问题
I am trying to combine the third column of several data frames, which are called and renamed in a nested for loop, within the same looping process.
# Sample Data
ecvec_msa6_1998=matrix( round(rnorm(200, 5,15)), ncol=4)
ecvec_msa6_1999=matrix( round(rnorm(200, 4,16)), ncol=4)
ecvec_msa6_2000=matrix( round(rnorm(200, 3,17)), ncol=4)
datasets=c("msa")
num_industrys=c(6)
years=c(1998, 1999, 2000)
alist=list()
for (d in 1:length(datasets)) {
dataset=datasets[d]
for (n in 1:length(num_industrys)){
num_industry=num_industrys[n]
for (y in 1:length(years)) {
year=years[y]
eval(parse(text=paste0("newly_added = ecvec_", dataset, num_industry, "_", year)))
# renaming the old data frames
alist = list(alist, newly_added) # combining them in a list
extracted_cols <- lapply(alist, function(x) x[3]) # selecting the third column
result <- do.call("cbind", extracted_cols) # trying to cbind the third colum
}
}
}
Can somebody show me the right way to do this?
回答1:
Your code almost works - here are a few changes...
alist=list()
for (d in 1:length(datasets)) {
dataset=datasets[d]
for (n in 1:length(num_industrys)){
num_industry=num_industrys[n]
for (y in 1:length(years)) {
year=years[y]
eval(parse(text=paste0("newly_added = ecvec_", dataset, num_industry, "_", year)))
#the next line produces the sort of list you want - yours was too nested
alist = c(alist, list(newly_added))
}
}
}
#once you have your list, these commands should be outside the loop
extracted_cols <- lapply(alist, function(x) x[,3]) #note the added comma!
result <- do.call(cbind, extracted_cols) #no quotes needed around cbind
head(result)
[,1] [,2] [,3]
[1,] 11 13 24
[2,] -26 -3 7
[3,] -1 -26 -14
[4,] 5 14 -15
[5,] 28 3 8
[6,] 9 -9 19
HOWEVER - a much more R-like (and faster) way of doing this would be to replace all of the above with
df <- expand.grid(datasets,num_industrys,years) #generate all combinations
datanames <- paste0("ecvec_",df$Var1,df$Var2,"_",df$Var3) #paste them into a vector of names
result <- sapply(datanames,function(x) get(x)[,3])
sapply
automatically simplifies the list into a dataframe if it can (lapply
always produces a list)
回答2:
Often it's recommended to avoid nested loops in R:
See Circle 2 of R's Inferno or here.
Maybe you should try to replace this part
extracted_cols <- lapply(alist, function(x) x[3]) # selecting the third column
result <- do.call("cbind", extracted_cols) # trying to cbind the third colum
with a list like Patrick Burns has done it in the first link (p. 14). It could be also much cleaner.
回答3:
Are you simply looking to extract and combine the third columns from each dataframe into a new one?
newdata <- cbind(ecvec_msa6_1998[,3],ecvec_msa6_1999[,3],ecvec_msa6_2000[,3])
来源:https://stackoverflow.com/questions/44725995/r-how-can-i-cbind-specific-columns-of-all-data-frames-of-a-nested-loop-within-t