更新R data.frame中的单个值（不是行） [英] Updating individual values (not rows) in an R data.frame

问题描述

我想根据一个简单的标准更新R data.frame mydata 中的 var3 值。

I would like to update values of var3 in an R data.frame mydata according to a simple criterion.

   var1  var2  var3
1  1     4     5
2  3     58    800
3  8     232   8 

我认为应该执行以下操作：

I would think that the following should do:

mydata$var3[mydata$var3 > 500,] <- NA

但是，这会将每个匹配记录的整行替换为NA（行中的所有单元格），而不仅仅是 var3 值（单元格）：

However, this replaces the entire row of every matching record with NA (all cells of the row), instead of just the var3 value (cell):

   var1  var2  var3
1  1     4     5
2  NA    NA    NA
3  8     232   8 

如何确保仅替换所选变量的值？ mydata 应该看起来像这样

How can I ensure that just the value for the selected variable is replaced? mydata should then look like

   var1  var2  var3
1  1     4     5
2  3     58    NA
3  8     232   8 

推荐答案

使用 which 和 arr.ind = TRUE

> mydata[which(mydata[,3]>500, arr.ind=TRUE), 3] <- NA
> mydata
  var1 var2 var3
1    1    4    5
2    3   58   NA
3    8  232    8

或者只是修改您以前的尝试...

Or just modify your previous attempt...

mydata[mydata$var3 > 500, 3] <- NA 

这也有效

mydata$var3[mydata$var3 > 500 ] <- NA   # note no comma is inside [ ]

中没有逗号，因为 mydata $ var3 给出一个向量，您正在使用 [mydata $ var3> 500，] ，因此会引发尺寸错误。差不多了，您所要做的就是删除代码中的逗号（请参阅我的最后一个选择）。

Your attempt didnt work because mydata$var3 gives a vector and you are indexing it as if it were a matrix by using [mydata$var3 > 500,] so a dimension error is thrown. You almost got it, all you have to do is remove the comma in your code (see my last alternative).

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