Category Archives: Programming Method

Calculating a square root value

A reader asked a question about how square roots can be calculated, referring to an example in my Volume 1 Introduction to App Inventor e-book. He was confused by the explanation of how it works – so I have written an expanded explanation of the method for calculating a square root of a number.

You could use the built in Math function to find the square root of a number – and generally that is what you should do. But if you would like to learn a method of calculating the square root of a number – then read on!

2,000 years ago, Greek mathematicians identified a method of finding approximate square roots. The method makes an initial guess of a square root (which is often not even close to the right value) and then repeats a sequence of dividing and averaging which gradually refines the initial guess into an accurate square root.  To find the square root of some number n, we set our initial guess to n (which we know is not correct), and then calculate an average of our guess with n divided by our guess.

To find the square root of 30, we initially guess the square root is 30 (which we know is wrong). We then take 30 and divide it by our guess, which in this case is 30. This gives us a value of 1. The average of our original number 30 and our divided value is 15.5. We select 15.5 as our new guess.

This little spreadsheet may help understand how the square root concept works.

To find the square root of 30, we initially guess the square root is 30 (which we know is wrong). We then take 30 and divide it by our initial guess, which is 30. This gives  a value of 1. The average of our original number 30 and 1 is 15.5. We select 15.5 as our new guess on the next row down in the Guess column.

Then we repeat this and take 30 divided by our new guess of 15.5. This gives us 1.93548. We average our 15.5 guess with our new value of 1.9458 giving us 8.717, which becomes our new guess on the next row down.

We repeat again using 8.717 as our new guess and divide 30 by 8.717, giving us 3.44. We average the 8.717 with 3.44 giving us 6.0795, use that as our next guess.

When do we stop this repeating cycle? We could repeat this a set number of times, say six, and then stop. But a better solution is to specify how many digits of accuracy we would like to have in our answer.

Let’s say that 5.4 is accurate enough for our purposes. We could stop as soon as our new guess is within 0.1 of our prior guess. In our spreadsheet above, our generated guesses are:

  • 15.5
  • 8.717742
  • 6.0795
  • 5.507058
  • 5.477306
  • 5.477226

As you can see, the correct answer has converged to 5.477 rather quickly. If we wanted an accuracy of just 1 digit to the right of the decimal point, we can stop once the difference between the current and previous guess is 0.1

The first digits of the last two guesses are the same 5.4 so we could stop with an accuracy of .1. In this particular calculation, we see that we are accurate to the thousandth’s place 5.477 as these values are no longer changing.

We stop the cycle when the difference between guesses becomes very small.

With our definition of small to 0.1, we stop at 5.4. If we set our definition of small difference to 0.001, then we stop at 5.477.

Here is some sample App Inventor code that calculates the square root. It fetches the number for which we find the square root from a user interface Text Box named txtDataEntry. We set NewValue (our initial guess) to this value. Then the code uses a while test loop to repeatedly do the calculation while the difference between our new guess and our previous guess is greater than 0.1 (you can change this to smaller values for greater accuracy).

This square root calculation repeats as long as the difference between our NewValue and OldValue are large.

As long as the difference remains large, the while test condition is “true”, and the code inside the while block continues to execute. However, once the difference becomes small, the test condition evaluates to “false” and the the execution of the “do” section stops. We have found our approximate square root.

An App Inventor recommended way of handling multiple similar screens

MIT App Inventor supports apps having multiple screens, but due to the way that App Inventor is implemented, they recommend that apps have no more than 10 screens total – or risk running out of memory or seeing the Blocks editor crash or hang!

They have a page of information about this issue and you should look at that if you are writing an app with more than about 10 screens.

They suggest it may be possible to simulate multiple screens using just one extra screen, depending on the application. They provide hints as to how to do those hints may be insufficient for new App Inventor programmers.

A reader asked for help on this so I created this tutorial.

Since this app has two screens, this tutorial is split into two sections, one for each screen, and then a third section discussing optional modifications.

