循环遍历数据框列名 - R

Question

我正在尝试遍历数据框的列名，并评估每一列是哪个类。

for (i in columns(df)){
  class(df$i)
}

我已经尝试了一切，除了正确的方法..

PS：我正在尝试这样做，因为之后我必须为每个班级设置不同的条件。

Answer 1

要回答确切的问题并修复给出的代码，请参见下面的示例

df <- iris # data

for (i in colnames(df)){
   print(class(df[[i]]))
}
# [1] "numeric"
# [1] "numeric"
# [1] "numeric"
# [1] "numeric"
# [1] "factor"

1. 需要使用colnames来获取df的列名。
2. 如果您想知道每一列的类别，您可以使用df[[i]] 访问每一列。 df[i] 属于 data.frame 类。

Answer 2

问题是循环遍历数据帧的列，另外一个问题是关于循环遍历数据帧的某些子集。我使用了 mtcars 数据集，因为它的数据列比 iris 数据集多。这提供了一个更丰富的例子。要遍历某些列子集，请在 for 循环中使用数值而不是使用列的名称。如果感兴趣的列是规则间隔的，则使用感兴趣的列创建一个向量。示例如下：

#Similar to previous answer only with mtcars rather than iris data.
df2<-mtcars
for (i in colnames(df2)){print(paste(i,"  ",class(df2[[i]])))}

#An alternative that is as simple but does not also print the variable names.
df2<-mtcars
for (i in 1:ncol(df2)){print(paste(i,"  ",class(df2[[i]])))}

#With variable names:
df2<-mtcars
for (i in 1:ncol(df2)){print(paste(i,"   ",colnames(df2[i]),"  ",class(df2[[i]])))}

#Now that we are looping numerically one can start in column 3 by:
df2<-mtcars
for (i in 3:ncol(df2)){print(paste(i,"   ",colnames(df2[i]),"  ",class(df2[[i]])))}

#To stop before the last column add a break statement inside an if
df2<-mtcars
for (i in 3:ncol(df2)){
  if(i>7){break}
  print(paste(i,"   ",colnames(df2[i]),"  ",class(df2[[i]])))}

#Finally, if you know the columns and they are irregularly spaced try this:
UseCols<-c(2,4,7,9,10)
for (i in UseCols){print(paste(i,"   ",colnames(df2[i]),"  ",class(df2[[i]])))}