purrr::map_int:无法将元素 1 从双精度强制转换为整数 [英] purrr::map_int: Can't coerce element 1 from a double to a integer

问题描述

我遇到了 purrr 包中的 map_int 最奇怪的错误.

I am having the weirdest bug with map_int from the purrr package.

# Works as expected
purrr::map_int(1:10, function(x) x)
#>  [1]  1  2  3  4  5  6  7  8  9 10

# Why on earth is that not working?
purrr::map_int(1:10, function(x) 2*x)
#> Error: Can't coerce element 1 from a double to a integer

# or that?
purrr::map_int(1:10, round)
#> Error: Can't coerce element 1 from a double to a integer

^{由 reprex 包 (v0.2.1) 于 2019 年 3 月 28 日创建}

^{Created on 2019-03-28 by the reprex package (v0.2.1)}

我在 Rocker 容器 (Debian) 中运行 3.5.2，并使用最新的 github 版本:

I run 3.5.2 in rocker container (Debian) with the latest github version of everything:

sessioninfo::package_info("purrr")
#>  package  * version    date       lib source                             
#>  magrittr   1.5.0.9000 2019-03-28 [1] Github (tidyverse/magrittr@4104d6b)
#>  purrr      0.3.2.9000 2019-03-28 [1] Github (tidyverse/purrr@25d84f7)   
#>  rlang      0.3.2.9000 2019-03-28 [1] Github (r-lib/rlang@9376215)       
#> 
#> [1] /usr/local/lib/R/site-library
#> [2] /usr/local/lib/R/library

推荐答案

help(map) 的文档说

.f 的输出将自动向上输入，例如逻辑 ->整数 -> 双精度 -> 字符

The output of .f will be automatically typed upwards , e.g. logical -> integer -> double -> character

它似乎遵循了 help(c) 中给出的更大的顺序.例如，这会产生错误 map_dbl(1:10, ~complex(real = .x, imaginary = 1)).

It appears to be following the larger ordering given in help(c). For example, this produces an error map_dbl(1:10, ~complex(real = .x, imaginary = 1)).

NULL <原始的逻辑 <整数＜双<复杂的<字符 <列表<表达

NULL < raw < logical < integer < double < complex < character < list < expression

正如您在该排序中所看到的，双精度到整数是向下转换.因此，该函数旨在不进行这种转换.

As you can see in that ordering, double-to-integer is a downward conversion. So, the function is designed to not do this kind of conversion.

解决方案是编写一个函数 .f 输出整数(或更低)分类的对象(如@Stéphane Laurent 的回答)，或者只使用 as.integer(map(.x, .f)).

The solution is to either write a function .f which outputs integer (or lower) classed objects (as in @Stéphane Laurent's answer), or just use as.integer(map(.x, .f)).

这是一种类型检查，这是一个有用的功能用于防止编程错误.

This is a kind of type-checking, which can be a useful feature for preventing programming mistakes.

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