写入RMD文件时，为什么得到“要替换的项目数不是替换长度的倍数”？

Question

编写RMD文件时，有时会收到“警告消息”。

控制台消息，如下所示。

> library('gapminder')
Warning messages:
1: In res[i] <- withCallingHandlers(if (tangle) process_tangle(group) else process_group(group),  :
number of items to replace is not a multiple of replacement length
2: In res[i] <- withCallingHandlers(if (tangle) process_tangle(group) else process_group(group),  :
number of items to replace is not a multiple of replacement length
> gapminder[gapminder$country=='Afghanistan', c('pop','gdpPercap')]
Warning messages:
1: In res[i] <- withCallingHandlers(if (tangle) process_tangle(group) else process_group(group),  :
number of items to replace is not a multiple of replacement length
...

奇怪的是，如果我创建一个R文件而不是RMD，则同一命令将不会输出警告消息。

所以我猜这条警告消息是由RMD引起的。

---
title: "test"
output:
  html_document:
    number_sections: true
    toc: true
    toc_depth: 6
    toc_float: true
  html_notebook:
    number_sections: true
    toc: true
    toc_depth: 6
    toc_float: true
---
```{r, eval=TRUE}
library('gapminder')
gapminder[gapminder$country=='Afghanistan', c('pop','gdpPercap')]
```