How do I use a variable with an ex command?

Question

I wrote a moderately complex routine for Vim and I need to programmatically set the height of windows.

The way to do this is with :resize [N] as the help says.

But my invocation didn't work until I employed exec:

exe `resize `.i[1]

The following doesn't work, it always resizes to a height of 1 instead of the required i[1]:

resize i[1]

So I understand that resize takes a Number type and I am apparently feeding it a string, but I can't figure out how to avoid exec.

Maybe there's a trick I can use to use the + operator with 0? In fact the Vim doc explicitly tells us to do this.

I tried it, but it doesn't work! It works with echo but not res.

Then I tried setting the value i[1] + 0 to a variable and then using that. Indeed it seems that when I use res, it does not attempt to expand the value of variables. Really perplexing.

Is there another canonical fast way to convert an integer string into a Number? Perhaps some other thing is causing this unexpected behavior?

Answer 1

You can't use a variable directly with :res. Compare the help text for :res, :s, :echo and :exe:

:res[ize] -N                                    :res :resize CTRL-W_-
CTRL-W -        Decrease current window height by N (default 1).

---

:[range]s[ubstitute]/{pattern}/{string}/[flags] [count]
                        For each line in [range] replace a match of {pattern}
                        with {string}.
                        For the {pattern} see pattern.
                        {string} can be a literal string, or something
                        special; see sub-replace-special.

---

                                                        :ec :echo
:ec[ho] {expr1} ..      Echoes each {expr1}, with a space in between.  The
                        first {expr1} starts on a new line.
                                                        :exe :execute
:exe[cute] {expr1} ..   Executes the string that results from the evaluation
                        of {expr1} as an Ex command.

---

The text uses expr1 for echo and exe, but nothing of the sort for the other commands. For :s, it's specially stated that the replacement can be something special:

                                        sub-replace-special :s\=
When the {string} starts with "\=" it is evaluated as an expression, see
sub-replace-expression.  You can use that for complex replacement or special
characters.

In short, unless the command is expected to be followed by an expression, don't expect variables to evaluated.

:exe is the way to go, in this case.

---

By the way, you don't need to use . here, the following will work the same:

:exe 'resize' i[1]

exe implicitly joins arguments with spaces. It's when you don't want the sapce that you should use . with exe arguments.