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Add new base R <-> purrr vignette. (#740)
Fixes #726 Co-authored-by: Hadley Wickham <h.wickham@gmail.com>
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--- | ||
title: "purrr <-> base R" | ||
output: rmarkdown::html_vignette | ||
vignette: > | ||
%\VignetteIndexEntry{purrr <-> base R} | ||
%\VignetteEngine{knitr::rmarkdown} | ||
%\VignetteEncoding{UTF-8} | ||
--- | ||
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```{r, include = FALSE} | ||
knitr::opts_chunk$set( | ||
collapse = TRUE, | ||
comment = "#>", | ||
fig.width = 7, | ||
fig.height = 4.5, | ||
fig.align = "center" | ||
) | ||
options(tibble.print_min = 6, tibble.print_max = 6) | ||
modern_r <- getRversion() >= "4.1.0" | ||
``` | ||
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# Introduction | ||
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This vignette compares purrr's functionals to their base R equivalents, focusing primarily on the map family and related functions. | ||
This helps those familiar with base R understand better what purrr does, and shows purrr users how you might express the same ideas in base R code. | ||
We'll start with a rough overview of the major differences, give a rough translation guide, and then show a few examples. | ||
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```{r setup} | ||
library(purrr) | ||
library(tibble) | ||
``` | ||
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## Key differences | ||
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There are two primary differences between the base apply family and the purrr map family: purrr functions are named more consistently, and more fully explore the space of input and output variants. | ||
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- purrr functions consistently use `.` as prefix to avoid [inadvertently matching arguments](https://adv-r.hadley.nz/functionals.html#argument-names) of the purrr function, instead of the function that you're trying to call. | ||
Base functions use a variety of techniques including upper case (e.g. `lapply(X, FUN, ...)`) or require anonymous functions (e.g. `Map()`). | ||
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- All map functions are type stable: you can predict the type of the output using little information about the inputs. | ||
In contrast, the base functions `sapply()` and `mapply()` automatically simplify making the return value hard to predict. | ||
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- The map functions all start with the data, followed by the function, then any additional constant argument. | ||
Most base apply functions also follow this pattern, but `mapply()` starts with the function, and `Map()` has no way to supply additional constant arguments. | ||
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- purrr functions provide all combinations of input and output variants, and include variants specifically for the common two argument case. | ||
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## Direct translations | ||
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The following sections give a high-level translation between base R commands and their purrr equivalents. | ||
See function documentation for the details. | ||
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### `Map` functions | ||
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Here `x` denotes a vector and `f` denotes a function | ||
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| Output | Input | Base R | purrr | | ||
|-----------------|-----------------|-----------------|--------------------| | ||
| List | 1 vector | `lapply()` | `map()` | | ||
| List | 2 vectors | `mapply()`, `Map()` | `map2()` | | ||
| List | \>2 vectors | `mapply()`, `Map()` | `pmap()` | | ||
| Atomic vector of desired type | 1 vector | `vapply()` | `map_lgl()` (logical), `map_int()` (integer), `map_dbl()` (double), `map_chr()` (character), `map_raw()` (raw) | | ||
| Atomic vector of desired type | 2 vectors | `mapply()`, `Map()`, then `is.*()` to check type | `map2_lgl()` (logical), `map2_int()` (integer), `map2_dbl()` (double), `map2_chr()` (character), `map2_raw()` (raw) | | ||
| Atomic vector of desired type | \>2 vectors | `mapply()`, `Map()`, then `is.*()` to check type | `pmap_lgl()` (logical), `pmap_int()` (integer), `pmap_dbl()` (double), `pmap_chr()` (character), `pmap_raw()` (raw) | | ||
| Side effect only | 1 vector | loops | `walk()` | | ||
| Side effect only | 2 vectors | loops | `walk2()` | | ||
| Side effect only | \>2 vectors | loops | `pwalk()` | | ||
| Data frame (`rbind` outputs) | 1 vector | `lapply()` then `rbind()` | `map_dfr()` | | ||
| Data frame (`rbind` outputs) | 2 vectors | `mapply()`/`Map()` then `rbind()` | `map2_dfr()` | | ||
| Data frame (`rbind` outputs) | \>2 vectors | `mapply()`/`Map()` then `rbind()` | `pmap_dfr()` | | ||
| Data frame (`cbind` outputs) | 1 vector | `lapply()` then `cbind()` | `map_dfc()` | | ||
