how to loop over vector elements in R with foreach-CodePudding

I am relatively new to R and cannot figure out how to turn my for loop into a foreach loop.

I have a vector of few elements, i.e.

> hpattern
[1] "sim0_pmax.tif" "sim0_vmax.tif" "sim1_pmax.tif" "sim1_vmax.tif"
> typeof(hpattern)
[1] "character"

First: I come from the python word, the hpattern item should be a list, since it is created as follows

list_h30 = data.frame(list.files(path = tr30dir, pattern=paste(depthfile,". $",sep=""), recursive = TRUE, full.names = FALSE))
hpattern <- sub(paste(".*",depthfile,sep=""), "", list_h30[,1])

How come the typeof hpattern is character and not list?

Second: if I run

for (hpi in hpattern) {print(hpi)}

I get

[1] "sim0_pmax.tif"
[1] "sim0_vmax.tif"
[1] "sim1_pmax.tif"
[1] "sim1_vmax.tif"

if I run

foreach(hpi=hpattern, .combine='c') %do% {print(hpi)}

I get

[1] "sim0_pmax.tif"
[1] "sim0_vmax.tif"
[1] "sim1_pmax.tif"
[1] "sim1_vmax.tif"
[1] "sim0_pmax.tif" "sim0_vmax.tif" "sim1_pmax.tif" "sim1_vmax.tif"

I do not understand why I get the last output.

CodePudding user response：

As Roland said, sub returns a character vector (in R, most things are vectors). Another useful function to check your object is str.

About foreach: it combines the output of each execution of the loop. You've specified that you want a vector (by .combine = 'c').

As print returns every object invisibly, each hpi gets printed inside the loop and then combined to the final vector. Compare it to when you don't print it:

foreach(hpi=hpattern, .combine='c') %do% {hpi}
[1] "sim0_pmax.tif" "sim0_vmax.tif" "sim1_pmax.tif" "sim1_vmax.tif"