Data Science Desktop Survival Guide by Graham Williams
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Selective Lines of Code

Raw Another useful formatting trick is to include only the top few and bottom few lines of a block of code. We can again do this using hooks. This time it is through a source hook.

opts_chunk$set(src.top=NULL)
opts_chunk$set(src.bot=NULL)
knit_hooks$set(source=function(x, options)
{
  # Split string into separate lines.
  x <- unlist(stringr::str_split(x, "\n"))
  # Trim to the number of lines specified.
  if (!is.null(n <- options$src.top))
  {
    if (length(x) > n)
    {
      # Truncate the output.
      if (is.null(m <-options$src.bot)) m <- 0
      x <- c(head(x, n+1), "\n....\n", tail(x, m+2))
   }
  }
  # Paste lines back together.
  x <- paste(x, collapse="\n")
  hook_source(x, options)
})

Now we repaet this code chunk in the source of this current document, but we set src.top=4 and src.bot=4:

opts_chunk$set(src.top=NULL)
opts_chunk$set(src.bot=NULL)
knit_hooks$set(source=function(x, options)
{
 # Split string into separate lines.

....

    }
  }
 # Paste lines back together.
  x <- paste(x, collapse="\n")
  hook_source(x, options)
})