More Expressions

When I previously wrote about Counting and Calculating with Vim I mentioned the use of the expression register for inserting the results of calculations: when in Insert mode, press CTRL-R = then type an expression. Expressions can do a lot more than simple mathematical calculations -- this article outlines a few interesting things you can do with them.

Interrogating and Experimenting

When experimenting with expressions, you sometimes just want to see the output before doing anything with the results. One easy way to explore registers is to use :echo to print the results first. When in Normal mode, type the following:

:echo $VIMRUNTIME

This will print the environmental variable, $VIMRUNTIME. To perform a simple calculation, use the same command:

:echo 10 * 3.14

Another thing you can do that is occasionally useful is print the contents of a register:

:echo @a

Options can be interrogated as well:

:echo &columns
:echo &tabstop

Expression Syntax

The syntax used for expressions (:help expression-syntax) should be fairly familiar to anyone who writes high-level programming languages. Expressions are parsed from left to right, and include simple boolean operators and even the C-like ternary operator:

:echo 1 == 0 ? 'Yep' : 'Nope'

All of the usual boolean operators are there, like greater and less than:

:echo 5 > 10
:echo 5 < 10

There are some operators for working with strings as well. Regular expressions can be used with =~, and case insensitive string comparisons are nicely succinct with ==?. The follow expressions are both true (1):

:echo 'alex' =~ '^al'
:echo 'alex' ==? 'Alex'