Why doesn't this `:execute` command work?

Question

I was looking at this question on Unix & Linux Stack Exchange, and I thought I could write an answer for it using Vim.

I ran the following for a quick test, and was surprised to find it didn't seem to do anything. I was expecting it to replace the first number on the first line with one less than the number of lines in the buffer.

:execute '1s/^[0-9]\+/' . line('$')-1 . '/'

Here is the text I had in the buffer at the time:

5 10
sample_1 gaatatccga
sample_2 gaatatccga
sample_3 gaatatcgca

Why didn't the :execute command shown above replace the 5 with a 3 as intended?

Answer 1

. has the same precedence as + and -

'1s/^[0-9]\+/' . line('$')-1 . '/' is the same as ('1s/^[0-9]\+/' . line('$')) - 1 . '/'—it's string - number . string, which is not what you want.

To solve your problem:

:execute '1s/^[0-9]\+/' . (line('$')-1) . '/'

• :h expr-..
• :h expression-syntax.