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An Introduction to

An Introduction to. Web resources. R home page: http://www.r-project.org/ R Archive: http://cran.r-project.org/ R FAQ ( frequently asked questions about R ): http://cran.r-project.org/doc/FAQ/R-FAQ.html R manuals: http://cran.r-project.org/manuals.html. The R environment.

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An Introduction to

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  1. An Introduction to

  2. Web resources • R home page: http://www.r-project.org/ • R Archive: http://cran.r-project.org/ • R FAQ (frequently asked questions about R): http://cran.r-project.org/doc/FAQ/R-FAQ.html • R manuals: http://cran.r-project.org/manuals.html

  3. The R environment • R command window (console) or Graphical User Interface (GUI) • Used for entering commands, data manipulations, analyses, graphing • Output: results of analyses, queries, etc. are written here • Toggle through previous commands by using the up and down arrow keys

  4. The R environment • The R workspace • Current working environment • Comprised primarily of variables, datasets, functions

  5. The R environment • R scripts • A text file containing commands that you would enter on the command line of R • To place a comment in a R script, use a hash mark (#) at the beginning of the line

  6. Some tips for getting started in R • Create a new folder on your hard drive for your current R session • Open R and set the working directory to that folder • Save the workspace with a descriptive name and date • Open a new script and save the script with a descriptive name and date

  7. Executing simple commands • The assignment operator <- • x <- 5assigns the value of 5 to the variable x • y <- 2*xassigns the value of 2 times x (10 in this case) to the variable y • r <- 4 • area.circle <- pi*r^2 • NOTE: R is case-sensitive (y ≠ Y)

  8. R object types • Vector • Matrix • Array • Data frame • Function • List

  9. Vectors and arrays • Vector: a one-dimensional array, all elements of a vector must be of the same type (numerical, character, etc) • Matrix: a two-dimensional array with rows and columns • Array: as a matrix, but of arbitrary dimension

  10. Entering data into R • Vectors: • The “c” command • combine or concatenate data • Data can be character or numeric v1 <- c(12, 5, 6, 8, 24) v2 <- c("Yellow perch", "Largemouth bass", "Rainbow trout", "Lake whitefish“)

  11. Entering data into R • Vectors: • Sequences of numbers c() seq() years<-c(1990:2007) x<-seq(0,100,10) x<-seq(0, 200, length=100)

  12. Entering data into R • Arrays: array() matrix() m1<-array(1:20, dim=c(4,5)) m2<-matrix(1:20, ncol=5, nrow=4)

  13. Entering data into R • Arrays: • Combine vectors as columns or rows cbind() rbind() Matrix1 <- cbind(v1, v2) Matrix2 <- rbind(v1, v2)

  14. Data frames • A data frame is a list of variables of the same length with unique row names • A collection of variables which share many of the properties of matrices and of lists • Used as the fundamental data structure by most of R's modeling software

  15. Data frames • Convert vectors or matrices into a data frame data.frame() df1<-data.frame(v1, v2) df2<-data.frame(matrix1)

  16. Data frames • Editing data frames in spreadsheet-like view edit() df2<-edit(df1)

  17. Let’s go to R and enter some vectors, arrays, and data frames Script : QFC R short course R object types_1_vectors and arrays.R

  18. Placing variables in the R search path • When variables in a data frame are used in R, the data frame name followed by a $ sign and then the variable name is required query1<-df1$v3 > 20

  19. Placing variables in the R search path • Alternatively, the attach() function can be used attach(df1) query1 <- v3 > 20 detach(df1)

  20. Accessing data from an array, vector, or data frame • Subscripts are used to extract data from objects in R • Subscripts appear in square brackets and reference rows and columns, respectively

  21. Subscripts df1 df[3,5] df[,3] df[5,] df[2:5,]

  22. Queries in R: Common logical arguments

  23. Queries in R • The use of logical tests query1<-df1$v3 > 20 df1[query1,] query2<-df1$v3 > 20 & df1$v4 < 30 (&=and) query2<-df1$v3 > 20 | df1$v4 < 30 (| = or)

  24. Queries in R Script : QFC R short course R object types_2_query arrays data frames.R

  25. Exercise 1

  26. Importing data from Excel(or other database management programs) • Export as text file (.txt) • Tips • Avoid spaces in variable and character names, use a period (e.g., fish.weight, not fish weight and Round.lake not Round lake) • Replace missing data with “NA” • See Excel example (MI STORET data RAW.xls)

  27. Importing data from Excel • read.table() data.frame.name <- read.table(“file path”, na.strings=”NA”, header=TRUE) df1<-read.table("C:\\R\\Example\\datafile1.txt", na.strings="NA", header=TRUE) Note the use of \\ instead of \ in path name

  28. Importing data from Excel • If your working directory is set, R will automatically look for the data text file there. • So, the read.table syntax can be simplified by excluding the file path name: • read.table(“data.txt”, na.strings=“NA”, header=T)

  29. Exporting data from R write.table() write.table(df, file = "Path Name\\file_name.csv", sep = ",", col.names = NA)

  30. Introduction to R functions • R has many built-in functions and many more that can be downloaded from CRAN sites (Comprehensive R Archive Network) • User-defined functions can also be created

  31. The R base package

  32. Introduction to R functions • Common functions names(): obtain variable names of a df summary(): summary of all variables in a df mean(): Mean var(): Variance sd(): standard deviation Script: QFC R short course R functions_1.R

  33. Introduction to R functions, cont head(): print first few rows of data frame sapply() and tapply(): column-wise summaries levels(): obtain levels of a character variable by(): produce summaries by group

  34. Introduction to R functions, cont tapply(variable, list(group1, group2), mean) Applies function to each element in ragged arrays sapply(variable, FUN=) Applies a function to elements in a list by(data, INDICES, FUN)

  35. Introduction to R loops Basin syntax: for (i in 1:n){ some code } *Excel example Script:QFC R short course R functions_2.R

  36. User-defined functions Function name <- function(x){ argument } Script: QFC R short course R user defined functions_3.R

  37. Exercise 2 (Part 1)

  38. R functions part 2: subset data • subset() function sub<- subset(data frame, criteria) sub1<-subset(fish, no.fish > 50) sub2<-subset(fish, no.fish>50 & position=="Below")

  39. R functions part 2: subset data • Select specific columns sub3<-subset(fish, select=c(stream, site, no.fish)) Script: QFC R short course R subset_4.R

  40. Exercise 2 (Part 2)

  41. Introduction to basic graphing http://addictedtor.free.fr/graphiques/

  42. Graphing basics Plotting commands • High-level functions: Create a new plot on the graphics device • Low-level functions: Add more information to an already existing plot, such as extra points, lines, and labels • Interactive graphing functions: Allow you to interactively add information to a graph

  43. Common high-level functions • plot(): A generic function that produces a type of plot that is dependent on the type of the first arguement • hist(): Creates a histogram of frequencies • barplot(): Creates a histogram of values • boxplot(): Creates a boxplot • pairs(): Creates a scatter plot matrix

  44. Common high-level functions plot() plot(x) plot(x,y) : scatter plot plot(y~x) : scatter plot plot(group, x) : box plot

  45. Common high-level functions hist(x) boxplot(x~group) pairs(z) pairs(df1[,3:7])

  46. Common high-level functions Script: QFC R course Graphing Basics_1.R

  47. Exercise 3

  48. END OF DAY 1

  49. Lower-level graphing functions

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