How can I use consecutive numbers in an Ex-style substitute command?

Question

For example, let's say I want to put a number before every line that starts with the word "Do". The command would look something like :%s/^(Do )/1. \1/, but what can I do so it will apply numbers consecutively instead of the same number each time?

Note that I'm well aware of the numerous ways to do this using macros and Ctrl-A, but for use in more complicated Ex-mode commands I would like to know the answer to exactly the title question.

For another example where a macro approach wouldn't be so easy, let's say I want to append a number to all instances of the word "Section", and have them consecutively numbered but restarting the count after each line that starts "Chapter". An Ex-mode command that can almost do this, save for actually incrementing the number to be added, is:

:g/^Chapter/;/^Chapter/-s/Section\zs/ 1/g

You can see that an easy method of incrementing numbers in the Ex-mode substitute command could be very useful.

Answer 1

This command does what you want:

:let i = 1|g/^Do/s/^/\=i/|let i = i + 1

Explanation…

• let i = 1 initializes counter i,
• g/^Do/s/^/\=i/ prepends i to each line starting with Do,
• let i = i + 1 increments i.

The trick is that the incrementation happens before the next substitution.

--- edit ---

If we used a single substitution, the counter would only be incremented once, after everything is done.

Since we are performing multiple substitutions — one for each matching line — instead of a single one, the counter is correctly incremented before the next substitution.