Welcome to a new week. This week we will be looking at lists. A list is a collection of items. An example of a list is a restaurant menu. It is a list of foods sold by the restaurant as well as their prices.
The first App Inventor application I created was Nigerian States and Capital. It was part of the requirements for finishing the edx course. Above is the image for the code used to create this project the initial project can be found here. Due to time constraints, I used only 5 out of the 36 states in Nigeria.
This project was also my first App Inventor application to be submitted to the Google Play Store. I expanded it to include 36 states in Nigeria along with the Federal Capital Territory. It is a quiz that test the knowledge of Nigerian states and their capitals. It can be found on the Google Play Store by clicking here.
This week I successfully uploaded AlphabetDraw to the Google Play Store. AlphabetDraw is a simple application designed to teach toddlers the letters of the alphabet. You can find it here.
If you can’t see the introductory image clearly, please download it and look through it. In the variables section, we have 4 variables and they are:
We have 2 procedures to use in this application. I covered procedures in the last topic so please read through it. I know it was long but a firm grasp of that topic would help you as you go along. Our two procedures are:
If you look at the rest of the code, you should notice that the naming convention I followed easily explains what my objects do. This is a good programming habit to imbibe. You should name objects and components of your application based on what they do.
The Screen1.Initialize block initializes the QuestionLabel to display the first question in the QuestionList.
The NextButton allows the user scroll through the questions in the QuestionList. If the user exceeds 5, the if block resets currentQuestionIndex to 1. This prevents the application from crashing.
The AnswerButton gives feedback to the user if the input values is correct or incorrect. Note the select list item block in the examples. It allows you to select the elements of a list.
Take the list of 5 states in Nigeria as shown below:
- Abia State
- Adamawa State
- Akwa Ibom State
- Anambra State
- Bauchi State
List indexing is the assignment of a number to each value in a list. For our list above, the index of the Anambra State value is 4. In App Inventor, list indexing starts from 1. The power of this is that if we know the index of a list, we can get the value at that index.
This concept is used a lot in working with lists so please don’t skip this part. As a test, the index for Bauchi State is?
In an earlier lesson, I covered the NICE Methodology. I follow it whenever I am designing applications. NICE means Name, Interface, Controls and Events. For our application, we are lucky that the name has already been chosen.
The controls for our application are:
- An image component to display the image of the state
- A question label to display the question to the user
- A textbox for the user to enter the answer
- A submit button to submit the answer entered by the user
- A feedback label to display if the entered value is correct or wrong
- A next button to display the next values for the application
The events that will make our application work are the click events for the Submit and Next buttons.
The first thing to do is to create your project at http://ai2.appinventor.mit.edu you already know how to do this so I won’t be showing you this. Call your project Nigerian States and Capitals. Once your project window shows up, create your user interface. To do this, follow my lead.
Go to the user interface drawer and drag out image component unto the screen. This is shown below:
Next we go to the media section of our screen and upload the images of our states. I got the images I used from Wikipedia so get them from there.
The Media section is shown below:
Click on the Upload File button. It will bring up the pop up shown below:
Click on the Choose File button. It will bring up a screen which will allow you navigate to where your image is stored. Select the image. Your screen should be as shown below:
Click on OK to upload the image to the App Inventor setup. Your image is then uploaded. You will see that your media section has changed to reflect this. You can view this below:
Now attach the image you just uploaded to the image component you just uploaded. This is similar to the process of adding an icon to an application which we have already covered. So click inside the Picture property and select the image you have uploaded.
Set the height of the image component to 250 pixels and the width of fill parent. Click on the ScalePictureToFit checkbox and you have successfully set all the properties of the image component. It is important you do this first because the layout.
Drag out the label component under the image component. Click the rename button in the components section and rename it to QuestionLabel. Please there should be no spaces in the name of your components.
Clear out the text property and make it empty. Like in the 3 number average, the label becomes a dash but we know it’s still there.
Next drag out the horizontal arrangement under the Layout drawer. Inside it, place a textbox and call it AnswerText and a button and call the button AnswerButton.
For your AnswerText, set the hint to Enter an answer and the text on the AnswerButton to Submit.
Drag a label and rename it to FeedbackLabel. In the image on this post, I called it RightWrongLabel but at the time I was working on a deadline and didn’t have time to think about my project. Clear the text in the FeedbackLabel. Your label should disappear but you know it is there.
