R: Dataframe colnames not being applied to dataframe-CodePudding

Long time user, first time poster. Something strange happened while I was looking at data. I made a data.frame, added columns, manipulated them, and renamed them. When I then looked at the colnames, the renaming appears to work. But when I looked at the df, only one of the column was actually renamed.

Here is a reproducible example.

var_tab <- data.frame(coef=c(1:4), p=rep(0.1, 4))
var_tab <- cbind (var_tab, c("one", "two", "three", "four"))
var_tab[4] <- "three"

> var_tab
  coef   p c("one", "two", "three", "four")    V4
1    1 0.1                              one three
2    2 0.1                              two three
3    3 0.1                            three three
4    4 0.1                             four three

> colnames(var_tab)
[1] "coef"                                    
[2] "p"                                       
[3] "c(\"one\", \"two\", \"three\", \"four\")"
[4] "V4"

All as expected...until I rename the columns. The colnames aren't all showing up correctly! The colnames change, but they only show up for the third variable, not the fourth variable.

var_tab[4] <- ifelse(var_tab[4] == var_tab[3], 1, 0)
colnames(var_tab)[3:4] <- c("model", "base")

> var_tab
  coef   p model V4
1    1 0.1   one  0
2    2 0.1   two  0
3    3 0.1 three  1
4    4 0.1  four  0

> colnames(var_tab)
[1] "coef"  "p"     "model" "base"

The problem can be solved by renaming column 4 before recalculating it, so the problem is easily avoidable.

colnames(var_tab)[3:4] <- c("model", "base")
var_tab[4] <- ifelse(var_tab[4] == var_tab[3], 1, 0)

> colnames(var_tab)
[1] "coef"  "p"     "model" "base" 

> var_tab
  coef   p model base
1    1 0.1   one    0
2    2 0.1   two    0
3    3 0.1 three    1
4    4 0.1  four    0

Though I can avoid the problem, I still cannot understand what solved the problem. And I cannot find any other reference to this issue. It vaguely reminds me of the R floating number problem. Does anyone here know what caused my colnames to not be applied to the dataframe?

Thanks in advance for your help!

CodePudding user response：

After your setup

var_tab <- data.frame(coef=c(1:4), p=rep(0.1, 4))
var_tab <- cbind (var_tab, c("one", "two", "three", "four"))
var_tab[4] <- "three"

note the difference between

str(var_tab[4] == var_tab[3])
str(var_tab[[4]] == var_tab[[3]])

The first one returns a data.frame. When you assign a data.frame with a single column to a column of a data frame, then things get weird. The outer data frame has a name for the column that contains a data frame. That inner data frame has its own name for its column.

If you assign a clean vector to a column of a data.frame then you don't have this problem.

So you should use

var_tab[4] <- ifelse(var_tab[[4]] == var_tab[[3]], 1, 0)
colnames(var_tab)[3:4] <- c("model", "base")