AUCTeX has autocompletion mechanisms different from most of the other LaTeX editors. In Emacs, when TeX-latex-mode
is activated, the sequence Ctrl-c Ctrl-e (the -
means that the second key has to be pressed while holding the first, while the space implies the release of both keys before the next combination) opens the mini-buffer dialog interface at the bottom of the page where one types the name of the environment desired. Press enter and Emacs then automatically inserts both \begin{}
and \end{}
tags and places the pointer in the line between said tags, usually even adding proper indentation. AUCTeX keeps the history of environments inserted in this way, so you can recall a recently inserted environment using the up and down arrow keys. In addition, the last inserted environment will be used by default, so if you insert an environment very often you don't need to type anything at all.
Moreover, as mentioned by @daleif, you can change the current environment with Ctrl-u Ctrl-c Ctrl-e and insert an \end{...}
matching the last open \begin{...}
with Ctrl-c ].
More in depth information can be found at the official project page.
Please note that in the documentation a non-conventional symbolism is used: e.g., C stands for Ctrl, RET for the enter (return) button, M for the diamond shaped Meta key in old keyboards (but alt does the job allright), etc.
Ctrl-c Ctrl-e
(the-
means that you press the second button while holding the first) opens the mini-buffer where you insert the environment and emacs then automatically inserts\begin{}
and\end{}
. It also remembers the history of the commands so that you don't have to always write it yourself