Store Data About Rows

2017-11-20

rstats keyholder

Introduction to keyholder package. Tools for keeping track of information about rows.

Prologue

During development of my other R package (ruler), I encountered the following problem: how to track rows of data frame after application of some user defined function? It is assumed that this function takes data frame as input, subsets it (with possible creation of new columns, but not rows) and returns the result. The typical example using dplyr and magrittr’s pipe:

suppressMessages(library(dplyr))

# Custom `mtcars` for more clear explanation
mtcars_tbl <- mtcars %>%
  select(mpg, vs, am) %>%
  as_tibble()

# A handy way of creating function with one argument
modify <- . %>%
  mutate(vs_am = vs * am) %>%
  filter(vs_am == 1) %>%
  arrange(desc(mpg))

# The question is: which rows of `mtcars_tbl` are returned?
mtcars_tbl %>% modify()
## # A tibble: 7 x 4
##     mpg    vs    am vs_am
##   <dbl> <dbl> <dbl> <dbl>
## 1  33.9     1     1     1
## 2  32.4     1     1     1
## 3  30.4     1     1     1
## 4  30.4     1     1     1
## 5  27.3     1     1     1
## # ... with 2 more rows

To solve this problem I ended up creating package keyholder, which became my first CRAN release. You can install its stable version with :

install.packages("keyholder")

This post describes basis of design and main use cases of keyholder. For more information see its vignette Introduction to keyholder.

Overview

suppressMessages(library(keyholder))

The main idea of package is to create S3 class keyed_df, which indicates that original data frame (or tibble) should have attribute keys. “Key” is any vector (even list) of the same length as number of rows in data frame. Keys are stored as tibble in attribute keys and so one data frame can have multiple keys. In other words, keys can be considered as columns of data frame which are hidden from subsetting functions but are updated according to them.

To achieve that, those functions should be generic and have method for keyed_df implemented. Look here for the list of functions supported by keyholder. As for version 0.1.1 they are all one- and two-table dplyr verbs for local data frames and [ function.

Create and manipulate keys

There are two distinct ways of creating keys: by assigning and by using existing columns:

# By assigning
mtcars_tbl_1 <- mtcars_tbl
keys(mtcars_tbl_1) <- tibble(rev_id = nrow(mtcars_tbl_1):1)
mtcars_tbl_1
## # A keyed object. Keys: rev_id 
## # A tibble: 32 x 3
##     mpg    vs    am
## * <dbl> <dbl> <dbl>
## 1  21.0     0     1
## 2  21.0     0     1
## 3  22.8     1     1
## 4  21.4     1     0
## 5  18.7     0     0
## # ... with 27 more rows

# By using existing columns
mtcars_keyed <- mtcars_tbl %>% key_by(vs)
mtcars_keyed
## # A keyed object. Keys: vs 
## # A tibble: 32 x 3
##     mpg    vs    am
## * <dbl> <dbl> <dbl>
## 1  21.0     0     1
## 2  21.0     0     1
## 3  22.8     1     1
## 4  21.4     1     0
## 5  18.7     0     0
## # ... with 27 more rows

To get keys use keys() (which always returns tibble) or pull_key() (similar to dplyr::pull() but for keys):

mtcars_keyed %>% keys()
## # A tibble: 32 x 1
##      vs
## * <dbl>
## 1     0
## 2     0
## 3     1
## 4     1
## 5     0
## # ... with 27 more rows

mtcars_keyed %>% pull_key(vs)
##  [1] 0 0 1 1 0 1 0 1 1 1 1 0 0 0 0 0 0 1 1 1 1 0 0 0 0 1 0 1 0 0 0 1

To restore keys (create respective columns in data frame) use restore_keys():

# Column `vs` didn't change in output because it was restored from keys
mtcars_keyed %>%
  mutate(vs = 2) %>%
  restore_keys(vs)
## # A keyed object. Keys: vs 
## # A tibble: 32 x 3
##     mpg    vs    am
##   <dbl> <dbl> <dbl>
## 1  21.0     0     1
## 2  21.0     0     1
## 3  22.8     1     1
## 4  21.4     1     0
## 5  18.7     0     0
## # ... with 27 more rows

To end having keys use unkey():

mtcars_keyed %>% unkey()
## # A tibble: 32 x 3
##     mpg    vs    am
## * <dbl> <dbl> <dbl>
## 1  21.0     0     1
## 2  21.0     0     1
## 3  22.8     1     1
## 4  21.4     1     0
## 5  18.7     0     0
## # ... with 27 more rows

Use cases

Track rows

To track rows after application of user defined function one can create key with row number as values. keyholder has a wrapper use_id() for this:

# `use_id()` removes all existing keys and creates key ".id"
mtcars_track <- mtcars_tbl %>%
  use_id()

mtcars_track %>% pull_key(.id)
##  [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
## [24] 24 25 26 27 28 29 30 31 32

Now rows are tracked:

mtcars_track %>%
  modify() %>%
  pull_key(.id)
## [1] 20 18 19 28 26  3 32

# Make sure of correct result
mtcars_tbl %>%
  mutate(id = seq_len(n())) %>%
  modify() %>%
  pull(id)
## [1] 20 18 19 28 26  3 32

The reason for using “key id” instead of “column id” is that modify() hypothetically can perform differently depending on columns of its input. For example, it can use dplyr’s scoped variants of verbs or simply check input’s column structure.

