#subsetting

2nd of 4 videos on basic data management in R. This video focuses on exploring datasets in R and basic data manipulation such … source

OOB stands for index out of bounds

[[]] and [] differ in their behavior when index is OOB

if a data frame (z) is entered: z<-data.frame(x=1:5,y=6:10)

if an atomic vector (a) is entered: a<-1:15

check the length of your object: (i.e. the # of indices)

> length(z) [1] 2

>length(z$x)

[1] 5

>length(z$y)

[1] 5

>length(c(z$x,z$y))

[1] 10

> length(a) [1] 15

now try to extract an object outside of these bounds using [[]] and []

> z[3] Error in `[.data.frame`(z, 3) : undefined columns selected

> z[[3]] Error in .subset2(x, i, exact = exact) : subscript out of bounds

> a[16] [1] NA

> DATA FRAMES

(if you only have 1 var in the data frame, treated like a list--with 2 or more vectors, df behaves like matrix)

Taken directly from site:  How many of your friends complain regularly about their bodies? ~None: 147 ~A few: 411 ~Some: 188 ~Most: 141 ~All: 53

construct an object that reflects it:

> body_complain<-data.frame(x=c('None','A few','Some','Most','All'),y=c(147,411,188,141,53))

object $ title of column

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>DATA FRAMES>STRUCTURAL QUALITIES

the structure of data frame subsets are unique 

examine the differences using this variable (a)

> a  x y z 1 1 1 a 2 2 2 b 3 3 3 c 4 4 4 d

> str(a) 'data.frame':   4 obs. of  3 variables: $ x: int  1 2 3 4 $ y: int  1 2 3 4 $ z: Factor w/ 4 levels "a","b","c","d": 1 2 3 4

Now examine the structural differences which subsetting initiates in data frames

> str(a[c(1)]) 'data.frame':   4 obs. of  1 variable: $ x: int  1 2 3 4

> str(a[,c(1,3)]) 'data.frame':   4 obs. of  2 variables: $ x: int  1 2 3 4 $ z: Factor w/ 4 levels "a","b","c","d": 1 2 3 4

> str(a[c(3),]) 'data.frame':   1 obs. of  3 variables: $ x: int 3 $ y: int 3 $ z: Factor w/ 4 levels "a","b","c","d": 3

To access a subscript of a data frame use $ and []

>a$y[3]

[1] 3

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>DATA FRAME > REASSIGNMENT

brackets [] are known as subsetting 

This is an exciting concept that will require an understanding of:

3 subsetting operators

the 6 types of subsetting

behavioral differences among different objects

subsetting with assignment