QuickSort routine for your App Inventor Apps!

Sorting data is a common need in many applications. If you have thought about writing your own sorting routine, you likely came up with a scheme that picked the first data item, and then compared it to each of the other elements to find a smaller value (if sorting into ascending order) or a larger value (if sorting into descending order). Finding a match, you switch the two values in your list of data.

Gradually, as you scan the list, the smallest (or largest) value bubbles up to the front of the list. Once the whole list is scanned, you have found the smallest (or largest) value and switched it to the beginning.

Then you would start with the next data value and compare it to the remaining data values. And so on.

Great idea if you came up with this – seriously! In computer science algorithms, this is known as the “bubble sort”. It works fine – but it has one problem – as the number of items to be sorted grows, the time it takes to do the sort grows even faster.

In fact, we say that a bubble sort runs in a time proportional to n^2 (n squared). If there are 10 items to sort, this will take 10^2 or 100 time units to sort.

If there are 30 items to sort, this will take 30^2 or 900 time units to sort. Just going from 10 items to 30 items adds 800 time units to our sorting time! Ouch!

As you can see, the bubble sort is simple but it can take a long time to run!

Computer scientists have invented other ways to sort data. One of the best known has the descriptive name “QuickSort”. In many cases, it’s sorting time is on the order of n ln n (that is n times the natural logarithm of n).

If we compare that to the Bubble Sort for n=10, we get 23 time units and for n=30, we get 103 time units. As you can see, QuickSort is much faster than the Bubble Sort.

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Part 1: Storing and accessing user interface components as variables

App Inventor programmers routinely store values, such as numbers or text strings (“Hello!”) in variables. For example,

stores the numeric value 6 in to the variable TOTALBUTTONS.

To illustrate by example, here is a global variable named SpecialButton. We can initialize it to anything we want at this point.

Next, inside our app, our blocks code assigns Button1 to the variable SpecialButton. SpecialButton now holds a reference to the actual user interface control Button1.

Since SpecialButton is a variable and not an actual button, we cannot directly use a SpecialButton.Click handler but we can use a feature of App Inventor to do the same thing in a different way. We will see how to do this in this a bit later.

You can store any App Inventor components – a Clock, a Bluetooth device – any component, in a variable.

Why would you want to do that? We will see in the example in this lesson.

This tutorial is in both written form and as an online video.

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Part 3: Bluetooth communications with 2 Arduino devices, using App Inventor

Please start with “Part 1: Basic Bluetooth communications using App Inventor” to learn how to configure, set up and program an App Inventor app that communicates over Bluetooth between two Android devices. Then, read “How to connect App Inventor apps to Arduino using Bluetooth” before going through this tutorial!

Then continue with this tutorial.

This tutorial shows how an App Inventor app can communicate with 2 (or more) Arduino boards and Bluetooth devices simultaneously. These instructions assume you are familiar with the code and hardware presented in Part 1 and Part 2 and How to connect App Inventor apps to Arduino using Bluetooth“. This tutorial uses the same Arduino source code as in that tutorial.

A follow up tutorial will show how to simplify some of this code for supporting multiple Bluetooth devices.

Brief Reminder

Bluetooth is a short range, low power, limited speed wireless communications technology. The original Bluetooth technology provided a serial communications link between two paired devices (as compared to an individual data packet sent between up to n devices using the much newer Bluetooth LE – see here and here for information on Bluetooth LE).

Arduino is a microcontroller board for building hardware projects. You can write software for Arduino using a programming language similar to the C++ programming language.

The code used in these examples has been tested with some specific Bluetooth modules connected to Arduino. These include the JY-MCU (Amazon (Prime), Amazon (non-Prime) and also some HC-05 and HC-06 based Bluetooth modules.

Getting Started

  • Read the prior tutorials (Part 1 and Part 2 and How to connect App Inventor apps to Arduino using Bluetooth“)
  • Build two Arduino boards each with an appropriate Bluetooth module as described in the prior tutorial.
  • Compile and load the Arduino software in to each of the Arduino boards.
  • Test and confirm that your basic LED lights flash for the original, single Bluetooth connection case.
  • Then, with two working boards, continue to this tutorial.

User Interface View

The original app supported just one device, so there was just a single “Connect” and “Disconnect” button. This version demonstrates how to connect more than one Bluetooth device so we need separate buttons for each device. Similarly, we must add a second status and data sending item to the screen:


Before running this app, be sure to use Android | Settings | Bluetooth to “pair” your Bluetooth devices with Android.

Then, run the app and select Connect to Device 1. This displays a list of available Bluetooth devices in the vicinity. Select your specific Bluetooth device for the connection. Do this for both Bluetooth devices.

Once connected, you can send some simple commands to the Arduino board. Commands are very simple – a single number – to tell the Arduino to do something (this confirms that the Bluetooth link is working). If we enter a single digit 1 and then press Send Numeric 1, the Arduino board will send back 2 bytes of data which will then be displayed on the app screen. If we enter a single digit 4 and then press Send Numeric 1, a value of 4 is transmitted over Bluetooth to the Arduino board, which responds by flashing the externally connected LED.

