Saving sensor data to a file

Overview

• In this lesson we are going to learn how to save our sensor data to a file.
• Saving data to a file will keep it safe.
• Once data is in a file, we can read the data again and make a graph.
• Reading, saving a graphing data is very important for all environmental and Citizen Science activities.
• Sensors often deliver data a numerical data. Some simple examples are temperature, time, speed, counts, etc.
• In this example we are going to save our data as a String (or text).
• Most files store data as text. In this example we also need to separate the data using commas (,).
• If data is separated by commas it makes each data element human readable and other programs, such as Spreadsheets, can also read the file more easily.

Learning Objectives

• Learn how convert numerical data into a String (or text)
• Learn how to save data to a new empty file
• Learn how to read a saved file and check for data integrity

Converting numerical data to a String

• In this example we will build on the python program from the previous lesson.
• Our python program was used to download data from a sensor (temperature, atmospheric pressure, battery voltage).
• We will now convert this data into a String and put all the data into a single variable named data.
• Each piece of data will be separated by a comma.

Open up the Python file named atm_sensor_get.py

• From the Raspberry Pi main menu drop down select Programming > Python3 (IDLE).

• This will open the Python Shell.

• From the File drop down menu select Recent Files.

• Choose the file named atm_sensor_get.py
• Edit the file so that it appears like the example below.

• Saving and Running the file should produce an output in the Python Shell like the output below.

Arranging numerical data in a String

• We can create (or define) a new empty variable named data to hold all the temperature, pressure and battery data using the statement data = “”

• We need to add or concatenate all the data together into one long String.
• To do this we need to:
  • convert the numerical data to String using the str() function
  • then add each data String together using the plus (+) symbol
• The final code is presented below.
• We also added a print(data) statement so that we can see data variable.

• Save and Run the program. The result should be like the following.

Adding spaces between data values

• The issue with the format of this data is that there is no gap between the data.
• To put a gap between the data we need to add a space using this additional code + “ “ +
• See the example below as a reference.

• The output of this program in the Shell is reproduced here.

Adding a human readable header to our data

• To make the data easier to read we can add an additional print() statement to act as a header for our data.
  • print(“Temp, Pres, Bat”)

• With the additional print() header, it makes the data easier to read.
• We also remove the data = “” statement, because we don’t really need it in Python.

Comma separated data

• Rather than having a space separating the data it is better to use a comma (,).
• Replace all the “ “ code with “,” as in the example below.

• After saving and running the program you should get an output like the one below.
• This is an important first step in preparing data to be saved to a file.

Saving data to a file

• Data can be saved in a text file so that we don’t lose the data, and so that other programs on our computer can access the data.
• With bigger data sets you can also save data in a specialised database or data warehouse.
• Saving data to a file is much easier and it also allows us to more easily check the data for integrity.
• In this example we will be creating an empty file named data.txt and then saving new data to this file.

Navigating and listing files using the Terminal

• Open the Terminal.
• When you open the Terminal you will be in your pi home directory. You should see the tilde (~) symbol.
• The path to the pi home directory is /home/pi yet for simplicity this is abbreviated to ~.
• Enter the command ls to list all the contents of the pi home directory.
• In my pi home directory, my project folder is named botanica-park-lake

• To move into the botanica-park-lake directory we enter the command cd botanica-park-lake
• cd is short for Change Directory

• One inside the botanica-park-lake directory we can also use the ls command to list all contents.
• Enter the command ls
• You should see a range of files similar to the ones displayed below.

Creating an empty file in the Terminal

• Before we can save data to a file, we need to create an empty file.
• Enter the command touch data.txt to create an empty file named data.txt
• To verify that the file has been created enter the command ls to list all directory contents.
• You should see the new file data.txt in the listing.

• Another way to list files is to add the option -l. This option lists the files in long format.
• Enter the command ls -l
• The file size value in bytes will be displayed in the column just before the month (e.g. Apr, May).
• We can see that the file size of data.txt is 0 bytes.
• For comparison, the file size of the birds.html file is 414 bytes.

Writing data to a file

• To write the data to a file we need to add three lines of code at the end of the python file.
  • f = open(‘/home/pi/botanica-park-lake/data.txt’,’a’)
  • f.write(data)
  • f.close()

• In the line f = open()
  • we identify the path to the file, in this case /home/pi/botanica-park-lake/data.txt
  • we also say that we want to append (a) new data to the file. This continually adds new data to the end of the file.
• f.write(data) – this statements writes the data to the file.
• f.close() – this statement closes the file. By closing the file other programs can then access or read the file.

• If we run this file we won’t see any changes to the output in the Python Shell.
• To see the new data we need to view the contents of the data.txt file. Hopefully it should hold some data now.
• To view the contents of a text file in Linux we enter the command cat data.txt
• cat is short for concatenate.
• We can see that the output shows that we have some numerical data stored in the file.

• If we go back to the Python Editor window (our program) and run the program again we start to notice a problem.
• You can see from the output below in the Terminal that the data starts to get jumbled together.
• Run the program again in IDLE3 (Python3) and then enter cat data.txt in the Terminal to see the results.
• Ideally, we would like to see each new data set on its own separate line.

• To fix this problem we need to add a special “new line” character at the end of our data.
• The new line special character is symbolised by “\n”
• Modify the line data = str(temp) + “,” + str(pres) + “,” + str(voltage) to the following:
  • data = str(temp) + “,” + str(pres) + “,” + str(voltage) + “\n”

• If we Run the program 3 or 4 times we will start to see the following output using the cat data.txt command.
• Now you can see that the new data we are adding now appears on a new line.