As part of my research for a different article, I recently collected data about my driving commute home via an accelerometer recorder app on my phone. The app generates a simple TSV file. (A TSV file is like a CSV file, but instead of a comma separator, it uses a TAB character to separate the values.) The raw data looks like this:Related from Analytics Experience 2018: Using your smartphone accelerometer to build a safe driving profile
With SAS, it's simple to import the file into a data set. Here's my DATA step code that uses the INFILE statement to identify the file and how to read it. Note that the DLM= option references the hexadecimal value for the TAB character in ASCII (09x), the delimiter for fields in this data.
data drive; infile "/home/chris.hemedinger/tsv/drivehome.tsv" dlm='09'x; length counter 8 timestamp 8 x 8 y 8 z 8 filename $ 25; input counter timestamp x y z filename; run;
In my research, I didn't stop with just my drive home. In addition to my commute, I collected data about 4 other activities, and thus accumulated a collection of TSV files. Here's my file directory in my SAS OnDemand for Academics account:
To import each of these data files into SAS, I could simply copy and paste my code 4 times and then replace the name of the file for each case that I collected. After all, copy-and-paste is a tried and true method for writing large volumes of code. But as the number of code lines grows, so does the maintenance work. If I want to add any additional logic into my DATA step, that change would need to be applied 5 times. And if I later come back and add more files to my TSV collection, I'll need to copy-and-paste the same code blocks for my additional cases.
Using a wildcard on the INFILE statement
I can read all of my TSV files in a single step by using the wildcard notation in my INFILE statement. I replaced the "drivehome.tsv" filename with *.tsv, which tells SAS to match on all of the TSV files in the folder and process each of them in turn. I also changed the name of the data set from "drive" to the more generic "accel".
data accel; infile "/home/chris.hemedinger/tsv/*.tsv" dlm='09'x; length counter 8 timestamp 8 x 8 y 8 z 8 filename $ 25; input counter timestamp x y z filename; run;
The SAS log shows which files have been processed and added into my data set.
With a single data set that has all of my accelerometer readings, I can easily segment these with a WHERE clause in later processing. It's convenient that my accelerometer app also captured the name of each TSV file so that I can keep these cases distinct. A quick PROC FREQ shows the allocation of records for each case that I collected.
Add the filename into the data set
If your input data files don't contain an eponymous field name, then you will need to use a different method to keep track of which records come from which files. The DATA step INFILE statement supports a special option for this -- it's the FILENAME= option. The FILENAME= option allows you to specify an automatic variable to store the name of the current input file. Like all automatic variables in the DATA step, this variable won't be written to the output data set by default. So, you need to include code that assigns the value to a permanent variable that you can save. Here's my code, refactored a little bit, now with the options for reading/storing the file names we're processing:
filename tsvs "/home/chris.hemedinger/tsv/*.tsv"; data accel; length casefile $ 100 /* to write to data set */ counter 8 timestamp 8 x 8 y 8 z 8 filename $ 25 tsvfile $ 100 /* to hold the value */ ; /* store the name of the current infile */ infile tsvs filename=tsvfile dlm='09'x ; casefile=tsvfile; input counter timestamp x y z filename; run;
In the output, you'll notice that we now have the fully qualified file name that SAS processed using INFILE.
Managing data files: fewer files is better
Because we started this task with 5 distinct input files, it might be tempting to store the records in separate tables: one for each accelerometer case. While there might be good reasons to do that for some types of data, I believe that we have more flexibility when we keep all of these records together in a single data set. (But if you must split a single data set into many, here's a method to do it.)
In this single data set, we still have the information that keeps the records distinct (the name of the original files), so we haven't lost anything. SAS procedures support CLASS and BY statements that allow us to simplify our code when reporting across different groups of data. We'll have fewer blocks of repetitive code, and we can accomplish more across all of these cases before we have to resort to SAS macro logic to repeat operations for each file.
As a simple example, I can create a simple visualization with a single PROC SGPANEL step.
ods graphics / width=1600 height=400; proc sgpanel data=accel; panelby filename / columns=5 noheader; series x=counter y=x; series x=counter y=y; series x=counter y=z; colaxis display=none minor; rowaxis label="m/s**2" grid; where counter<11000; run;
Take a look at these 5 series plots. Using just what you know of the file names and these plots, can you guess which panel represents which accelerometer case?
Leave your guess in the comments section. Update: answers revealed (via SAS Visual Analytics) in this post!