gistfile1.r

truncate_prettify = function(s, n = 20)
{
  # s - json character string
  # n - number of lines

  library(stringr)
  
  stopifnot(length(s)==1)
  stopifnot(is.character(s))
  
  # find carraige returns
  pos = str_locate_all(s,"\n")[[1]]
  
  #
  n = min(c(nrow(pos),n))

  # truncate prettify string
  new_s = str_sub(s,1,pos[n, 2]+1)
  
  # avoid ellipsis if not truncating
  if (n != nrow(pos))
    new_s = paste0(new_s, "\n...\n\n")
  
  return(new_s)
}


# Usage examples:

library(rjson)
library(jsonlite)

l = list(a = list(1,2,3:4),b = TRUE, c="ABC")
s = prettify(rjson::toJSON(l))


cat(truncate_prettify(s,10))
cat(truncate_prettify(s,11))