How can I detect the end of the buffer when iterating over lines or headings in elisp?

The TLDR is the code:

(while (?????)
    (org-next-visible-heading 1)
    (org-todo 'todo)
)

What should I put in the condition ????? in order to stop at the end of the file, or after the last visible heading?

The longer context:

I'm using trying to use elisp to modify org-mode files for emacs. I'm new to elisp and finding it hard to find the relevant documentation for some basic things.

I want to iterate over the headings in an org-mode buffer and modify the status (i.e. todo) keyword. I've seen it suggested that if I want to modify the buffer, it is better not to use org-element-map but instead to use something like org-next-visible-heading or org-forward-same-level, and at each point use org-todo to modify the heading.

I know how to write a while loop, but I don't know how to write the condition as I haven't been able to find out how to detect if I'm at the end of the buffer, even though this seems to be a trivial thing to want to do.

I'm happy to be told there is a better approach to my problem but I would still like to know how to detect the end of a buffer in general.

Solution

Rather than looping, you can use org-map-entries. Here is an example, pretty much from the doc, that marks each headline as a TODO item:

(org-map-entries '(org-todo "TODO") t 'file 'archive 'comment)

It specifies a function to apply to each headline: (org-todo "TODO"); no tag or property matching; file scope; and skips archive and comment headlines.

Read the linked doc and also do C-h f org-map-entries, but don't get too scared by the doc string of the function: org-map-entries is very powerful, but it will take some time to understand how to do things with it and harness its power.