-24. The answer choices presented to the user for the first question
When the user chooses, you check the answer with the blocks shown in
Figure 19-25. Checking whether the user chose the correct answer
In these blocks, the user’s selection from the ListPicker is compared to the correct
answer, which is stored in a different list, AnswerList (because answerChoices provides
only the choices and does not denote the correct answer).
Lists are used in almost every app you can think of. Understanding how they work is
fundamental to programming. In this chapter, we explored one of the most common
programming patterns: using an index variable that starts at the beginning of the list
and is incremented until each list item is processed. If you can understand and
customize this pattern, you are indeed a programmer!
We then covered some of the other mechanisms for list manipulation, including
typical forms for letting the user add and remove items. Such programming requires
yet another level of abstraction, as you have to envision the dynamic data before it
really exists. After all, your lists are empty until the user puts something in them. If you
can understand this, you might even think of quitting your day job.
We concluded the chapter by introducing a complex data structure, a list of lists.
This is defnitely a difficult concept, but we explored it by using fixed data: the answer
choices for a multiple-choice quiz. If you mastered that and the rest of the chapter,
your final test is this: create an app that uses a list of lists but with dynamic data. One
example would be an app with which people can create their own multiple-choice
quizzes, extending even further the MakeQuiz app in Chapter 10. Good luck!
While you think about how you’ll tackle that, understand that our exploration of
lists isn’t done. In the next chapter, we continue the discussion and focus on list
iteration with a twist: applying functions to each item in a list.