chapters/21.markdown @ c639d2c11892
typo-the-then
Close branch.
author |
Steve Losh <steve@stevelosh.com> |
date |
Fri, 07 Oct 2016 13:11:30 +0000 |
parents |
f09f87e10570 |
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Conditionals
============
Every programming language has a way to branch, and in Vimscript that method is
the `if` statement. The `if` statement is the core of branching in Vim.
There's no `unless` statement like Ruby, so any decision making you do in your
coding will be done with `if`s.
Before we talk about Vim's `if` statement we need to take a short detour to talk
about syntax so we're all on the same page.
Multiple-Line Statements
------------------------
Sometimes you can't fit a piece of Vimscript on a single line of code. We saw this
when we talked about autocommand groups. Here's a chunk of code we used before:
:::vim
:augroup testgroup
: autocmd BufWrite * :echom "Baz"
:augroup END
You can write this as three separate lines in a Vimscript file, which is ideal,
but it gets tedious to write this way when running commands manually. Instead
you can separate each line with a pipe character (`|`). Run the following
command:
:::vim
:echom "foo" | echom "bar"
Vim will treat that as two separate commands. Use `:messages` to check the log
if you didn't see both lines appear.
For the rest of this book if you want to manually run a command but don't want
to bother typing in the newlines and colons, feel free to put it all on one line
separated by pipes.
Basic If
--------
Now that we've got that out of the way, run the following commands:
:::vim
:if 1
: echom "ONE"
:endif
Vim will display `ONE`, because the integer `1` is "truthy". Now try these
commands:
:::vim
:if 0
: echom "ZERO"
:endif
Vim will *not* display `ZERO` because the integer `0` is "falsy". Let's see how
strings behave. Run these commands:
:::vim
:if "something"
: echom "INDEED"
:endif
The results may surprise you. Vim does *not* necessarily treat a non-empty
string as "truthy", so it will not display anything!
Let's dive a bit further down the rabbit hole. Run these commands:
:::vim
:if "9024"
: echom "WHAT?!"
:endif
This time Vim *does* display the text! What's going on here?
To try to wrap our heads around what's going on, run the following three commands:
:::vim
:echom "hello" + 10
:echom "10hello" + 10
:echom "hello10" + 10
The first command causes Vim to echo `10`, the second command echoes `20`, and
the third echoes `10` again!
After observing all of these commands we can draw a few informed conclusions
about Vimscript:
* Vim will try to coerce variables (and literals) when necessary. When `10 +
"20foo"` is evaluated Vim will convert `"20foo"` to an integer (which
results in `20`) and then add it to `10`.
* Strings that start with a number are coerced to that number, otherwise they're
coerced to `0`.
* Vim will execute the body of an `if` statement when its condition evaluates to
a non-zero integer, *after* all coercion takes place.
Else and Elseif
---------------
Vim, like Python, supports both "else" and "else if" clauses. Run the following
commands:
:::vim
:if 0
: echom "if"
:elseif "nope!"
: echom "elseif"
:else
: echom "finally!"
:endif
Vim echoes `finally!` because both of the previous conditions evaluate to zero,
which is falsy.
Exercises
---------
Drink a beer to console yourself about Vim's coercion of strings to integers.