數據框的下標與子集的提取

數據框的下標與子集的提取與矩陣基本相同. 不同的是: 對于列我們可以使用變量的名稱, 仍以數據集Puromycin進行舉例說明.

?1、提取單個元素

> Puromycin[1, 1]

[1] 0.02

2、提取一個子集，例如第1, 3, 5行，第1, 3列

> Puromycin[c(1, 3, 5), c(1, 3)]

conc state

1 0.02 treated

3 0.06 treated

5 0.11 treated

> Puromycin[c(1, 3, 5), ]

conc rate state

1 0.02 76 treated

3 0.06 97 treated

5 0.11 123 treated

常使用變量名稱來指定列的位置，上面的命令等價于

> Puromycin[c(1, 3, 5), c("conc", "state")]]

3、提取一列(變量的值). 一個數據框的變量對應了數據框的一列, 如果變量有名稱, 則可直接使用“數據框名$變量名”這種格式指向對應的列. 例如

> Puromycin$conc # 等價于 Puromycin[,1]

[1] 0.02 0.02 0.06 0.06 0.11 0.11 0.22 0.22 0.56 0.56

[11] 1.10 1.10 0.02 0.02 0.06 0.06 0.11 0.11 0.22 0.22

[21] 0.56 0.56 1.10

> Puromycin$state

[1] treated treated treated treated treated

[6] treated treated treated treated treated

[11] treated treated untreated untreated untreated

[16] untreated untreated untreated untreated untreated

[21] untreated untreated untreated

Levels: treated untreated

4、提取滿足條件的子集

> subset(Puromycin, state == "treated" & rate > 160)

conc rate state

9 0.56 191 treated

10 0.56 201 treated

11 1.10 207 treated

12 1.10 200 treated

> subset(Puromycin, conc > mean(conc))

conc rate state

9 0.56 191 treated

10 0.56 201 treated

11 1.10 207 treated

12 1.10 200 treated

21 0.56 144 untreated

22 0.56 158 untreated

23 1.10 160 untreated

下期請期待在數據框中添加新變量