Because the text box for data entry has its property set to NumbersOnly, a pop up numeric keypad displays when entering data, rather than the usual Android text keyboard.

Video Demonstration this App

I created a short video showing this app in operation. There are two versions of the video – one is standard 2D format and the other is in VR 3D format for viewing on Google Cardboard-like viewers used with smart phones to watch VR videos.

2D (normal) version: https://youtu.be/BU2gIAxbY_o

VR 3D SBS version: https://www.youtube.com/watch?v=UJIggzZgld4

That version is in 3D, for viewing with VR 3D viewers or 3D TVs or monitors.

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Using buttons to simulate a bar chart in App Inventor Code

Our prior post showed how to use user interface button components to simulate a column chart.

We can apply the same trick to create a bar chart. In a bar chart, the data is represented as horizontal bars, whereas in the column chart, the data appears in vertical columns.

Implementing the bar chart requires just a few minimal changes to the original column chart app.

User Interface

This screen shows the basic output, with the data represented as horizontal bars in the chart. To simplify, the slider control and column #6 that appeared in the original column chart version, have been removed.


A new feature has been added, as an example illustration. Since each bar in the chart is actually a button, you can press on the bar. For fun, a Click event handler has been added to bar #1 in the chart. Pressing bar #1 causes the bar to change to a randomly selected color. This feature has been implemented only for bar #1, but if you wish, you can add Click event handlers for the other buttons.

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Using buttons to simulate a column chart in App Inventor Code

Clark Hochgraf has a blog post on using button controls as a simple way to draw a column chart. This is a clever idea. I created a sample program to illustrate the idea in a bit more depth, with multiple columns.

User Interface View

Let us look at our sample program’s user interface. The screen displays a colorful column chart – but those columns are actually buttons stretched vertically.

Our demo app displays two kinds of column chart features. The first is that columns 1 through 5 are drawn based on the data values (separated by spaces) entered in the text box, at top, followed by pressing the Draw Chart With Data button.


The sixth column (labeled imaginatively “6”!) is controlled by the Slider control. Adjust the control to the left, and column 6 becomes shorter; adjust to the right and column 6 becomes taller. That idea was shown in Clark Hochgraf’s example too.

Designer View

We set up our app by creating a horizontal layout, at top, for a label (“Data (5 values):”) and a text box for data entry. Next, we add a button and label it “Draw Chart With Data”.

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Demonstration app for using FirebaseDB in AppInventor

FirebaseDB provides for sharing between users all running the exact same app on their device. Read “What is FirebaseDB?” to learn more about FirebaseDB and what it does for your applications.

This is a quick and very short app that demonstrates the fundamental operation of FirebaseDB when used in MIT App Inventor. I hope to create a more interesting demo app a bit later.

Caution: FirebaseDB is an experimental component offered by MIT App Inventor. FirebaseDB remains under development and is subject to change; apps written today might not work in the future. Apps containing the FirebaseDB component will not work in the emulator – run on your phone or tablet instead. At this time, the cloud-based database is a shared database, shared among multiple users, and cannot – yet – be linked to your personal Google account.

FirebaseDB is Similar to TinyWebDB

The programming interface for FirebaseDB is nearly the same as that used for TinyWebDBTinyWebDB is a simple cloud-based database – to use, you need to set up the TinyWebDB on your own server or on Google’s servers. With your data stored in the “cloud”, your data may be shared among many apps. For the FirebaseDB demo, you do not need to set up your own server, nor do you need to use TinyWebDB:

For details on setting up and using TinyWebDB – including some tricks that enable sharing of TinyWebDB data between apps – please see my book,

  • App Inventor 2 Databases and Files (Volume 3 e-book)
    Step-by-step TinyDB, TinyWebDB, Fusion Tables and Files
    Buy from: Amazon, Google Books, Kobo Books

For more information, including a sample chapter, please see my App Inventor books page.

Sample App User Interface

Our simple demonstration app stores and retrieves a text value to and from the FirebaseDB. As with TinyDB or TinyWebDB, enter a “tag” value to use to look up the value. For example, a tag value could be a part number, and the value could be the text description of the part’s name. Or the tag could be a phone number and the value could be the name of the person who has that phone number.


The program is operated by entering a tag and a value and then pressing the Store Value button. The value entered is written to the FirebaseDB database in the cloud.

After a value has been stored, you retrieve values by entering the original tag and pressing Retrieve Value. The data corresponding to the tag is retrieved from FirebaseDB and display in the Value field, on screen.

If the app is run simultaneously on other devices, any data updates made on the other devices result in all devices receiving a data changed notification. When the data in the FirebaseDB is changed, the new data is displayed on all devices.

Designer View

A combination of vertical and horizontal layouts is used to organize the positions of the controls (see the Components list, below, or download the sample code).

