Data Science Desktop Survival Guide by Graham Williams
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Truncating Too Many Lines

Raw Another issue we sometimes want to deal with is limiting the number of lines of output displayed from knitr. We can add a hook to knitr 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 the output function. Notice we take a copy of the current output 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 x 7
##    Location MinTemp MaxTemp Rainfall Evaporation Sunshine WindGustDir
##    <chr>      <dbl>   <dbl>    <dbl>       <dbl>    <dbl> <ord>      
##  1 Canberra     5      24.3      0           3.4      6.3 NW         
##  2 Canberra    14      26.9      3.6         4.4      9.7 ENE        
##  3 Canberra    13.7    23.4      3.6         5.8      3.3 NW         
....

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

weather[2:8]

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