Repetitive Action Over Ten Matrices in R - r

Question

I have ten datasets, and each dataset contains "ratings" and "occupation" columns. From each of those ten datasets I want to find out the "average" of "ratings" per three occupation groups (i.e. artists, technician, marketing).
The code I have written is as follows:
Average.Rating.per.Interval <- data.frame(interval=as.numeric(),
occupation=as.character(),
average.rating=as.numeric(),
stringsAsFactors=FALSE)
##interval number refers to the dataset number (e.g. for 'e.1' it is 1, for 'e.2' it's 2)
Average.Rating.per.Interval <- as.matrix(Average.Rating.per.Interval)
e.1.artist <- e.1[which(e.1[,"occupation"]=='artist', arr.ind = TRUE),]
mean(e.1.artist$rating)
Average.Rating.per.Interval <- rbind(Average.Rating.per.Interval,
c(interval=1,occupation="artist",average.rating=mean(e.1.artist$rating)))
e.1.technician <- e.1[which(e.1[,"occupation"]=='technician', arr.ind = TRUE),]
mean(e.1.technician$rating)
Average.Rating.per.Interval <- rbind(Average.Rating.per.Interval,
c(1,"technician",mean(e.1.technician$rating)))
e.1.marketing <- e.1[which(e.1[,"occupation"]=='marketing', arr.ind = TRUE),]
mean(e.1.marketing$rating)
Average.Rating.per.Interval <- rbind(Average.Rating.per.Interval,
c(1,"marketing",mean(e.1.marketing$rating)))
This is clearly not efficient at all, because for ten datasets, I have to rewrite the same code 9 more times to get the average ratings for each of those occupations groups for all of my ten datasets. Is there a better way to do this? I cannot think of anything better! I found out that apply/lapply can be a way to do this, but I could not figure out how they can work for my case.
Two of my datasets (e1 and e2) can be found here. (I have only included 10% of the entire observations in each)

Answer 1

You can use the tidyverse package to summarize each of your data frames. First, you'll want to put them in a list. Then you can iterate over each of the data frames in the list, summarizing by occupation:
library(tidyverse)
# Create sample data
set.seed(2353)
sample_data <- rerun(10, tibble(
occupation = sample(c("Artist", "Technician", "Marketing"), 100, replace = TRUE),
ratings = sample(1:100, 100, replace = TRUE)
))
# Summarize by occupation
summarized_data <- sample_data %>%
map(~ .x %>% group_by(occupation) %>% summarize(avg_rating = mean(ratings)))

Answer 2

Another option, with base. First load the files into a list, then use lapply to calculate the means for each dataset
# Set directory to a file that contains the files
files <- list.files()
# Load all the data at once into a single list
l <- lapply(files, dget)
names(l) <- substr(files, 1, 2) # gives meaningful names to list elements (datasets)
# Calculate the mean by group for each dataset
all_group_means <- lapply(l, function(x) tapply(x$rating, x$occupation, mean, na.rm = TRUE))
# Subset all the group means to just those you're interested in
sapply(all_group_means, function(x) x[c("artist", "technician", "marketing")])
d1 d2
artist 3.540984 3.612048
technician 3.519512 3.651106
marketing 3.147208 3.342569
Note that if your data are already all loaded, you could just put them into a list (rather then loading all the data directly into a list) and then use the lapply function and it should still work.
Edit
I just realized you only wanted the means for the three groups. I've edited the code above to subset all means to only the three groups.

Answer 3

I recommend the "plyr" package for this kind of manipulation; it is well worth the investment of an hour or so to learn. In your case, I loaded your first example dataset in "d1", and I can summarise it like so:
ddply(d1, .(occupation), summarise, mean_rating=mean(rating))
This shows the results for all occupations, and you only wanted a specific three, so we can filter it to those:
ddply(subset(d1, occupation %in% c('artist','technician','marketing')), summarise, mean_rating=mean(rating))
Now we just need to generalize it to running over 10 datasets without cut and paste. Let's store our data frames inside a list:
dataset_list <- list(d1=d1) # you would put all of them here; I just have one
Now we can run the same code on all of them, with lapply, and get a list back out:
filtered_occupations <- c('artist','technician','marketing')
lapply(dataset_list, function(dataset) {
ddply(subset(dataset,occupation %in% filtered_occupations),
.(occupation), summarise, mean_rating=mean(rating))} )
Result:
$d1
occupation mean_rating
1 artist 3.540984
2 marketing 3.147208
3 technician 3.519512