Here are some good habits to get into that I learned from experience to make working with R easier.

## Project structure

1. For every new task, client or project, create an RStudio project (top right)
2. Create a script and data folder in the project straight away
3. Put your R scripts ./scripts/your-script.R in the script folder, and raw or processed data written to the data folder (./data/your_data.csv)
4. Never change your working directory (setwd()) in the scripts; it ruins portability
5. If using Git (and you should!) then make one Github repo per R project.

## Code style

1. Follow a code style - any will do, just be consistent. Here is Google’s. I am closer to this one.
2. Descriptively name your function and variables.
3. Comment everything - you can even start code with comments.
4. Pretend the reader of your code is a homicidal maniac who will axe you if they don’t understand it.
5. Avoid copy-pasting: if you repeat yourself more than twice, write a function!
6. Fail fast: use stop() to write informative error messages.
7. Check that the types of your input and output variables are what you expected.
8. “Clever” code or “code golf” makes you feel good in the short term, but confused long term.
9. Clarity is most important.
10. If you write functions, document them using the roxygen format (#' lines above function).

## Always check the type

1. 4 times out of 10 the errors in code are from you copying something wrong (rowname instead of row.name, for instance).
2. 4 times out of 10 the errors in code are because a function is expecting a different type from what you are trying to pass it (e.g., class(obj) == "list" not class(obj) == "data.frame"). Read the function documentation carefully.
3. Use str() a lot.

## Useful functions to know

1. str()
2. Get help via ?[function name].
3. paste0 - shortcut for paste("blah", "blah", sep = "")
4. Learn to love list() - it’s a way to group related objects:
stuff = list(mydata = data,
the_author = "Bob",
created = Sys.Date())

## can then access items via $stuff$mydata
stuff$the_author stuff$created
1. Once you love lists, you will adore lapply(), apply(), sapply(), etc.
2. data.frames are special lists…just with equal length objects.
3. You can do lookups via subsetting of named vectors:
## example with some user Ids
lookup <- c("Bill","Ben","Sue","Linda","Gerry")
names(lookup) <- c("1231","2323","5353","3434","9999")
lookup
##    1231    2323    5353    3434    9999
##  "Bill"   "Ben"   "Sue" "Linda" "Gerry"
## this is a big vector of Ids you want to lookup
big_list_of_ids <- c("2323","2323","3434","9999","9999","1231","5353","9999","2323","1231","9999")

lookup[big_list_of_ids]
##    2323    2323    3434    9999    9999    1231    5353    9999    2323
##   "Ben"   "Ben" "Linda" "Gerry" "Gerry"  "Bill"   "Sue" "Gerry"   "Ben"
##    1231    9999
##  "Bill" "Gerry"
1. Reduce() operates repeatedly on a list, adding the result to its previous. A good example is for reading a folder full of files:
## say you have lots of files in folder "./data/myfolder"
## we can use lapply on write.csv to read in all the files:

folder <- "./data/myfolder"
filenames <- list.files(folder)

## a list of data.frames read from the csv

## operate rbind (bind the rows) on the list, iterativly
all_files <- Reduce(rbind, df_list)

## all_files is now one big dataframe, all_files

## in one line:
all_files <- Reduce(rbind, lapply(filenames, read.csv))
1. A handy way to get rid of the first column is to use the_data[-1] where the_data is your data.frame. This is equivalent to the_data[,-1]
2. If you want to include only certain columns from a big data frame, it can be a bit of a chore to write out all the column names you want to keep rather than dropping the ones you want. Two options: Use library(dplyr) and the select() function that lets you specify dropped columns like select(-notthisone, -notthisonetoo) or in base R use setdiff() against the names:
names(mtcars)
##  [1] "mpg"  "cyl"  "disp" "hp"   "drat" "wt"   "qsec" "vs"   "am"   "gear"
## [11] "carb"
new_data <- mtcars[ , setdiff(names(mtcars), c("mpg","disp","drat"))]
names(new_data)
## [1] "cyl"  "hp"   "wt"   "qsec" "vs"   "am"   "gear" "carb"