The basic app is called ManyScreens. The optional modifications are in ManyScreens2_Buttons (source code links at end of this post)

User View

This demonstration app features 4 buttons – the first button displays “Screen 1”, the second button displays “Screen 2” and so on. These buttons could have any label on them appropriate for your application, such as “Register Account”, “Reserve a space”, “Display reservations” or what ever make sense for your app.

Pressing Screen 1 displays this screen – note the label at the top and the photo on the page.

This screen appears when pressing Screen 3. Note that the text label references Screen 3.

The button Do Something! is a placeholder for adding code to do something unique for this screen. For this application all it does is close the screen and return to the main app screen, Screen1.

Continue reading An App Inventor recommended way of handling multiple similar screens

How to center an image on the screen using App Inventor

A reader asked, “How can I center an image on the screen in an App Inventor app?”

There is an easy way to do this and there is a more complicated method (using the Canvas and other graphics objects). In this tutorial, we show the easy way to center an image on screen!

Our app uses the camera to take a photo and then displays that photo on screen. A secondary feature of the app demonstrates how the “image object” in App Inventor is the filename where the image is stored. In this way, the “image object” can be stored in TinyDB – its not really the binary data representing the photo or image – it is, in fact, just the filename. (TinyDB only stores text).

User Interface View

To demonstrate image centering, we use the camera to take a photo (creating the image) and then a Show pictures button to display the image on screen.

When you press Take a picture, the app launches the Android camera app to take the photo. When done taking the photo, click on the “check mark” (as it appears on my Android 7 phone) at upper right of the screen. The camera app returns to our app, which displays the file name and location on our Android device where the JPG image file has been stored.

Press “Show pictures” to display the photo. The default image size is small – but centered! We will see how to show the photo at a larger size.

Designer View

Continue reading How to center an image on the screen using App Inventor

Can App Inventor have a button or control that rotates on screen?

A reader asked: Is there is a way to have a control, such as a button, rotate on the screen?

With a simple trick, the answer is Yes!

The trick is to use ImageSprites that are made to look like buttons. Then, instead of .Click event handler, as used on a Button, we use one of the “touch” events defined for the ImageSprite class. To rotate the sprite, change the sprite’s Heading property. Easy!

If you would like to learn more about ImageSprites, please read my online tutorial “Using ImageSprites for animated App Inventor Graphics”.

I’ve also written an inexpensive 227 page e-book all about graphics, animation and charts available from many online e-book outlets. You may find that text helpful to you as well.

Tutorial Video

The tutorial is shown here as a video – or page down for a written version of the tutorial.

User View

Watch this video to see what a rotating button looks like:

Designer View

Creating the user interface is the same as creating other App Inventor user interfaces – except we use a graphical drawing area called the Canvas and add an ImageSprite to the Canvas.

Continue reading Can App Inventor have a button or control that rotates on screen?

Google introduces Blockly 1.0 for Android, iOS and web

If you have used MIT App Inventor (and of course you have!), you can quickly adapt to using Blockly for programming.  Blockly is basically a “Drag and Drop” code editor (like in App Inventor), providing a visual programming system. It uses the same ideas as MIT App Inventor.

To see it in operation, visit MIT’s Scratch project to create a simple program.

Google has released code libraries that, when combined with other tools, enable use of Blockly to create code for the web, for iOS and Android. For iOS you also need XCode and for Android you need Android Studio. Blockly is not a programming language itself; it outputs code in JavaScript, Python, Lua and other formats.

Source: Google Developer’s Blog

WATCH THIS VIDEO!

(FYI I am half way through my period of traveling too much and not getting much programming done!)

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.

Continue reading QuickSort routine for your App Inventor Apps!

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.

Continue reading Part 1: Storing and accessing user interface components as variables

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.

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

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.

Screenshot_20161011-134517

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.

Continue reading Using buttons to simulate a bar chart in App Inventor Code

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.

Screenshot_20161010-162157

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”.

Continue reading Using buttons to simulate a column chart in App Inventor Code