**R – Win-Vector Blog**, and kindly contributed to R-bloggers)

This article is on writing sweet `R`

code using the `wrapr`

package.

## The problem

Consider the following `R`

puzzle. You are given: a `data.frame`

, the name of a column that you wish to find missing values (`NA`

) in, and the name of a column to land the result. For instance:

d <- data.frame(x = c(1, NA)) print(d) # x # 1 1 # 2 NA cname <- 'x' print(cname) # [1] "x" rname <- paste(cname, 'isNA', sep = '_') print(rname) # [1] "x_isNA"

How do you write generic code to populate the column `x_isNA`

with which rows of `x`

are missing?

### The “base R” solution

In “base `R`

” (R without additional packages) this is easy.

When you know the column names while writing the code:

d2 <- d d2$x_isNA <- is.na(d2$x) print(d2) # x x_isNA # 1 1 FALSE # 2 NA TRUE

And when you don’t know the column names while writing the code (but know they will arrive in variables later):

d2 <- d d2[[rname]] <- is.na(d2[[cname]])

The “base R” solution really is quite elegant.

### The “all in” non-standard evaluation `dplyr::mutate`

solution

As far as I can tell the “all in” non-standard evaluation `dplyr::mutate`

solution is something like the following.

When you know the column names while writing the code:

library("dplyr") d %>% mutate(x_isNA = is.na(x))

And when you don’t know the column names while writing the code (but know they will arrive in variables later):

d %>% mutate_(.dots = stats::setNames(list(lazyeval::interp( ~ is.na(VAR), VAR = as.name(cname) )), rname))

### The sweet `wrapr::let`

`dplyr::mutate`

solution

We will only work the “when you don’t yet know the column name” (or parametric) version:

library("wrapr") let(list(COL = cname, RES = rname), d %>% mutate(RES = is.na(COL)) )

I think that this is pretty sweet, and can really level up your `dplyr`

game.

If function behavior depends on variable names, then convenient control of functions is eventually going to require convenient control of variable names; so needing to re-map at some point is inevitable.

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