swirl 6: Subsetting Vectors

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> x[x>0]
[1] NA 1.212753086 1.226772040 1.859602576 0.763847244 NA 1.791336311 NA NA
[10] NA NA NA NA NA 0.439779205 2.324976502 NA NA
[19] NA NA NA 0.009213532 NA 0.141221402 NA 0.524453811 NA
[28] NA NA 0.922737419 NA

Since NA is not a value, but rather a placeholder for an unknown quantity, the expression NA > 0 evaluates to NA. Hence we get a bunch of NAs mixed in with our positive numbers when we do this.

> x[-c(2,10)]
[1] NA 1.212753086 1.226772040 -0.332124446 1.859602576 0.763847244 NA 1.791336311
[9] NA NA NA NA -0.262890059 NA NA -0.539133551
[17] NA -0.940835275 -0.396005495 -0.556808402 0.439779205 2.324976502 NA NA
[25] NA NA NA 0.009213532 NA 0.141221402 NA 0.524453811
[33] -0.207030473 NA NA NA 0.922737419 NA

“-”标示去除

> names(vect)
[1] "foo" "bar" "norf"

names()

> identical(names(vect),names(vect2))
[1] TRUE
identical() The identical() function tells us if its first two arguments are, well, identical.

> vect["bar"]
bar
2

"name of vector"向量中元素名，（加引号）可以直接返回元素名元素值

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