Johns Hopkins University series of data Science courses--r language: extracting subsets

In the R language, there are three types of operators that can extract subsets of objects:
? "[" Typically returns an object of the same type as the original object; It can also return multiple elements in an object
? [[] Extracts an object from a list or data frame (data.frame), or extracts a single element from a list or data frame, and the type of the returned object may not be a list or data frame.
? "$" can extract elements from lists and data frames by name, if only from the angle of the extracted elements, and "[[" No Difference
(1) Vector
Vectors can use "[]" to extract individual elements of the corresponding position, or multiple elements, or to set conditions in "[]" to extract the corresponding elements.

> x <-C ("a","B","C","D","E","F") > x[2]# #单个提取[1]"B"> x[2:5]# #批量提取[1]"B" "C" "D" "E"> x[x>"C"]# #条件提取[1]"D" "E" "F"> u<-x>"B"    # #逻辑判断> u[1]FALSE FALSE  TRUE  TRUE  TRUE  TRUE> X[u]# #逻辑提取[1]"C" "D" "E" "F"

(2) matrix
You can also extract the corresponding element in the matrix, which is returned by default as a cell vector, if the attribute Drop=false (the drop default is True), a 1x1 matrix is returned, and the entire row and whole column elements can be returned by missing values:

>x<- Matrix(1: 6, 2, 3) >x[2,3][1]6>x[up][1]3 # #返回单个元素 >x[1,2,drop=false]     [, 1][1,]3 # #返回1 X1 Matrix >x[1,][1]1 3 5 # #返回第一行 and quantify it >x[1,,drop=false]     [, 1] [, 2] [, 3]# #返回第一行的矩阵形式[1,]1 3 5

(3) List

> X<-list (one=1:4, two=5) > x$one[1]1 2 3 4$two [1]5> x[1] # #返回名称与值, type list$one[1]1 2 3 4> x[[1]]# #返回元素的值, type is the type of the corresponding position element, not the list type [1]1 2 3 4> x[["One"]]# #也可使用名称索引对应位置的元素 [1]1 2 3 4> x$one[1]1 2 3 4> x[[2]][[1]]# #可以返回特定位置的单个元素 [1]5> x[[C (2,1)]][1]5> X=list (data=1:5) > X[["Data"]][1]1 2 3 4 5> x[["D"]]Null> x[["D", Exact=false]]# #list默认元素名称全局匹配, but the element name can be locally matched when property exact is set to true [1]1 2 3 4 5

(4) Missing value removal

> x<-c(1,2,3,NA,5,NA)> bad<-is.na(x)> bad[1FALSEFALSEFALSE  TRUEFALSE  TRUE> x[!bad][11235

You can also use the complete.cases () function to determine the missing values.

> x <- c(12NA4NA5)> good<-complete.cases(x)> good[1]  TRUE  TRUEFALSE  TRUEFALSE  TRUE> x[good][11245

Complete.cases () can also handle matrix data

> airquality[1:6,] Ozone SOLAR.R Wind TempMonth  Day1     A      the  7.4    the     5   12     $     118  8.0    the     5   23     A     149 12.6    About     5   34     -     313 11.5    +     5   45Na Na14.3    About     5   56     -NA14.9    the     5   6> Airquality[good, [1:6,] Ozone SOLAR.R Wind TempMonth  Day1     A      the  7.4    the     5   12     $     118  8.0    the     5   23     A     149 12.6    About     5   34     -     313 11.5    +     5   47     at     299  8.6    $     5   78     +       About 13.8    -     5   8

Johns Hopkins University series of data Science courses--r language: extracting subsets