Work with strings with stringr:: CHEAT SHEET Detect Matches strdetect(string, pattern) Detect the presence of a pattern match in a string. Strdetect(fruit, 'a') strwhich(string, pattern) Find the indexes of strings that contain a pattern match. Strwhich(fruit, 'a') strcount(string, pattern) Count the number of matches in a string. This cheat sheet summarizes common Stata commands for econometric analysis and provides their equivalent expression in R. References for importing/cleaning data, manipulating variables, and other basic commands include Hanck et al. (2019), Econometrics with R, and Wickham and Grolemund (2017), R for Data Science.
FunctionName - just writing the name of the function returns the function source code help with math: ?Control - Help on control ow statements (e.g. If, for, while) ?Extract - Help on operators acting to extract or replace subsets of vectors ?Logic - Help on logical operators ?regex - Help on regular expressions used in R.
Flashcards#
I suggest making your own flashcards to study, but if you don’t want to you can use my flashcard deck here.
A function is just like a function in programming. It takes an input, performs some maths on it, then outputs the result.
The syntax of a function is f(x) = 2 * x. This means given x, the function will return 2*x.
A function cannot have more than one output if given a single input. However, multiple inputs can lead to the same output.
Functions can be written in two ways, either f(x) = 2 * x or f: R→ Rwhich defines the input and output to be real numbers.
Domain, Codomain, Range#
Let function f(x) = 2x. The set, x, is the Domain; the range of all possible inputs. The set 2x is the codomain. The values produced by the function is the range.
In other syntax, f: A → B. A is called the domain, B is the **co-domain **and the **range **of f(A) is F(A) = {f(x) | x ∈ A}
Finding the domain of a function#
It is best to show an example of how to find the domain of a function.
f(x) = sqrt(2x-8)We know that the domain of a function is the set of all possible inputs into a function. We know that the above function is only defined when it’s taking the square root of a non-negative number so it’s only going to be defined when:2x-8 ≥ 0.Now this is a simple algebraic problem to find x.2x ≥ 8x ≥ 4So the domain here is the set of all real numbers ≥ 4.
Sometimes it is easier to find what cannot be in the domain than to find every item in the domain.
Given the function f(x) = 2/x-3, let’s try find the range.
Is there anything that X CANNOT be? Yes! If X is 0 it would cause an error as you cannot divide by 0!
Types of functions#
Label the left hand set A and the right hand set B and ignore the lines.
A has many B#
Adobe photoshop mac dmg. Not a function, because function inputs cannot map to more than one output.
B can have many A#
It is a function because a single output can be produced from different inputs.
B can’t have many A (Injective)#
Not all outputs of B can be produced by A.
Every B has some A (Surjective)#
Every output in B has at least one input in A, sometimes more.
A to B, perfectly (Bijective)#
Bijective is the combination of injective and surjective. Every A matches perfectly to an output.https://www.mathsisfun.com/sets/injective-surjective-bijective.html Adobe zii 2020 5.1 download.
Cardinality of sets#
The cardinality of a set is how many items are in the set, denoted as |a| Macos catalina version 10.15 download.
Powersets#
The powerset is the set containing every single subset of set A, where A is any set. The powerset of a = {1, 2, 3} is:
Pow(A)= { {1}, {2}, {3}, {1,2}, {1,3}, {2, 3}, {1, 2, 3}, {Ø} }
The cardinality of a power set is always 2^n where n is |a|. The same cardinality rule applies to bit vector sets.
Infinite sets#
Infinite sets can also be bijective, injective or surjective.
Uncountable sets#
There exists sets that are not countable, such as Cantor’s set derived from Cantor’s diagonal argument.
The Pigeon Hole Principle#
The Pigeon Hole Principle states that if |A| > |B| then at least one value of F occurs more than once.
In other words, if there are N holes and we have N+1 pigeon then 2 pigeons must occupy the same hole.
If you spend time with R regularly then you should have the basics of data manipulation & plotting down within a couple of weeks. The more esoteric functions will take time to master. These are PDF cheat sheets that can be printed out. Keep them handy as you work with R.
Free Printable Excel Cheat Sheets
help.start() - This first link doesn't actually take you to another site because it isn't a resource on the web, it's contained with R. To access it just type help.start() at the prompt and a collection of manuals will appear in your browser window. Pretty handy reference tool, eh?
Basic R Functions Cheat Sheet
R Reference Card - This is the most popular reference card. This same document is available from a number of sites and includes info on input & output, getting help, working with variables & arrays, plotting & more. You want this one.
R for Data Mining - This document was written by RDataMining.com. It includes info on classification, clustering, social network analysis, big data and more. This is very handy if you're interested in knowledge discovery & data mining.
Basic R Syntax Cheat Sheet
R Style Guide - This resource is more than a cheat sheet. Google's internal R user community put together this guide for clean R code that covers syntax & conventions that are unique to R. I include it here because I've refered to it quite a bit in my own work. Your code will be easy to read & maintain if you follow these guidelines.