In this vignette we showcase the arguments of flow_view() and flow_run() that affect the way the diagrams are displayed.


prefix is used to display special comments on top of code blocks, it must start with "#".


factorial <- function(x) {
  ## is the input valid ?
  if(!is.integer(x) && length(x) == 1) {
    ## fail explicitly
    stop("`x` must be integer!")
  ## initialize result
  res <- 1
  for(i in 2:x) {
    ## update result
    res <- res * i
    ## is the result to big ?
    if(res > 1000) {
      ## fail explicitly
      stop("too big!!!") 

flow_view(factorial, prefix = "##")

  • prefix can be of length > 1 in which case all given prefixes are considered
  • The unprefixed comments are ignored.
  • Prefixed comments right before control flow calls (here if) will serve as headers for control flow header blocks.
  • Prefixed comments before other code will serve as headers for the subsequent code chunks.
  • Adding special comments in the middle of a code block code will split the block in two
  • We need to use {} after if (n == 0L) if we want to use a prefixed comment

This argument is ignored when using the plantuml engine.


code is used to choose whether to display the code in code blocks or only the header, if NA the code will be displayed only if no header is defined by special comments.

flow_view(factorial, code = FALSE)

flow_view(factorial, prefix = "##", code = FALSE)

flow_view(factorial, prefix = "##", code = NA) # pay attention to the last block

This argument is ignored when using the plantuml engine.


narrow is used to narrow makes sure the diagram stays centered on one column, it will be longer but won’t shift, which might be easier to display on web pages for instance.

kg_to_lb <- function(mass) {
  if(mass < 0) {
    stop("mass should not be negative!")
  } else {
      mass <- 2.20462262185 * mass


flow_view(kg_to_lb, narrow = TRUE)

This argument is ignored when using the plantuml engine.


truncate is used to provide the maximum number of characters to be printed per line, it works for code and headers.

fun <- function() {
  ## short header
  x <- "__________________________long string__________________________"
  ## __________________________long header__________________________
flow_view(fun, prefix = "##")

flow_view(fun, truncate = 15, prefix = "##")


By default calls like x <- if(cond) a else b are automatically transformed into if(cond) x <- a else x <- b so we can display the logic in a proper diagram.

If this is not acceptable this can be turned off by setting swap to FALSE.

fun <- function(cond) {
  x <- if(cond) this else that


flow_view(fun, swap = FALSE)