How to index a purrr map call within a for loop

March 1, 2023

I am working with list columns. One column is the the data of interest, and another is a list of row indices for the the data. I would like to dynamically add and title new columns to the List column. Each new column should be a subset of the data sliced by the indices stored in the other column. I am able to create one new list column, but if I try and index a mapping variable it does not work. The example below reproduces what I have done so far.

library(tidyverse)

# This sample mimics the structure of my data

LC <- mtcars %>% 
  as_tibble() %>% 
  group_by(gear) %>% 
  nest() %>% 
  rowwise() %>% 
  mutate(inds = list(as.integer(ceiling(runif(n = gear))))) %>% 
  group_by(gear) %>% 
  print()

And the following map function works if I want to mutate 1 new column

LC.slice <- LC %>%
  mutate(slices = map2(.x = data,
                       .y = inds,
                       ~ .x %>% 
                         slice((.y[[1]] - 1): (.y[[1]] + 1)))) %>% 
  print()

But my goal is to dynamically create and name many columns. So I tried a function like this:

LC.slicer <- function(df, var1, var2){
  for (i in 1:3){
    df <- df %>% 
      mutate("slice_{i}" := map2(.x = {{ var1 }},
                                 .y = {{ var2 }},
                                 ~ .x %>% 
                                   slice((.y[[i]] - 1): (.y[[i]] + 1))))
  }
}

# and then call the function
LC.sliced <- LC.slicer(LC, data, inds) %>% print()

but this function returns NULL.
How can I successfully index to make a function?

>Solution :

Just return the object after the for loop

LC.slicer <- function(df, var1, var2){
  for (i in 1:3){
    df <- df %>% 
      mutate("slice_{i}" := map2(.x = {{ var1 }},
                                 .y = {{ var2 }},
                                 ~ .x %>% 
                                   slice((.y[[i]] - 1): (.y[[i]] + 1))))
  }
df
}

-testing

> LC.slicer(LC, data, inds) 
# A tibble: 3 × 6
# Groups:   gear [3]
   gear data               inds      slice_1           slice_2           slice_3          
  <dbl> <list>             <list>    <list>            <list>            <list>           
1     4 <tibble [12 × 10]> <int [4]> <tibble [2 × 10]> <tibble [2 × 10]> <tibble [2 × 10]>
2     3 <tibble [15 × 10]> <int [3]> <tibble [2 × 10]> <tibble [2 × 10]> <tibble [2 × 10]>
3     5 <tibble [5 × 10]>  <int [5]> <tibble [2 × 10]> <tibble [2 × 10]> <tibble [2 × 10]>