1. input/output
   1. part1
      1. sink()
      2. cat()
      3. scan()
         1. default separator: white space
      4. print()
   2. part2
      1. readClipboard()
      2. read.table()/write.table()
      3. read.csv()/write.csv()
   3. part3
      1. xlsx packages
         1. Adrian Dragulescu
         2. read.xlsx()/write.xlsx()
      2. XLConnect
         1. Martin Studer
         2. readWorksheetFromFile()
         3. writeWorksheetToFile()
      3. readxl
         1. Hadley Wickham
         2. read_excel()
         3. excel_sheets()
   4. part4
      1. file_type
         1. SAS
         2. SPSS
         3. Stata
         4. Matlab
         5. MySQL
      2. package
         1. foreign
         2. foreign
         3. foreign
         4. R.matlab
         5. RMySQL
      3. file
         1. read.ssd()/write.foreign()
         2. read.spss()/write.foreign()
         3. read.dta()/write.foreign()
         4. readMat()/writeMat()
         5. dbConnect()/dbReadTable()/dbWiteTable()
   5. ref.
      1. R Data Import/Export
      2. This R Data Import Tutorial Is Everything You Need
2. check data
   1. str()/class()/head()/tail()/identical()/all.equal()
   2. special values
      1. missing value
         1. NA
            1. is.na()/complete.cases()
            2. na.fail()/na.omit()/na.exclude()/na.pass()
            3. na.rm=TRUE
      2. Inf, -Inf(positive and negative infinity)
      3. NaN(Not a Number)
   3. graphic
      1. boxplot
         1. outlier
      2. histogrm
         1. distribution
      3. etc.
3. sort
   1. sort()
   2. rank()
      1. ties.method
   3. order()
      1. how to break ties?
4. basic manipulate
   1. data type
      1. vector
      2. matrix
      3. array
      4. factor
      5. data.frame
      6. list
      7. 判別/轉換
         1. is.datatype()
         2. as.datatype()
   2. what
      1. (ref. check data)
   3. how
      1. create
      2. add
         1. rbind()/cbind()
      3. remove
      4. factor level
   4. functions
      1. all()/any()
      2. duplicated()/unique()
      3. set operations
         1. union()
         2. intersect()
         3. setdiff()
         4. setequal()
         5. is.element()
      4. etc.
   5. Operators
      1. Arithmetic
         1. ?'*'
      2. Logical
         1. ?'&'
      3. Relational
         1. ?'>'
      4. Assignment
         1. ?'='
5. select
   1. indices
      1. []
      2. [[]]
      3. $
   2. functions
      1. which
      2. subset
      3. match, %in%
      4. merge
      5. cut
      6. sample
      7. etc.
6. apply family
   1. apply
   2. lapply
      1. sapply
      2. vapply
   3. mapply
   4. rapply
   5. tapply
   6. etc.