从lapply或带print语句的函数中调用kable时，其行为异常 [英] Unexpected behavior of kable when called from lapply or from function with print statement
问题描述
在尝试使用knitr软件包编织HTML时(在Ubuntu 14.04上的RStudio 0.98.977中)，我试图了解kable函数的以下两个意外行为:
I am trying to understand the two following unexpected behaviors of the kable function when knitting HTML using the knitr package (in RStudio 0.98.977 on Ubuntu 14.04):
- 从lapply内两次调用kable时，只有第一个调用会在最终的HTML中产生漂亮的显示.
- 当在同样使用print语句的函数中进行两次kable调用时，只有最后一次调用会在最终HTML中产生漂亮的显示.
下面是一个示例代码:
Load library:
```{r init}
library("knitr")
```
Define dataframe:
```{r define_dataframe}
df <- data.frame(letters=c("a", "b", "c"), numbers=c(1, 2, 3))
rownames(df) <- c("x", "y", "z")
```
### Example 1: pretty display with simple call
The dataframe is displayed nicely twice when knitting HTML with the following code:
```{r pretty_display1, results="asis"}
kable(df)
kable(df)
```
### Example 2: unexpected display with lapply
The dataframe is displayed nicely only the first time when knitting HTML with the following code:
```{r unexpected_display1, results="asis"}
lst <- list(df, df)
lapply(lst, kable)
```
### Example 3: pretty display with function
The dataframe is displayed nicely twice when knitting HTML with the following code:
```{r pretty_display2, results="asis"}
foo1 <- function (df) {
kable(df)
}
foo2 <- function (df) {
foo1(df)
foo1(df)
}
foo2(df)
```
### Example 4: unexpected display with function containing print statements
The dataframe is displayed nicely only the second time when knitting HTML with the following code:
```{r unexpected_display2, results="asis"}
foo1 <- function (df) {
kable(df)
}
foo2 <- function (df) {
print("first display")
foo1(df)
print("second display")
foo1(df)
}
foo2(df)
```
您对这些奇怪的行为有何解释，以及如何规避它们?
Do you have an explanation to these strange behaviors and how to circumvent them?
推荐答案
kable的输出是副作用.您可以将输出的值存储在变量中，但是仅运行kable就会将某些内容输出到控制台.两次运行kable(df)时，这不是问题，您什么也不存储，该函数两次将输出转储到控制台.
The output of kable is a side-effect; you can store the value of the output in a variable, but just running kable will output something to console. When you run kable(df) twice, this isn't a problem, you aren't storing anything, and the function dumps the output to the console twice.
但是，当您运行lapply(lst, kable)时，该函数将输出转储到控制台，然后显示列表的值.尝试仅在控制台中运行它:
However, when you run lapply(lst, kable), the function dumps the output to the console and then the value of the list is displayed. Try running this just in your console:
lst <- list(df, df)
lapply(lst, kable)
您应该完全明白这一点:
You should get exactly this:
| |letters | numbers|
|:--|:-------|-------:|
|x |a | 1|
|y |b | 2|
|z |c | 3|
| |letters | numbers|
|:--|:-------|-------:|
|x |a | 1|
|y |b | 2|
|z |c | 3|
[[1]]
[1] "| |letters | numbers|" "|:--|:-------|-------:|"
[3] "|x |a | 1|" "|y |b | 2|"
[5] "|z |c | 3|"
[[2]]
[1] "| |letters | numbers|" "|:--|:-------|-------:|"
[3] "|x |a | 1|" "|y |b | 2|"
[5] "|z |c | 3|"
注意如何输出正确的减价，然后显示您创建的列表的实际值.这就是造成不良输出的原因.
Notice how the correct markdown is output, and then the actual value of the list you created is displayed. This is what creates the bad output.
功能范式在副作用方面效果不佳，因此有两种选择.您可以通过将output参数设置为FALSE来存储kable的结果，或者可以仅使用for浏览list，或者可以防止显示结果列表.这是一些可行的示例.
The functional paradigm doesn't work particularly well with side-effects, so you have a couple of options. You can store the results of kable by setting the output parameter to FALSE, or you can just use a for to go through your list, or you could prevent the displaying of the result list. here are some examples that will work.
```{r nograpes1, results="asis"}
lst <- list(df, df)
for(x in lst) kable(x) # Don't create a list, just run the function over each element
```
```{r nograpes2, results="asis"}
lst <- list(df, df)
invisible(lapply(lst, kable)) # prevent the displaying of the result list.
```
```{r nograpes3, results="asis"}
lst <- list(df, df)
l <- lapply(lst, kable) # Store the list and do nothing with it.
```
我认为，这是在R中应使用for 的一个很好的例子，因为它最清楚地表达了您要如何使用基于副作用的函数.
In my opinion, this is a nice example of when for should be used in R, as it most cleanly expresses how you want to use a side-effect based function.
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