What is R语言

参考网站：

http://machinelearningmastery.com/what-is-r/(简介与入门课程下载)

https://www.zhihu.com/question/19611094(知乎：R 语言的优劣势是什么？)

http://blog.csdn.net/qq_17478541/article/details/51201726(CSDN：R安装)

http://www.cnblogs.com/jamesf/p/4751598.html(RStudio，R最好的IDE)

What is R?

R is an open source environment for statistical programming and visualization.

R is a number of things, which might be confusing at first.

R is the most popular platform among professional data scientists for applied machine learning.

R is a computer language. It is a variant of Lisp and you can write programs in it.
R is an interpreter. It can parse and execute R scripts (programs) that are typed in directly or loaded from a file with a .R extension.
R is a platform. It can create graphics to be displayed on the screen or saved to file. It can also prepare models that can be queried and updated.

You may want to write R scripts in files and run them in batch mode using the R interpreter to get results such as tables or graphics. You may want to open the R interpreter and type in commands to load data, explore and model it in an ad hoc manner.

There are graphical environments, but the simplest and most common usage of R is from the R console (like a REPL). If you are just starting out with R, I would recommend learning R on the console.

What is REPL?

A read–eval–print loop (REPL), also known as an interactive toplevel or language shell, is a simple, interactive computer programming environment that takes single user inputs (i.e. single expressions), evaluates them, and returns the result to the user; a program written in a REPL environment is executed piecewise. The term is most usually used to refer to programming interfaces similar to the classic Lisp machine interactive environment. Common examples include command line shells and similar environments for programming languages, and is particularly characteristic of scripting languages.

In a REPL, the user enters one or more expressions (rather than an entire compilation unit) and the REPL evaluates them and displays the results. The name read–eval–print loop comes from the names of the Lisp primitive functions which implement this functionality:

The read function accepts an expression from the user, and parses it into a data structure in memory. For instance, the user may enter the s-expression (+ 1 2 3), which is parsed into a linked list containing four data elements.
The eval function takes this internal data structure and evaluates it. In Lisp, evaluating an s-expression beginning with the name of a function means calling that function on the arguments that make up the rest of the expression. So the function + is called on the arguments 1 2 3, yielding the result 6.
The print function takes the result yielded by eval, and prints it out to the user. If it is a complex expression, it may be pretty-printed to make it easier to understand. In this example, though, the number 6 does not need much formatting to print.

The development environment then returns to the read state, creating a loop, which terminates when the program is closed.

如果能有个网站可以在线执行代码：我们只要打开浏览器，进入网页，敲入要执行的代码，点击执行按钮就看到执行结果。那是一件多畅快的事情！

对于这种交互式的开发环境我们叫做 REPL（Read-Eval-Print Loop），我收集了一些 REPL Online（有些网站可能被"墙"了，你懂的），欢迎大家留言补充：

http://blog.csdn.net/redraiment/article/details/6941121

多种语言在线REPL：http://www.shucunwang.com/RunCode/r/

R语言最好的IDE——Rstudio

Where R Came From?

R was created by Ross Ihaka and Robert Gentleman at University of Auckland, New Zealand as an implementation of the S programming language. Development started in 1993. A version was made available on FTP released under the GNU GPL in 1995. The larger core group and open source project was setup in 1997.

It started as an experiment by the authors to implement a statistical test bed in Lisp using a syntax like that provided in S. As it developed, it took on more of the syntax and features of S, eventually surpassing it in capability and scope.

For an interesting and detailed treatment of the history of R, check out the technical report R: Past and Future History (PDF).

R 语言的优劣势是什么？

R语言的优势是免费开源易操作，劣势是慢、慢、慢！