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 ifs.

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:

: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:

: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:

:if 1
:    echom "ONE"
:endif

Vim will display ONE, because the integer 1 is "truthy". Now try these commands:

: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:

: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:

: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:

: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:

: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.

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