R Reference

Basic Operations

  • Use variable <- value to assign a value to a variable.
  • # starts a comment.
  • Statements in a block must be surrounded by curly braces { }.
  • ?thing displays the help for thing.
  • length(thing) produces the length of a collection.
  • c(value1, value2, value3, ...) creates a vector.
  • vector_name[i] selects the i'th value from a vector.
  • mean, max, and min() calculate simple statistics.
  • plot creates simple visualizations.
  • list.files(pattern = "txt") returns the names of all files that contain "txt" in their name.

Data frames

  • dim(dat) gives the dimensions of a data frame.
  • dat[x, y] selects a single element from a data frame.
  • dat[i, ] selects the i'th row; dat[, i] selects the i'th column.
  • low:high specifies a slice including elements from low to high.
  • apply(dat, 1, mean) calculates the mean of each row.
  • apply(dat, 2, mean) calculates the mean of each column.


  • name <- function(...args...) defines a new function.
  • name <- function(arg = default) specifies a default value for a parameter.
  • Call a function using name(...values...).

Control Flow

  • Create a for loop to process elements in a collection one at a time:

    for (variable in collection) {
  • Create a conditional using if and else:

    if (condition_1) {
    } else if (condition_2) {
    } else {
  • Use == to test for equality.
  • X & Y is only true if both X and Y are true.
  • X | Y is true if either X or Y, or both, are true.
  • Use stopifnot(condition) to check that something is true when the program is running.

Using R from the command-line

  • commandArgs(trailingOnly = TRUE) returns the command-line arguments.
  • file("stdin") reads from standard input.
  • cat(vec, sep = "\n") writes to standard ouput each element of vec on its own line.