| Data frame (`cbind` outputs) | 2 vectors | `mapply()`/`Map()` then `cbind()` | `map2_dfc()` | | ||
| Data frame (`cbind` outputs) | \>2 vectors | `mapply()`/`Map()` then `cbind()` | `pmap_dfc()` | | ||
| Any | Vector and its names | `l/s/vapply(X, function(x) f(x, names(x)))` or `mapply/Map(f, x, names(x))` | `imap()`, `imap_*()` (`lgl`, `dbl`, `dfr`, and etc. just like for `map()`, `map2()`, and `pmap()`) | | ||
| Any | Selected elements of the vector | `l/s/vapply(X[index], FUN, ...)` | `map_if()`, `map_at()` | | ||
| List | Recursively apply to list within list | `rapply()` | `map_depth()` | | ||
| List | List only | `lapply()` | `lmap()`, `lmap_at()`, `lmap_if()` | | ||
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### Shorthands | ||
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When an anonymous function is required in `*apply` or `map` functions, purrr offers shorthands to make the anonymous function more readable and easier to write. | ||
Here `l` denotes a list of arguments, and `f` denotes some expression involving arguments of the anonymous function. | ||
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| Input | base R | purrr | | ||
|------------------|--------------------------|----------------------------| | ||
| 1 vector | `function(x) f(x)` | `~ f(.x)` | | ||
| 2 vectors | `function(x, y) f(x, y)` | `~ f(.x, .y)` | | ||
| More than 2 vectors | `function(x, y, z, ...) f(x, y, z, ...)` or `function(l) f(l[[1]], l[[2]], l[[3]], ...)` or `function(l) do.call(f, args = l)` | `~ f(..1, ..2, ..3, ...)` | | ||
| Extract `i`th element of each vector | `lapply(x, function(y) tryCatch(y[["a"]], error = function(e) NA))`, | `map(x, "a", default = NA)` | | ||
| Extract `i`th element of each vector with default value | `lapply(x, function(y) tryCatch(y[[3]], error = function(e) NA))` | `map(x, 3, .default = NA)` | | ||
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### Predicates | ||
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Here `p`, a predicate, denotes a function that returns `TRUE` or `FALSE` indicating whether an object fulfills a criterion, e.g. `is.character()`. | ||
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| Description | base R | purrr | | ||
|-----------------------------|--------------------|-----------------------| | ||
| Find a matching element | `Find(p, x)` | `detect(x, p)`, | | ||
| Find position of matching element | `Position(p, x)` | `detect_index(x, p)` | | ||
| Do all elements of a vector satisfy a predicate? | `all(sapply(x, p))` | `every(x, p)` | | ||
| Does any elements of a vector satisfy a predicate? | `any(sapply(x, p))` | `some(x, p)` | | ||
| Does a list contain an object? | `any(sapply(x, identical, obj))` | `has_element(x, obj)` | | ||
| Keep elements that satisfy a predicate | `x[sapply(x, p)]` | `keep(x, p)` | | ||
| Discard elements that satisfy a predicate | `x[!sapply(x, p)]` | `discard(x, p)` | | ||
| Negate a predicate function | `function(x) !p(x)` | `negate(p)` | | ||
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### Other vector transforms | ||
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| Description | base R | purrr | | ||
|-----------------------------|--------------------|-----------------------| | ||
| Accumulate intermediate results of a vector reduction | `Reduce(f, x, accumulate = TRUE)` | `accumulate(x, f)` | | ||
| Recursively combine two lists | `c(X, Y)`, but more complicated to merge recursively | `list_merge()`, `list_modify()` | | ||
| Reduce a list to a single value by iteratively applying a binary function | `Reduce(f, x)` | `reduce(x, f)` | | ||
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## Examples | ||
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### Varying inputs | ||
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#### One input | ||
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Suppose we would like to generate a list of samples of 5 from normal distributions with different means: | ||
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```{r} | ||
means <- 1:4 | ||
``` | ||
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There's little difference when generating the samples: | ||
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- Base R uses `lapply()`: | ||
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```{r} | ||
set.seed(2020) | ||
samples <- lapply(means, rnorm, n = 5, sd = 1) | ||
str(samples) | ||
``` | ||
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- purrr uses `map()`: | ||
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```{r} | ||
set.seed(2020) | ||
samples <- map(means, rnorm, n = 5, sd = 1) | ||
str(samples) | ||
``` | ||
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#### Two inputs | ||
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Lets make the example a little more complicated by also varying the standard deviations: | ||
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```{r} | ||
means <- 1:4 | ||
sds <- 1:4 | ||