Finally, drag out a button component to the screen, rename it to NextButton. We also change its text value to Next. Now out user interface is complete. Our components drawer should look as shown below:
To get started, we need to first declare our variables. If you look carefully at the code for this program, our first variable currentQuestionIndex is set to 1. You already know how to do this.
Go to the blocks area and create a variable called currentQuestionIndex your workspace should only have one block that looks like this:
For our three remaining variables QuestionList, PictureList and AnswerList, creating them requires using the Lists Drawer.
I will demonstrate how to do this using the QuestionList variable as an example. First we go to the variables drawer and drag out the initialize global block. Change the name to QuestionList. Your block should now look like this:
Go to the List drawer and drag out the make a list block. Attach it to the end of the block above. Your block should now look like this:
Now we need a list of 5 items. So we click the part of the list block that looks like an eye. It should open up like this:
Now drag the item outside into the list block to make it up to 5. You have done this before so I expect your block now looks like this:
Now all we have to do is attach our questions to it. To do this, go to the Text drawer and drag out an empty text block. Attach it 5 times. Your list should now look like this:
Now enter your questions and your block should look like this:
Now our QuestionList variable is complete. Duplicate this block and do the same for AnswerList and PictureList. So AnswerList look like this:
Before we can create out PictureList variable, please upload remaining pictures for the states in Nigeria. Note how they are stored in the Media area and match them to the index they represent. Your PictureList should now look something like this:
Now that we are done with our variables, the hard work is out of the way. Now we need to start the program.
To start the program we need to program the code that would run when the program starts. This will ensure that the first question is displayed when the program is first run. To do this click on Screen1 and drag out the initialize block.
Next click on the QuestionLabel. Drag out the set QuestionLabel Text to block and snap it to the initialize block of Screen1. Your block should look like shown below:
Now we need to refer to the content of the first index of the QuestionList. This is the first question we will ask the user. Our goal is that once the application runs, we display the first question to the user. To do this, go to the Lists drawer and drag out the select list item. Your block will now be as shown below:
Go to the variables drawer and drag out the get block. Drag out 2 of them. In the first block, select the QuestionList and for the second one, select the currentQuestionIndex and attach it to the respective blocks. Our blocks should now look like this:
Our variable currentQuestionIndex is used to move through the list. The goal would be to increase it until we get to 5 then we reset its value to 1.
Now would be a good time to do a build of our application to see how far we have gone. You know how to build an application so there is no need for me to cover it again. Just ensure that your Screen Orientation for Screen1 is set to Portrait.
Once your build finishes, run it on your setup. I will run mine on BlueStacks. My application looks like this:
So we are close to getting to the original application. Now let’s move on and create the rest of this application.
To complete our application, we need to first create our procedures. Then we will use them in the event listeners.
Our first procedure is the clearText procedure. It clears the text in the AnswerText textbox and the FeedbackLabel label. This will allow us to clear them when we move to the next question. Its easy so I will do it for you. The code to do this is shown below:
Our next procedure is called moveThroughList and what it does is to allow us move through our QuestionList and PictureList as the user presses the next button.
The next thing to do is to create the events for our buttons. We start with the NextButton. When the NextButton is clicked by the user, we want to display the image of the next state and the question for that state. This event also resets our currentQuestionIndex when it is greater than 5.
The code to do this is shown below:
The if block can be found in the Control drawer. It is used to test if the value of the currentQuestionIndex is greater than 5. We place it after we have increased the value of the currentQuestionIndex.
Finally we create the event listener for our AnswerButton. When the user types in a value into the AnswerText textbox and presses the AnswerButton comparision is made between the value entered by the user and the correct answer. Based on this comparision, the text in the FeedbackLabel changes.
The code to do this is shown below:
The comparison is made using the block from the Logic drawer. The normal if block is used for the comparison but we click on the sink and bring out the else part of it. This is shown below:
So we are done creating our application. Build it and test it out. Mine works perfectly. If yours didn’t, take a break for a day. Come back tomorrow and redo the tutorial.
Science shows us that learning involves increasing the intelligence so give yourself time to grow.
Congratulation on making it to 4th part of this course. I commend you for hanging in there. The next topic we will be covering will be databases. I haven’t created the application I intend to use to demonstrate the concept of databases so I will take my time in doing it.
You can check up on our progress by visiting this page. Have a great week and a great month ahead.