Store Value and Retrieve Value are buttons. Tag and Value are labels, followed by text boxes for data entry.  Data Changed Event and the status message are both labels.

Drag the FirebaseDB component from the Experimental section of the Designer controls palette. You will receive a warning that FirebaseDB is experimental.



Blocks View

(Sorry for the image quality on these three blocks – the screen capture utility I used for these did not do a very good job)

The btnStoreValue event handler reads the enter tag and data value from the text boxes on screen, and then stores those to the FirebaseDB. Find the FirebaseDB StoreValue component by clicking on the FirebaseDB component in the Blocks list.


Fetching a store value is simple – call FirebaseDB’s GetValue method and pass to it the tag. Unlike TinyDB (but similar to TinyWebDB), the value is not read instantaneously but instead, once the data is read and available, an event called GotValue occurs.  A GotValue event handler processed the incoming data; in our simple app, the data is stored back in to the Value text box, on screen.


A unique feature of FirebaseDB is the database’s ability to alert apps that data inside the database has been changed. This alert caused a DataChanged event to occur – and which delivers the tag and value that were updated to the app.



FirebaseDB is experimental and incomplete, is subject to change, and should not be relied upon at this time for production code. However, you may use it for learning and experimentation.

Download Source Code

Download: FirebaseDB_Demo.aia

After downloading to your computer, you may upload the file to your App Inventor account using Projects | Import project (.aia) from my computer

Related Tutorials


Rounding a number to a specific number of decimal places

I was working on a small app that needed to round some numeric values. For example, given a number such as 123.456789, I wanted to round this off to two decimal places such as 123.46. Why .46 instead of .45? Because the value is rounded at the n-th decimal place (.456 rounds up to .46).

MIT App Inventor does not have a function to do this sort of round off – well, not exactly. Actually, it has a formatting function that can accomplish the same thing but it is intended for converting numeric values to text strings.

I created a simple little program to demonstrate how this works, plus a couple of other methods to implement this feature.

Update: This post has been updated with thanks to Taifun for noting that App Inventor does have a “raise to a power function”. See below for more details and a link to his great web site!

User Interface View

The user interface prompts for a number, and the number of decimal places to which it should be rounded. Press the Round off number button to see the result.

RoundOffUIHere you can see that 123.4567 has been rounded to 123.457.

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Aligning the text that appears in ListPicker

Readers post questions on the FB page or the blog. Sometimes I can answer them but sometimes I cannot answer them right away. For those that I cannot answer, I add the question to a list of future tutorial ideas. If someone is not sure how to solve a problem, chances are that there are others who may need help with the same issue!

I am beginning to go through my list – watch for more tutorials based on reader questions. Note – I do not have time to solve specific or custom applications. I try to abstract the basic elements of the problem and create a generic solution that can apply to a wide variety of use cases.

ListPicker Text Alignment

A reader asked how to align the text that appears in the ListPicker box. The ListPicker displays a set of items on screen so that the user may select an item from the list. When the list appears on screen, all the items are “left justified” which means they appear on the left side of the screen.

To demonstrate, our ListPicker, below, displays a list of auto manufacturers:

Screenshot_20160502-115325Is there a way to center or right justify the items that appear in the ListPicker list like this? The first 4 items in this list are right justified and the last two are centered:

Screenshot_20160502-114442The answer to that question is basically “yes”, but it may not be perfect – as we will see.

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Can you “gray out” a button until data entry is complete?

A reader asked  if it might be possible to “gray out” a button so that pressing it has no action, until appropriate data has been entered?

The answer is “Yes, we can do this.” After some thought, I came up with the following simple solution.

Update 1: Check the comments to this post for a reader’s great solution for doing this for Location services dependent function.

Update 2: Also, you can set the button component’s Enabled property to false, so that the button will not function. Then set Enabled to true once the data entry meets your app’s requirements.

User Interface

What we want to do is have the button look like it is “grayed out” and unusable until after some data is entered into the field. In the text box, I have set the  “hint” value to “Button available when data entered”:

Screenshot_20160317-202312After the user has entered some data, the button becomes “active” as shown here:


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A “switch board” user interface panel for App Inventor apps

In the last post, we introduced some concepts for building “creative” App Inventor user interfaces that feature visually appealing user interface controls rather than the usual bland buttons.

In this post, we look at creating an array of toggle switches. Tapping a switch flips the switch from left to right, or right to left.


In developing this user interface, we learn two concepts:

  1. We expand on the previous post and its use of images to create custom buttons.
  2. We see how a user interface control can be stored in a list and referenced like a variable within our apps.

Source code:

The User Interface

I called my app “Mission Control” because any good mission control panel needs lots of switches!

The user interface features 9 toggle switches in a 3 x 3 array. The purpose of this app is to demonstrate how to implement this type of interface – the app does not otherwise do anything interesting.

Tapping any toggle switch causes the switch lever to move to the other side of the switch. Here is a screen shot showing some toggle switches to the left and some to the right.

Screenshot_20160204-140323The Designer View

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