reference.md

layout	reference
permalink	/reference/

Basic Operation

• # this is a comment in R
• Use x <- 3 to assign a value, 3, to a variable, x
• R counts from 1, unlike many other programming languages (e.g., Python)
• length(thing) returns the number of elements contained in the variable collection
• c(value1, value2, value3) creates a vector
• container[i] selects the i'th element from the variable container

List objects in current environment ls()

Remove objects in current environment rm(x)

Remove all objects from current environment rm(list = ls())

Control Flow

• Create a conditional using if, else if, and else

if(x > 0){
	print("value is positive")
} else if (x < 0){
	print("value is negative")
} else{
	print("value is neither positive nor negative")
}

{: .r}

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

for (i in 1:5) {
	print(i)
}

{: .r}

This will print:

1
2
3
4
5

{: .output}

• Use == to test for equality
  • 3 == 3, will return TRUE,
  • 'apple' == 'orange' will return FALSE
• X & Y is TRUE is both X and Y are true
• X | Y is TRUE if either X or Y, or both are true

Functions

• Defining a function:

is_positive <- function(integer_value){
	if(integer_value > 0){
	   TRUE
	}
	else{
	   FALSE
	{
}

{: .r}

In R, the last executed line of a function is automatically returned

• Specifying a default value for a function argument

increment_me <- function(value_to_increment, value_to_increment_by = 1){
	value_to_increment + value_to_increment_by
}

{: .r}

increment_me(4), will return 5

increment_me(4, 6), will return 10

• Call a function by using function_name(function_arguments)

• apply family of functions: apply(), sapply(), lapply(), and mapply()

apply(dat, MARGIN = 2, mean) will return the average (mean) of each column in dat

Packages

• Install package by using install.packages("package-name")
• Update packages by using update.packages("package-name")
• Load packages by using library("package-name")

Glossary

{:auto_ids} argument : A value given to a function or program when it runs. The term is often used interchangeably (and inconsistently) with parameter.

call stack : A data structure inside a running program that keeps track of active function calls. Each call's variables are stored in a stack frame; a new stack frame is put on top of the stack for each call, and discarded when the call is finished.

comma-separated values (CSV) : A common textual representation for tables in which the values in each row are separated by commas.

comment : A remark in a program that is intended to help human readers understand what is going on, but is ignored by the computer. Comments in Python, R, and the Unix shell start with a # character and run to the end of the line; comments in SQL start with --, and other languages have other conventions.

conditional statement : A statement in a program that might or might not be executed depending on whether a test is true or false.

dimensions (of an array) : An array's extent, represented as a vector. For example, an array with 5 rows and 3 columns has dimensions (5,3).

documentation : Human-language text written to explain what software does, how it works, or how to use it.

encapsulation : The practice of hiding something's implementation details so that the rest of a program can worry about what it does rather than how it does it.

for loop : A loop that is executed once for each value in some kind of set, list, or range. See also: while loop.

function body : The statements that are executed inside a function.

function call : A use of a function in another piece of software.

function composition : The immediate application of one function to the result of another, such as f(g(x)).

index : A subscript that specifies the location of a single value in a collection, such as a single pixel in an image.

loop variable : The variable that keeps track of the progress of the loop.

notional machine : An abstraction of a computer used to think about what it can and will do.

parameter : A variable named in the function's declaration that is used to hold a value passed into the call. The term is often used interchangeably (and inconsistently) with argument.

pipe : A connection from the output of one program to the input of another. When two or more programs are connected in this way, they are called a "pipeline".

return statement : A statement that causes a function to stop executing and return a value to its caller immediately.

silent failure : Failing without producing any warning messages. Silent failures are hard to detect and debug.

slice : A regular subsequence of a larger sequence, such as the first five elements or every second element.

stack frame : A data structure that provides storage for a function's local variables. Each time a function is called, a new stack frame is created and put on the top of the call stack. When the function returns, the stack frame is discarded.

standard input (stdin) : A process's default input stream. In interactive command-line applications, it is typically connected to the keyboard; in a pipe, it receives data from the standard output of the preceding process.

standard output (stdout) : A process's default output stream. In interactive command-line applications, data sent to standard output is displayed on the screen; in a pipe, it is passed to the standard input of the next process.

string : Short for "character string", a sequence of zero or more characters.

while loop : A loop that keeps executing as long as some condition is true. See also: for loop.