Select list item: Attempt to get item 4 of a list of length 3

[cruzleogabriel]

Hello, I'm a newcomer to the MIT App Inventor community and I need help with our capstone project

Our capstone project involves an app that we needed to create and it has 3 features; temperature monitor, humidity monitor, and on/off status monitor of a relay connected to our machine. We made an temperature/humidty/relay status App designed for monitoring the state of our refrigerator with the use of arduino. It involves a potentiometer that sets a temperature to turn off the relay to turn off the compressor of the refrigerator.

We are almost done with the app, but every time it gets to the set temperature of the potentiometer, I get a Runtime Error that says, "Select list item: Attempt to get item number 4 of a list of length 3." what could be the problem and solution for this?

[Kevinkun]

are you sure the lista has 4 items? how?

[ABG]

Be sure to use println() at the end of each message to send from the sending device, to signal end of message. Do not rely on timing for this, which is unreliable.

In the AI2 Designer, set the Delimiter attribute of the BlueTooth Client component to 10 to recognize the End of Line character.

Also, return data is not immediately available after sending a request,
you have to start a Clock Timer repeating and watch for its arrival in the Clock Timer event. The repeat rate of the Clock Timer should be faster than the transmission rate in the sending device, to not flood the AI2 buffers.

In your Clock Timer, you should check

  Is the BlueTooth Client still Connected?
  Is Bytes Available > 0?
     IF Bytes Available > 0 THEN
       set message var  to BT.ReceiveText(-1) 

This takes advantage of a special case in the ReceiveText block:

ReceiveText(numberOfBytes)
Receive text from the connected Bluetooth device. If numberOfBytes is less than 0, read until a delimiter byte value is received.

If you are sending multiple data values per message separated by | or comma, have your message split into a local or global variable for inspection before trying to select list items from it. Test if (length of list(split list result) >= expected list length) before doing any select list item operations, to avoid taking a long walk on a short pier. This bulletproofing is necessary in case your sending device sneaks in some commentary messages with the data values.

Some people send temperature and humidity in separate messages with distinctive prefixes like "t:" (for temperature) and "h:" (for humidity).
(That's YAML format.)

The AI2 Charts component can recognize these and graph them. See Bluetooth Client Polling Rate - #12 by ABG

To receive YAML format messages, test if the incoming message contains ':' . If true, split it at ':' into a list variable, and find the prefix in item 1 and the value in item 2.

[cruzleogabriel]

temp_hum.aia (460.8 KB)

here's the app

[ABG]

The sketch is also needed.

[cruzleogabriel]

Capstone.ino (1.8 KB)

here it is

[ABG]

Here is an updated blocks sample illustrating these ideas ...

BlueTooth_delimiter_sample.aia (3.4 KB)

[ABG]

I read that.ino file.

You should too.

; Or , or |

Pick one and use it every where

Use println() to end line.

[cruzleogabriel]

I tried to follow your instructions but how can I get the variables from the function after the code has found out at what index it is

[ABG]

Put your own Label components in that global list.

Annotate labels with adjacent labels.

[Mugilan123]

strong text

[ABG]

Be sure to use println() at the end of each message to send from the sending device, to signal end of message.

Only use print() in the middle of a message.

Be sure not to println() in the middle of a message, or you will break it into two short messages and mess up the item count after you split the message in AI2.

Do not rely on timing for this, which is unreliable.

In the AI2 Designer, set the Delimiter attribute of the BlueTooth Client component to 10 to recognize the End of Line character.

Also, return data is not immediately available after sending a request,
you have to start a Clock Timer repeating and watch for its arrival in the Clock Timer event. The repeat rate of the Clock Timer should be faster than the transmission rate in the sending device, to not flood the AI2 buffers.

In your Clock Timer, you should check

  Is the BlueTooth Client still Connected?
  Is Bytes Available > 0?
     IF Bytes Available > 0 THEN
       set message var  to BT.ReceiveText(-1) 

This takes advantage of a special case in the ReceiveText block:

ReceiveText(numberOfBytes)
Receive text from the connected Bluetooth device. If numberOfBytes is less than 0, read until a delimiter byte value is received.

If you are sending multiple data values per message separated by | or comma, have your message split into a local or global variable for inspection before trying to select list items from it. Test if (length of list(split list result) >= expected list length) before doing any select list item operations, to avoid taking a long walk on a short pier. This bulletproofing is necessary in case your sending device sneaks in some commentary messages with the data values.

Some people send temperature and humidity in separate messages with distinctive prefixes like "t:" (for temperature) and "h:" (for humidity).
(That's YAML format.)

The AI2 Charts component can recognize these and graph them. See Bluetooth Client Polling Rate - #12 by ABG

To receive YAML format messages, test if the incoming message contains ':' . If true, split it at ':' into a list variable, and find the prefix in item 1 and the value in item 2.

[ABG]

Here is a simple BlueTooth text receiver sample, for single value per line:

[ABG]

Here is an updated blocks sample illustrating these ideas ...

BlueTooth_delimiter_sample.aia (3.4 KB)