在 RMarkdown 代码块内循环

Question

我有一个类似这样的文件：

```{r, include = F}
require(data.table)

dt <- data.table(
  A = c(1, 4, 1, 4, 2, 4, 8),
  B = c(2, 2, 3, 4, 3, 1, 5)
)
```


```{r, echo = F}
col <- "A"

knitr::asis_output(paste0("### Column ", col, "\n"))

knitr::asis_output("Counts\n")

dt[, .(n = .N), by = col]

knitr::asis_output("Statistics\n")

summary(dt[, get(col)])
```


```{r, echo = F}
col <- "B"

knitr::asis_output(paste0("### Column ", col, "\n"))

knitr::asis_output("Counts\n")

dt[, .(n = .N), by = col]

knitr::asis_output("Statistics\n")

summary(dt[, get(col)])
```

虽然这很好用，但显然这是非常重复的。我尝试使用循环而不是复制粘贴，如下所示：

```{r, echo = F}
for (col in c("A", "B")) {
  knitr::asis_output(paste0("### Column ", col, "\n"))

  knitr::asis_output("Counts\n")

  dt[, .(n = .N), by = col]

  knitr::asis_output("Statistics\n")

  summary(dt[, get(col)])
}
```

然而，这根本不打印任何输出。如何在 RMarkdown 文档中循环并生成 Markdown 和 R 输出？

Answer 1

对于summary 结果，您需要一个print 语句。您的代码将更改为以下内容，其中仅将 print 语句添加到 summary 输出：

```{r, echo = F}
for (col in c("A", "B")) {
  print(knitr::asis_output(paste0("### Column ", col)))

  print(knitr::asis_output("Counts"))

  print(dt[, .(n = .N), by = col])

  print(knitr::asis_output("Statistics"))

  print(summary(dt[, get(col)]))
}
```

这会产生以下结果：

## [1] "### Column A"
## attr(,"class")
## [1] "knit_asis"
## attr(,"knit_cacheable")
## [1] NA
## [1] "Counts"
## attr(,"class")
## [1] "knit_asis"
## attr(,"knit_cacheable")
## [1] NA
##    A n
## 1: 1 2
## 2: 4 3
## 3: 2 1
## 4: 8 1
## [1] "Statistics"
## attr(,"class")
## [1] "knit_asis"
## attr(,"knit_cacheable")
## [1] NA
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##   1.000   1.500   4.000   3.429   4.000   8.000 
## [1] "### Column B"
## attr(,"class")
## [1] "knit_asis"
## attr(,"knit_cacheable")
## [1] NA
## [1] "Counts"
## attr(,"class")
## [1] "knit_asis"
## attr(,"knit_cacheable")
## [1] NA
##    B n
## 1: 2 2
## 2: 3 2
## 3: 4 1
## 4: 1 1
## 5: 5 1
## [1] "Statistics"
## attr(,"class")
## [1] "knit_asis"
## attr(,"knit_cacheable")
## [1] NA
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##   1.000   2.000   3.000   2.857   3.500   5.000

您可能希望在打印时重新格式化和删除属性。

编辑

根据@user2554330 的建议，这里是一个使用knitr::knit_print 的示例：

```{r, echo = F}
for (col in c("A", "B")) {
  knitr::knit_print(paste0("### Column ", col))
  knitr::knit_print("Counts")
  knitr::knit_print(dt[, .(n = .N), by = col])
  knitr::knit_print("Statistics")
  knitr::knit_print(summary(dt[, get(col)]))
}
```

给出以下输出

## [1] "### Column A"
## [1] "Counts"
##    A n
## 1: 1 2
## 2: 4 3
## 3: 2 1
## 4: 8 1
## [1] "Statistics"
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##   1.000   1.500   4.000   3.429   4.000   8.000 
## [1] "### Column B"
## [1] "Counts"
##    B n
## 1: 2 2
## 2: 3 2
## 3: 4 1
## 4: 1 1
## 5: 5 1
## [1] "Statistics"
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##   1.000   2.000   3.000   2.857   3.500   5.000

编辑 2

以下替代解决方案将更准确地打印 SO 要求的内容。它全局删除“##”前缀并将其添加回data.frame（即仅在需要时）。它还使用cat 代替knitr::knit_print 来避免打印数组索引（即[1]）。

```{r, echo = F, comment=NA}
for (col in c("A", "B")) {
  cat(paste0("Column ", col), "\n")
  cat("Counts\n")
  ## Add comment prefix
  tdf <- as.data.frame(dt[, .(n = .N), by = col]) ## Convert to data.frame for printing
  rownames(tdf) <- paste("## ", 1:nrow(tdf))      ## Add comments for printing.
  knitr::knit_print(tdf)
  cat("Statistics\n")
  ## Add comment prefix
  knitr::knit_print(summary(dt[, get(col)]))
}
```

下面的输出更多的是 SO 的要求（基于这篇文章的 cmets）。它在data.frame 输出前加上“##”前缀，但摘要输出没有“##”前缀；如果需要，可以添加。

Column A 
Counts
      A n
##  1 1 2
##  2 4 3
##  3 2 1
##  4 8 1
Statistics
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
  1.000   1.500   4.000   3.429   4.000   8.000 
Column B 
Counts
      B n
##  1 2 2
##  2 3 2
##  3 4 1
##  4 1 1
##  5 5 1
Statistics
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
  1.000   2.000   3.000   2.857   3.500   5.000