R programming I expressions, variables, and data types. Luigi Cerulo University of Sannio

Transcription

1 R programming I expressions, variables, and data types Luigi Cerulo University of Sannio

2 R scripts An R program is known also as an R script A script is a piece of code that you can run and that will hopefully produce the desired output (some data analysis, produce some graphs) To begin we will concentrate on writing R code one line at a time and understanding what each command means When writing longer code it is very useful if the code is well written and formatted, both for you to understand when you come back to it after a period of time, and for anyone else that might be taking a look

3 Getting familiar with the R console R is a command line environment, this means that you type in an instruction and R interprets this and either stores the result or writes it back on the screen for you You can run R in a very simple command shell window, but for all practical purposes it s much easier to use a dedicated piece of software which makes it easy to work within the R environment (eg. Rstudio)

4 Rstudio

5 Math expressions is a finite combination of symbols that is well-formed according to rules that depend on the context

6 Math expressions is a finite combination of symbols that is well-formed according to rules that depend on the context

7 Math expressions in R only parentheses (not square or curly braces) In R the objective is to find the value of the expression

8 Try it in R

9 Saving results (variables) > 27 / 5 [1] 5.4 These calculations have just produced output in the console - no values have been saved. To save a value, it can be assigned to a variable name. > x < The symbol <- is generally used as the assignment operator. For now we'll use x, y and z as names of data structures, though more informative names can be used.

10 Assignment instruction A variable is a memory location where a data is stored. The data inside a variable can be changed with the assignment operator Assignment variable name = data <- Alternative symbol

11 Data types In R we can have different data types numeric alphanumeric (characters or string) Hello, how are you? Hello world and 300 are not the same data logic TRUE FALSE

12 Assign a data to a variable Numbers A = 3 B = Strings (must be delimited by quotes) title = Il nome della rosa author = Umberto Eco Logic find = TRUE exists = FALSE C = TRUE

13 Reuse previously stored variables Now that values have been assigned to x and y they can be used in calculations. > x < > y <- 3 > x + y [1] 14 > x * y [1] 33 > z <- x * y > z [1] 33

14 Showing what is stored The Environment or Workspace show the current data stored in memory through variables

15 Reassign a variable R is case sensitive so x and X are not the same. If you try to print the value of X out into the console an error will be returned as X has not been used so far in this session. > X Error: object 'X' not found If you use the same data structure name as one that you have previously used then R will overwrite the previous information with the new information. > y <- 3 > y [1] 3 > y <- 12 > y [1] 12

16 Naming variables Variables can be named anything you like (within reason), though they cannot begin with a number or symbol. Having informative, descriptive names is useful and this often involves using more than one word. Providing there are no spaces between your words you can join them in various ways, using dots, underscores and capital letters. > mouse.age <- 112 > mouse.weight <- 25 > tail.length <- 54 Although you may not wish to use such convoluted names, the following are perfectly valid: > tail.length.mouse.1 <- 41 > tail.length.mouse2.condition4.ko <- 35

17 What is the effect of such instructions? X = 5 Y = X+10 What is stored in Y at this point of execution?

18 What is the effect of such instructions? X = 5 Y = X+10 X = 3 And at this point, what is stored in Y?

19 Exercise Try to write a sequence of assignment instruction in order to switch the content of two variables Memory (before) A 10 B 20 Memory (after) A 20 B 10 Memory (before) A Mario B Maria Memory (after) A Maria B Mario