What is ‘Stack’ing and ‘Unstack’ing in R
Stacking and unstacking are function that help in reshaping data in
R. Data is not always in the format you require. For example, say you
have data from 3 temperature sensors in different areas of chicago. And
the data you get from these sensors is horizontal.
Even if you wanted to compare the temperatures across sensors, this is not a good format. Instead, if all the temperature data were in a single column, with the second column identifying where the sensor data came from, you can analyze the data much better. This is one of the data reshaping steps.
# Let's take the first 5 rows.
> sensors
sensor_1 sensor_2 sensor_3
1 105 98 90
2 105 113 98
3 102 92 110
4 95 101 103
5 97 113 96
How do we stack these 3 columns into the same column ? Try this
> sensors = stack(sensors)
> sensors
values ind
1 105 sensor_1
2 105 sensor_1
3 102 sensor_1
4 95 sensor_1
5 97 sensor_1
6 98 sensor_2
7 113 sensor_2
8 92 sensor_2
9 101 sensor_2
10 113 sensor_2
11 90 sensor_3
12 98 sensor_3
13 110 sensor_3
14 103 sensor_3
15 96 sensor_3
R created a new column automatically, called ind , to indicate which column the row came from. If you did not want to stack all the columns, you can very well specify the columns.
# say you only wanted the first and third sensor's data
> sensors = stack(sensors, select = c("sensor_1","sensor_3"))
> sensors
values ind
1 105 sensor_1
2 105 sensor_1
3 102 sensor_1
4 95 sensor_1
5 97 sensor_1
6 90 sensor_3
7 98 sensor_3
8 110 sensor_3
9 103 sensor_3
10 96 sensor_3
Unstacking
Unstacking the data is just as easy. Let’s unstack the dataframe we just stacked.
> sensors = unstack(sensors)
> sensors
sensor_1 sensor_3
1 105 90
2 105 98
3 102 110
4 95 103
5 97 96
That was easy. Wasn’t it ?