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.

Functions

• 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) {
      ...body...
  }
  

• Create a conditional using if and else:

  if (condition_1) {
      ...body...
  } else if (condition_2) {
      ...body...
  } else {
      ...body...
  }
  

• 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.