Working with lists that contain a variable number of data frames

Question

I'm new into R programming and I'm looking for suggestions for the next problem.

I have a list that contains a variable number of data frames, to contextualize, let's say we have "n" sensors and all data is contained in a list that contains "n" data frames, where each data frame contains 10 variables that deliver these sensors.

First of all, the data I have comes in a data frame with the following structure:

head(rawData, 3)

  EmailUsuario                                    Fecha Hcho Humidity  Latitude Longitude NombreUsuario Pm25 SensorName Temperature
1         null Fri Feb 01 2019 10:40:51 GMT-0300 (CLST) null     null -34.42584 -72.03271          null   40        C08        null
2         null Fri Feb 01 2019 10:40:56 GMT-0300 (CLST) null     null -34.42584 -72.03271          null   35        C08        null
3         null Fri Feb 01 2019 10:41:01 GMT-0300 (CLST) null     null -34.42642 -72.03216          null   35        C08        null

In this data frame all the sensors are found, in the SensorName column only one is observed, "C08", but there is a variable number of sensors in that column.

The next thing I do is create a list where all the data frames are contained but separated by sensor.

n <- levels(rawData$SensorName) # sensor names vectors
s <- split(x = rawData, f = rawData$SensorName) # List of DFs per sensor

If I look at the second element in the "s" list, I will get the following data frame:

head(s[[2]], 3)


    EmailUsuario                                   Fecha Hcho Humidity Latitude Longitude NombreUsuario Pm25 SensorName Temperature
560         null Wed Feb 06 2019 14:49:17 GMT+0000 (GMT) null     null -70.6667    -33.45          null   12        C17        null
561         null Wed Feb 06 2019 14:49:22 GMT+0000 (GMT) null     null -70.6667    -33.45          null   12        C17        null
562         null Wed Feb 06 2019 14:49:27 GMT+0000 (GMT) null     null -70.6667    -33.45          null    9        C17        null

You can see that it corresponds to the sensor "C17".

Now what I want is, for example, to create a new data frame but only with some variables (columns) of all the data frames contained in the "s" list. In this case I want to create a data frame where the names of the columns are equal to the name of the sensor and the values of the column are equal to the value of the column "Pm25" of that sensor. So I can work with just that data.

Is this approach good? What solution do you recommend? Do you recommend another solution?

Thanks

It depends on what you want to do with the data. A good tell if this is a good approach would be if you have the same number of records for every sensor. Anyhow you can get to the format with `SensorName` on the columns without the list. Feel free to edit your question for further help — josemz, Apr 04 '19 at 23:25
@josemz Hi, thanks for your response. No, I do not have the same number of records in each sensor. Is not the creation of the list necessary? Is there an easier way? — Wolkuz, Apr 04 '19 at 23:31
check the answers [here](https://stackoverflow.com/questions/5890584/how-to-reshape-data-from-long-to-wide-format), you currently have it in "long" format so you can use either `spread` or `dcast` to put `SensorName` in the columns and `Pm25` as the values. Notice I didn't include `reshape` since it's way too old. Look at the answers below the accepted one. Good luck! — josemz, Apr 04 '19 at 23:40

Working with lists that contain a variable number of data frames

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