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I wish to calculate how each set of data in a tibble is different to a baseline dataset.

to plan, I wrote this R code to subtract a tibble from another of the same size:

# this works
tbl_a <- tibble(a1 = 1, a2 = 2, a3 = 3)
tbl_b <- tibble(a1 = 4, a2 = 5, a3 = 6)
tbl_a + tbl_b

# > tbl_a + tbl_b
#   a1 a2 a3
# 1  5  7  9

Now I'm turning it into a set of tibbles

# compare multiple datasets to baseline of same shape
tbl_a1 <- tibble(id = "i", a1 = 1, a2 = 2, a3 = 3)
tbl_a2 <- tibble(id = "ii", a1 = 2, a2 = 3, a3 = 4)
tbl_a3 <- tibble(id = "iii", a1 = 3, a2 = 4, a3 = 5)
tbl_base <- tibble(id_base = "baseline", a1 = 4, a2 = 5, a3 = 6)
tbls <- bind_rows(tbl_a1, tbl_a2, tbl_a3)

tbls_compare <- tbls %>%
  nest(set = starts_with("a")) %>%
  bind_cols(tbl_base) %>%
  nest(set_baseline = starts_with("a"))
# id    set              id_base  set_baseline    
# <chr> <list>           <chr>    <list>          
#   1 i     <tibble [1 × 3]> baseline <tibble [1 × 3]>
#   2 ii    <tibble [1 × 3]> baseline <tibble [1 × 3]>
#   3 iii   <tibble [1 × 3]> baseline <tibble [1 × 3]>

I expected to be able to perform subtraction like I could in tbl_a + tbl_b example.

However, I'm greeted with an error:

> tbls_compare %>%
+   mutate(diff_to_base = set_baseline - set)
Error in `mutate()`:
! Problem while computing `diff_to_base = set_baseline - set`.
Caused by error in `set_baseline - set`:
! non-numeric argument to binary operator
Run `rlang::last_error()` to see where the error occurred.

I tried using purrr:map but could not by myself work out a solution.

Could somebody please enlighten me?

taiyodayo
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2 Answers2

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I think you need to add rowwise, otherwise set_baseline - set will attempt to subtract a list of tibbles from a list of tibbles.

tbls_compare <- tbls %>%
  nest(set = starts_with("a")) %>%
  bind_cols(tbl_base) |>
  nest(set_baseline = starts_with("a")) |>
  rowwise() |>
  mutate(diff_to_base = list(as_tibble(set_baseline - set)))

tbls_compare

# A tibble: 3 × 5
# Rowwise: 
  id    set              id_base  set_baseline     diff_to_base    
  <chr> <list>           <chr>    <list>           <list>          
1 i     <tibble [1 × 3]> baseline <tibble [1 × 3]> <tibble [1 × 3]>
2 ii    <tibble [1 × 3]> baseline <tibble [1 × 3]> <tibble [1 × 3]>
3 iii   <tibble [1 × 3]> baseline <tibble [1 × 3]> <tibble [1 × 3]>

This is how it looks when unnested:

tbls_compare |>
  unnest(cols = diff_to_base)

+ # A tibble: 3 × 7
  id    set              id_base  set_baseline        a1    a2    a3
  <chr> <list>           <chr>    <list>           <dbl> <dbl> <dbl>
1 i     <tibble [1 × 3]> baseline <tibble [1 × 3]>     3     3     3
2 ii    <tibble [1 × 3]> baseline <tibble [1 × 3]>     2     2     2
3 iii   <tibble [1 × 3]> baseline <tibble [1 × 3]>     1     1     1
Stefano Barbi
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  • Thank you!!! This is the perfect solution with updated dplyr 1.0.0 syntax. – taiyodayo Feb 25 '22 at 09:59
  • `rowwise()` was key, and code works with just that inserted in the pipeline followed by `unnest()`. But `list(as_tibble(set_baseline - set))` is a great touch in keeping the intermediate object clean and in the same form as the original `set` column (list of tibbles). I learned so much from this. Please have a great day! – taiyodayo Feb 25 '22 at 10:02
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    @taiyodayo Yes, you need to wrap `set_baseline - set` into a `list` otherwise `mutate` will bind whatever columns are in there. Have a great day too ! – Stefano Barbi Feb 25 '22 at 10:05
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There might be no need for such complex data processing, for example you can just do matrix subtraction:

a_matrix <- matrix(c(1, 2, 3, 2, 3, 4, 3, 4, 5), nrow = 3, byrow = TRUE)
a <- c(4, 5, 6)

t(a - a_matrix)

     [,1] [,2] [,3]
[1,]    3    3    3
[2,]    2    2    2
[3,]    1    1    1
det
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  • Thank you for the suggestion! But the original data is much larger with many more cols. Keeping it in dplyr syntax really helps myself and the team in communicating ideas. – taiyodayo Feb 25 '22 at 10:04