Vim has context-sensitive completion, based on the current mode. When in Insert mode, there's actually a whole slew of completion types (:help ins-completion). The one most people know is CTRL-P: keyword completion (:help i_CTRL-P). This finds the previous match for a word that starts with the characters in front of the cursor -- CTRL-N is the opposite and will find the next match. Pressing the return key will insert the match, and pressing CTRL-P/N again will cycle through the completion menu.

This example shows the difference: typing eng then CTRL-P matches engines, while CTRL-N matches engine:

These commands look for matches based on the complete option (:help 'complete'). The default value is .,w,b,u,t,i -- let's break that down:

• .: The current buffer
• w: Buffers in other windows
• b: Other loaded buffers
• u: Unloaded buffers
• t: Tags
• i: Included files

Working with the complete Option

The complete option is just a comma-separated list, so flags can be easily added or removed. Typing :set complete will display the current list. Typing :set complete+=k will add dictionary scanning, and :set complete-=k will remove it. Incidentally, if you really want CTRL-P to scan a dictionary file, then try :set dictionary=/usr/share/dict/words. Multiple dictionaries can be used as well.

The completeopt setting controls how completions will appear. It's another comma-separated list, and the default is menu,preview. Other editors often prioritise the longest match, so I've seen a lot of people use the longest option.

The i_CTRL-X Sub-Mode

When in Insert mode, typing CTRL-X actually triggers a sub-mode that accepts several commands. For example, typing CTRL-X CTRL-L will cause Vim to complete an entire line:

These Insert mode completion commands can be quite useful -- I don't want CTRL-P to match based on the dictionary, but I don't mind doing this occasionally by typing CTRL-X CTRL-K. See :help ins-completion for a full list of these commands.

Omni Completion

Omni completion supports filetype-specific completion, so it's greatly suited to programming. Here I've told Vim to use an omni function (omnifunc) and I'm using filetype plugin to turn on filetype detection and load plugins based on the current file:

filetype plugin on
set omnifunc=syntaxcomplete#Complete

Now programming-language specific completion is available by typing CTRL-X CTRL-O:

This behaves a lot like the SDK-based completion found in graphical IDEs.