plotting polygons inside loop in R, why this ambiguous behavior?

user_1099369 0
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So, I want to plot multiple polygons over one polygon using a loop in r. I have read that you need to use print in order to succesively display the polygons over the original polygon. However, that is not what R seems to do. Besides, in a hardly reproducible example, what i find is that either using print or not, the polygons will not plot within the loop. I am using a mac with OS monterrey, btw.

Here is a reproducible example, using print gives a bunch of NULL messages, but does print, not using print still plots. Yet a more complicated example will never plot from within the loop despite plotting everything when i request line by line from within the loop.

require(maptools)
data(wrld_simpl)

plot(wrld_simpl)
for (i in 1:12){
region=wrld_simpl[wrld_simpl$NAME==wrld_simpl$NAME[i],]
 print(plot(region,border="red",add=T))
 }
polygon r loops
1 Answers

Maybe you're confusing print and plot? The plot function creates (or adds elements to) a plot. Whereas print function prints its arguments to the console. Since plot doesn't return a value, print has nothing to print and hence prints NULL.

Btw. you don't even need a loop. Since the loaded data wrld_simpl is of class SpatialPolygonsDataFrame, the object is subsettable like a dataframe you can just plot the needed indices:

plot(wrld_simpl)
plot(wrld_simpl[1:12,], border="red", add=T)

If you need help on a more complicated example, I suggest you share it?

user_29921990
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