Restore information

During development of tools for data analysis one can have a need to ensure that certain columns don’t change after application of some function. This can be achieved by keying those columns and restoring them later (note that this can change the order of columns.):

weird_modify <- . %>% transmute(new_col = vs + 2 * am)

# Suppose there is a need for all columns to stay untouched in the output
mtcars_tbl %>%
  key_by(everything()) %>%
  weird_modify() %>%
  # This can be replaced by its scoped variant: restore_keys_all()
  restore_keys(everything()) %>%
  unkey()
## # A tibble: 32 x 4
##   new_col   mpg    vs    am
##     <dbl> <dbl> <dbl> <dbl>
## 1       2  21.0     0     1
## 2       2  21.0     0     1
## 3       3  22.8     1     1
## 4       1  21.4     1     0
## 5       0  18.7     0     0
## # ... with 27 more rows

Hide columns

In actual data analysis the following situation can happen: one should modify all but handful of columns with dplyr::mutate_if().

is_integerish <- function(x) {all(x == as.integer(x))}

if_modify <- . %>% mutate_if(is_integerish, ~ . * 10)

mtcars_tbl %>% if_modify()
## # A tibble: 32 x 3
##     mpg    vs    am
##   <dbl> <dbl> <dbl>
## 1  21.0     0    10
## 2  21.0     0    10
## 3  22.8    10    10
## 4  21.4    10     0
## 5  18.7     0     0
## # ... with 27 more rows

Suppose column vs should appear unchanged in the output. This can be achieved in several ways, which differ slightly but significantly. The first one is to key by vs, apply function and restore vs from keys.

mtcars_tbl %>%
  key_by(vs) %>%
  if_modify() %>%
  restore_keys(vs)
## # A keyed object. Keys: vs 
## # A tibble: 32 x 3
##     mpg    vs    am
##   <dbl> <dbl> <dbl>
## 1  21.0     0    10
## 2  21.0     0    10
## 3  22.8     1    10
## 4  21.4     1     0
## 5  18.7     0     0
## # ... with 27 more rows

The advantage is that it doesn’t change the order of columns. The disadvantage is that it actually applies modification function to column, which can be undesirable in some cases.

The second approach is similar, but after keying by vs one can remove this column from data frame. This way column vs is moved to last column.

mtcars_hidden_vs <- mtcars_tbl %>% key_by(vs, .exclude = TRUE)

mtcars_hidden_vs
## # A keyed object. Keys: vs 
## # A tibble: 32 x 2
##     mpg    am
## * <dbl> <dbl>
## 1  21.0     1
## 2  21.0     1
## 3  22.8     1
## 4  21.4     0
## 5  18.7     0
## # ... with 27 more rows

mtcars_hidden_vs %>%
  if_modify() %>%
  restore_keys(vs)
## # A keyed object. Keys: vs 
## # A tibble: 32 x 3
##     mpg    am    vs
##   <dbl> <dbl> <dbl>
## 1  21.0    10     0
## 2  21.0    10     0
## 3  22.8    10     1
## 4  21.4     0     1
## 5  18.7     0     0
## # ... with 27 more rows

Conclusions

  • It might be a good idea to extract some package functionality into separate package, as this can lead to one more useful tool.
  • Package keyholder offers functionality for keeping track of arbitrary data about rows after application of some user defined function. This is done by creating special attribute “keys” which is updated after every change in rows (subsetting, ordering, etc.).
sessionInfo()
sessionInfo()
## R version 3.4.2 (2017-09-28)
## Platform: x86_64-pc-linux-gnu (64-bit)
## Running under: Ubuntu 16.04.3 LTS
## 
## Matrix products: default
## BLAS: /usr/lib/openblas-base/libblas.so.3
## LAPACK: /usr/lib/libopenblasp-r0.2.18.so
## 
## locale:
##  [1] LC_CTYPE=ru_UA.UTF-8       LC_NUMERIC=C              
##  [3] LC_TIME=ru_UA.UTF-8        LC_COLLATE=ru_UA.UTF-8    
##  [5] LC_MONETARY=ru_UA.UTF-8    LC_MESSAGES=ru_UA.UTF-8   
##  [7] LC_PAPER=ru_UA.UTF-8       LC_NAME=C                 
##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
## [11] LC_MEASUREMENT=ru_UA.UTF-8 LC_IDENTIFICATION=C       
## 
## attached base packages:
## [1] methods   stats     graphics  grDevices utils     datasets  base     
## 
## other attached packages:
## [1] keyholder_0.1.1 bindrcpp_0.2    dplyr_0.7.4    
## 
## loaded via a namespace (and not attached):
##  [1] Rcpp_0.12.13     bookdown_0.5     assertthat_0.2.0 digest_0.6.12   
##  [5] rprojroot_1.2    R6_2.2.2         backports_1.1.1  magrittr_1.5    
##  [9] evaluate_0.10.1  blogdown_0.2     rlang_0.1.4      stringi_1.1.5   
## [13] rmarkdown_1.7    tools_3.4.2      stringr_1.2.0    glue_1.2.0      
## [17] yaml_2.1.14      compiler_3.4.2   pkgconfig_2.0.1  htmltools_0.3.6 
## [21] bindr_0.1        knitr_1.17       tibble_1.3.4

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