17.33. Use R scripts in Processing

Kontribusi modul oleh Matteo Ghetta - dibiayai oleh Scuola Superiore Sant'Anna

Processing (with the Processing R Provider plugin) makes it possible to write and run R scripts inside QGIS.

Peringatan

R has to be installed on your computer and the PATH has to be correctly set up. Moreover Processing just calls the external R packages, it is not able to install them. So be sure to install external packages directly in R. See the related chapter in the user manual.

Catatan

If you have package problems, it may be related to missing mandatory packages required by Processing, like sp, rgdal and raster.

17.33.1. Adding scripts

Adding a script is simple. The easiest way is to open the Processing toolbox and choose Create new R script... from the R menu (labelled with an R icon) at the top of the Processing Toolbox. You can also create the script in for instance a text editor and save it in your R scripts folder (processing/rscripts). When it has been saved there, it will be available for editing by right-clicking on the script name in the processing toolbox and then choose Edit Script...).

Catatan

If you cannot see R in Processing, you have to activate it in Processing ► Options ► Providers

It opens a script editor window in which you have to specify some parameters before you can add the script body.

17.33.2. Creating plots

In this tutorial we are going to create a boxplot of a vector layer field.

Open the r_intro.qgs QGIS project under the exercise_data/processing/r_intro/ folder.

Script parameters

Open the editor and start writing at the beginning of it.

You must specify some parameters before the script body:

The name of the group (plots in this case) in which you want to put your script (if the group does not exist, it will be created):
```
##plots=group
```
You will find your script in the plots R group in the Processing toolbox.
You have to tell Processing that you want to display a plot (in this example):
```
##showplots
```
You will then find a link to the plot in the Result Viewer panel (can be turned on / off in View ► Panels and with Processing ► Results Viewer).
You also need to tell Processing about your input data. In this example we want to create a plot from a field of a vector layer:
```
##Layer=vector
```
Processing now knows that the input is a vector. The name Layer is not important, what matters is the vector parameter.
Finally, you have to specify the input field of the vector layer (using the name you have provided above - Layer):
```
##X=Field Layer
```
Processing now knows that you need a field of Layer, and that you will call it X.
It is also possible to define the name of your script using name:
```
##My box plot script=name
```
If not defined, the file name will be used as the name of the script.

Badan skrip

Now that you have set up the heading of the script you can add the function:

boxplot(Layer[[X]])

boxplot is the name of the R function, the parameter Layer is the name that you have defined for the input dataset and X is the name you have defined for the field of that dataset.

Peringatan

The parameter X has to be within double square brackets ([[]]).

The final script looks like this:

##Vector processing=group
##showplots
##Layer=vector
##X=Field Layer
boxplot(Layer[[X]])

Save the script in the default path suggested by Processing (processing/rscripts). If you have not defined a name in the script heading, the file name you choose will become the name of the script in the Processing toolbox.

Catatan

You can save the script wherever you like, but Processing will then not be able to include it in the processing toolbox automatically, so you have to upload it manually.

Now just run it using the button on the top of the editor window:

Once the editor window has been closed, use the text box of Processing to find your script:

You can now fill the parameters required in the Processing algorithm window:

for Layer choose sample_points
for the X field choose value

Click on Run.

The Result window should be automatically opened, if not, just click on Processing ► Result Viewer....

Click on the link in the viewer and you will see:

Catatan

You can open, copy and save the image by right clicking on the plot.

17.33.3. Create a vector

You can also create a vector layer and have it automatically loaded into QGIS.

The following example has been taken from the Random sampling grid script that can be found in the online collection of R scripts (the scripts in this online collection can be found in https://github.com/qgis/QGIS-Processing/tree/master/rscripts).

The aim of this exercise is to create a random point vector layer using an input vector layer to restrict the extent using the spsample function of the sp package.

Script parameters

As before we have to set some parameters before the script body:

Specify the name of the group in which you want to put your script, in this case Point pattern analysis:
```
##Point pattern analysis=group
```
Define an input parameter (a vector layer) that will constrain the placement of the random points:
```
##Layer=vector
```
Set an input parameter for the number of points that are going to be created (Size, with a default value of 10):
```
##Size=number 10
```
Catatan

Since a default value (10) is defined, the user can change this number or can leave the parameter without a number.
Specify that there is an output vector layer (called Output):
```
##Output=output vector
```

Badan skrip

Now you can add the body of the function:

Use the spsample function:
```
pts=spsample(Layer, Size, type="random")
```
The function uses the Layer to constrain the placement of the points (if it is a line layer, a points will have to be on one of the lines in the layer, if it is a polygon layer, a point will have to be within a polygon). The number of points is taken from the Size parameter. The sampling method is random.

Generate the output (the Output parameter):

Output=SpatialPointsDataFrame(pts, as.data.frame(pts))

The final script looks like this:

##Point pattern analysis=group
##Layer=vector
##Size=number 10
##Output=output vector
pts=spsample(Layer, Size, type="random")
Output=SpatialPointsDataFrame(pts, as.data.frame(pts))

Save it and run it, clicking on the run button.

In the new window type in the right parameters:

and click on run.

The result layer will be added to the table of contents and its points will be displayed on the map canvas:

17.33.4. Text and graph output from R - syntax

Processing (with the Processing R Provider plugin) uses special syntax to get the results out of R:

> before your command, as in >lillie.test(Layer[[Field]]) means the result should be sent to R output (Result viewer)
+ after a plot enables overlay plots. For example plot(Layer[[X]], Layer[[Y]]) + abline(h=mean(Layer[[X]]))