在小标题中添加两列并将总和保存到第三列使第三列成为 dataframe

Question

I am working on generating a report, upon trying to write the tibble using xlsx package's write.xlsx, it gave an error (even after me specifying as.data.frame(tibble) in write.xlsx). Upon checking the tibble, I realized that when I added multiple columns and stored the result in another column in the tibble, the total column has become a dataframe.

Example:

> marks <- tibble(math = c(90,90,85,90),
+                 physics = c(90,85,95,80),
+                 Total = c(rep(NA,4)))
> marks
# A tibble: 4 x 3
   math physics Total
  <dbl>   <dbl> <lgl>
1    90      90 NA   
2    90      85 NA   
3    85      95 NA   
4    90      80 NA   
> class(marks)
[1] "tbl_df"     "tbl"        "data.frame"
> str(marks)
Classes ‘tbl_df’, ‘tbl’ and 'data.frame':   4 obs. of  3 variables:
 $ math   : num  90 90 85 90
 $ physics: num  90 85 95 80
 $ Total  : logi  NA NA NA NA
> marks$Total <- marks[,1] + marks[,2]
> str(marks)
Classes ‘tbl_df’, ‘tbl’ and 'data.frame':   4 obs. of  3 variables:
 $ math   : num  90 90 85 90
 $ physics: num  90 85 95 80
 $ Total  :'data.frame':    4 obs. of  1 variable:
  ..$ math: num  180 175 180 170
> 

As we can see above, I thought I can use vectorized operations of R but the "Total" column has changed to dataframe after summing up two columns and storing the result in Total column.

Could someone let me know why this is happening, also, how to perform the above operation.

Edited: OK seems like because tibble doesn't drop dimension, it was not like adding two vectors.

Answer 1

I think this is a result of the fact that by defaul tibbles don't drop the 2nd dimension when you access part of them with [] , whereas dataframes do. Compare:

> marks[, 1]
# A tibble: 4 x 1
   math
  <dbl>
1    90
2    90
3    85
4    90
> marks_df = as.data.frame(marks)
> marks_df[ , 1]
[1] 90 90 85 90

So marks[, 1] + marks[, 2] is adding a tibble to a tibble and the result is a tibble.

To avoid this, you can either drop the 2nd dimension explicitly, or just use the column names:

marks$Total <- marks[,1, drop = TRUE] + marks[, 2, drop = TRUE]
marks$Total <- marks$math + marks$physics