6.4 数据转换

transform 对数据框中的某些列做计算，取对数，将计算的结果单存一列加到数据框中

transform(iris[1:6, ], scale.sl = (max(Sepal.Length) - Sepal.Length) / (max(Sepal.Length) - min(Sepal.Length)))

##   Sepal.Length Sepal.Width Petal.Length Petal.Width Species scale.sl
## 1          5.1         3.5          1.4         0.2  setosa    0.375
## 2          4.9         3.0          1.4         0.2  setosa    0.625
## 3          4.7         3.2          1.3         0.2  setosa    0.875
## 4          4.6         3.1          1.5         0.2  setosa    1.000
## 5          5.0         3.6          1.4         0.2  setosa    0.500
## 6          5.4         3.9          1.7         0.4  setosa    0.000

验证一下 scale.sl 变量的第一个值

(max(iris$Sepal.Length) - 5.1) / (max(iris$Sepal.Length) - min(iris$Sepal.Length))

## [1] 0.7777778

Warning: This is a convenience function intended for use interactively. For programming it is better to use the standard subsetting arithmetic functions, and in particular the non-standard evaluation of argument transform can have unanticipated consequences.