28.29 Truncating Too Many Lines

REVIEW Another issue we sometimes want to deal with is limiting the number of lines of output displayed from knitr (Xie 2025). We can add a hook to knitr (Xie 2025) so that whenever we have reached a particular line count for the output of any command, we replace all the remaining lines with `...''. The hook again extends theoutputfunction. Notice we take a copy of the currentoutput` hook and then run that after our own processing.

opts_chunk$set(out.lines=4)
hook_output <- knit_hooks$get("output")
knit_hooks$set(output=function(x, options)
{
  if (options$results != "asis")
  {
    # Split string into separate lines.
    x <- unlist(stringr::str_split(x, "\n"))
    # Trim to the number of lines specified.
    if (!is.null(n <- options$out.lines))
    {
      if (length(x) > n)
      {
        # Truncate the output.
        x <- c(head(x, n), "....\n")
      }
    }
    # Paste lines back together.
    x <- paste(x, collapse="\n")
  }
  hook_output(x, options)
})

We can then illustrate it:

weather[2:8]

## # A tibble: 366 × 7
##    Location MinTemp MaxTemp Rainfall Evaporation Sunshine WindGustDir
##    <chr>      <dbl>   <dbl>    <dbl>       <dbl>    <dbl> <ord>      
##  1 Canberra     5      13.9      0            NA       NA NNW        
##  2 Canberra    -4.1    15.7      0            NA       NA SSW        
##  3 Canberra    -3.3    13.4      0.2          NA       NA SE         
##  4 Canberra     3      12        0            NA       NA N          
##  5 Canberra     7.4    15        2.8          NA       NA NW         
##  6 Canberra    -0.6    12.4      0.2          NA       NA N          
##  7 Canberra     2.5    11.8      0.2          NA       NA NW         
##  8 Canberra     5.6     9.9      0            NA       NA NW         
##  9 Canberra     6.2    11.8      4.4          NA       NA NW         
## 10 Canberra     6.5    12.2      0            NA       NA WNW        
## # ℹ 356 more rows

Now we set out.lines=2 to only include the first two lines.

weather[2:8]

## # A tibble: 366 × 7
##    Location MinTemp MaxTemp Rainfall Evaporation Sunshine WindGustDir
....

References

———. 2025. Knitr: A General-Purpose Package for Dynamic Report Generation in r. https://yihui.org/knitr/.

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