R

## Vectorization - Loops are slow — use `apply()`, `lapply()`, `sapply()`, or `purrr::map()` - Vectorized functions operate on whole vectors — `sum(x)` not `for (i in x) total <- total + i` - `ifelse()` is vectorized — `if` is not, use `ifelse()` for vector conditions - Column operations faster than row — R is column-major ## Indexing Gotchas - R is 1-indexed — first element is `x[1]`, not `x[0]` - `x[0]` returns empty vector — not error, silent bug - Negative index excludes — `x[-1]` removes first element - `[[` extracts single element — `[` returns subset (list stays list) - `df[, 1]` drops to vector — use `df[, 1, drop = FALSE]` to keep data frame ## NA Handling - NA propagates — `1 + NA` is `NA`, `NA == NA` is `NA` - Use `is.na()` to check — not `x == NA` - Most functions need `na.rm = TRUE` — `mean(x)` returns NA if any NA present - `na.omit()` removes rows with any NA — may lose data unexpectedly - `complete.cases()` returns logical vector — rows without NA ## Factor Traps - Old R converted strings to factors by default — use `stringsAsFactors = FALSE` or modern R - `levels()` shows categories — but factor values are integers internally - Adding new value not in levels gives NA — use `factor(x, levels = c(old, new))` - `as.numeric(factor)` gives level indices — use `as.numeric(as.character(factor))` for values - Dropping unused levels: `droplevels()` — or `factor()` again ## Recycling - Shorter vector recycled to match longer — `c(1,2,3) + c(10,20)` gives `11, 22, 13` - No error if lengths aren't multiples — just warning, easy to miss - Single values recycle intentionally — `x + 1` adds 1 to all elements ## Data Frames vs Tibbles - Tibble never converts strings to factors — safer defaults - Tibble never drops dimensions — `df[, 1]` stays tibble - Tibble prints better — shows type, doesn't flood console - `as_tibble()` to convert — from `tibble` or `dplyr` package ## Assignment - `<-` is idiomatic R — `=` works but avoided in style guides - `<<-` assigns to parent environment — global assignment, usually a mistake - `->` right assignment exists — rarely used, confusing ## Scope - Functions look up in parent environment — can accidentally use global variable - Local variable shadows global — same name hides outer variable - `local()` creates isolated scope — variables don't leak out ## Common Mistakes - `T` and `F` can be overwritten — use `TRUE` and `FALSE` always - `1:length(x)` fails on empty x — gives `c(1, 0)`, use `seq_along(x)` - `sample(5)` vs `sample(c(5))` — different! first gives 1:5 permutation - String splitting: `strsplit()` returns list — even for single string