Substitution.md

Substitution

rquery allows substitution of variable values for column/variable names in several places. This is particularly useful in the extend() and select_rows() operations.

The substitution is performed using bquote “.()” notation. The values being substituted must be of class name, to specify they are refering to column names (and not string constants).

Here is an example.

d <- data.frame(
  x = c(1, 1, 2),
  y = c(5, 4, 3),
  z = c(6, 7, 8)
)

knitr::kable(d)

x	y	z
1	5	6
1	4	7
2	3	8

library("rquery")

## Loading required package: wrapr

condition_variable <- as.name('x')
new_value_variable <- as.name('y')
old_value_variable <- as.name('z')

ops <- local_td(d) %.>%
  select_rows(.,
              .(condition_variable) == 1) %.>%
  extend(.,
         .(new_value_variable) := .(old_value_variable) + 1)

cat(format(ops))

## mk_td("d", c(
##   "x",
##   "y",
##   "z")) %.>%
##  select_rows(.,
##    x == 1) %.>%
##  extend(.,
##   y := z + 1)

And we can execute the operations as follows.

d %.>% 
  ops %.>%
  knitr::kable(.)

x	y	z
1	7	6
1	8	7

This also works in the wrap/ex pattern.

d %.>%
  wrap %.>%
  select_rows(.,
              .(condition_variable) == 1) %.>%
  extend(.,
         .(new_value_variable) := .(old_value_variable) + 1) %.>%
  ex %.>%
  knitr::kable(.)

x	y	z
1	7	6
1	8	7

And the method can also be used with project.

d %.>%
  wrap %.>%
  select_rows(.,
              .(condition_variable) == 1) %.>%
  project(.,
         .(new_value_variable) := max(.(old_value_variable))) %.>%
  ex %.>%
  knitr::kable(.)

y
7

We want to avoid using strings instead of names, as strings don’t always give us the expected result. rquery tries to work around and catch many of these cases. For example:

condition_name <- 'x'

ops2 <- local_td(d) %.>%
  select_rows(.,
              .(condition_name) == 1)

## Warning in warn_about_filter_conditions(parsed): rquery::select_rows: expression
## "x" == 1 refers to no columns (so is a constant)

cat(format(ops2))

## mk_td("d", c(
##   "x",
##   "y",
##   "z")) %.>%
##  select_rows(.,
##    "x" == 1)

New to rquery 1.4.3 we have a new “- to strip off quotes” feature. What this is: .(-v) converts v from a character type to a name. Meaning we don’t have to use the as.name() calls if we don’t want to.

condition_name <- 'x'

ops3 <- local_td(d) %.>%
  select_rows(.,
              .(-condition_name) == 1)

cat(format(ops3))

## mk_td("d", c(
##   "x",
##   "y",
##   "z")) %.>%
##  select_rows(.,
##    x == 1)