``` | ||
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- This is relatively tricky in base R because we have to adjust a number of `mapply()`'s defaults. | ||
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```{r} | ||
set.seed(2020) | ||
samples <- mapply( | ||
rnorm, | ||
mean = means, | ||
sd = sds, | ||
MoreArgs = list(n = 5), | ||
SIMPLIFY = FALSE | ||
) | ||
str(samples) | ||
``` | ||
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Alternatively, we could use `Map()` which doesn't simply, but also doesn't | ||
take any constant arguments, so we need to use an anonymous function: | ||
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```{r} | ||
samples <- Map(function(...) rnorm(..., n = 5), mean = means, sd = sds) | ||
``` | ||
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In R 4.1 and up, you could use the shorter anonymous function form: | ||
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```{r, eval = modern_r} | ||
samples <- Map(\(...) rnorm(..., n = 5), mean = means, sd = sds) | ||
``` | ||
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- Working with a pair of vectors is a common situation so purrr provides the `map2()` family of functions: | ||
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```{r} | ||
set.seed(2020) | ||
samples <- map2(means, sds, rnorm, n = 5) | ||
str(samples) | ||
``` | ||
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#### Any number of inputs | ||
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We can make the challenge still more complex by also varying the number of samples: | ||
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```{r} | ||
ns <- 4:1 | ||
``` | ||
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- Using base R's `Map()` becomes more straightforward because there are no constant arguments. | ||
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```{r} | ||
set.seed(2020) | ||
samples <- Map(rnorm, mean = means, sd = sds, n = ns) | ||
str(samples) | ||
``` | ||
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- In purrr, we need to switch from `map2()` to `pmap()` which takes a list of any number of arguments. | ||
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```{r} | ||
set.seed(2020) | ||
samples <- pmap(list(mean = means, sd = sds, n = ns), rnorm) | ||
str(samples) | ||
``` | ||
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### Outputs | ||
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Given the samples, imagine we want to compute their means. A mean is a single number, so we want the output to be a numeric vector rather than a list. | ||
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- There are two options in base R: `vapply()` or `sapply()`. `vapply()` requires you to specific the output type (so is relatively verbose), but will always return a numeric vector. `sapply()` is concise, but if you supply an empty list you'll get a list instead of a numeric vector. | ||
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```{r} | ||
# type stable | ||
medians <- vapply(samples, median, FUN.VALUE = numeric(1L)) | ||
medians | ||
# not type stable | ||
medians <- sapply(samples, median) | ||
``` | ||
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- purrr is little more compact because we can use `map_dbl()`. | ||
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```{r} | ||
medians <- map_dbl(samples, median) | ||
medians | ||
``` | ||
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What if we want just the side effect, such as a plot or a file output, but not the returned values? | ||
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- In base R we can either use a for loop or hide the results of `lapply`. | ||
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```{r, fig.show='hide'} | ||
# for loop | ||
for (s in samples) { | ||
hist(s, xlab = "value", main = "") | ||
} | ||
# lapply | ||
invisible(lapply(samples, function(s) { | ||
hist(s, xlab = "value", main = "") | ||
})) | ||
``` | ||
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- In purrr, we can use `walk()`. | ||
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```{r, fig.show='hide'} | ||
walk(samples, ~ hist(.x, xlab = "value", main = "")) | ||
``` | ||
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### Pipes | ||
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You can join multiple steps together either using the magrittr pipe: | ||
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```{r} | ||
set.seed(2020) | ||
means %>% | ||
map(rnorm, n = 5, sd = 1) %>% | ||
map_dbl(median) | ||
``` | ||
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Or the base pipe R: | ||
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```{r, eval = modern_r} | ||
set.seed(2020) | ||
means |> | ||
lapply(rnorm, n = 5, sd = 1) |> | ||
sapply(median) | ||
``` | ||
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(And of course you can mix and match the piping style with either base R or purrr.) |