Symbol for definition :=

Question

Sometimes the symbol := is used to denote a definition. For example, X:=Y+Z means that X is defined to be Y+Z.

When using LaTeX, can I just use $:=$, or do I need to do something special?

Do you also know how to add the equal sign with a 'def' above it? — Someone, Jan 18 '20 at 15:06
@Anatoly, try \newcommand{\defeq}{\overset{\mathrm{def}}{=\joinrel=}}, so $x\defeq 2$ should do what you want. — Algeboy, Feb 25 '22 at 05:40

score 54 · Accepted Answer · edited Mar 09 '24 at 19:52

54

Many do, but then the colon isn't correctly centered. Better to use the package mathtools and \coloneq. See the mathtools documentation for more information.

Note: In Summer 2022, this macro was changed to \coloneq (the older \coloneqq is still supported as legacy duplicate name).

edited Mar 09 '24 at 19:52

postylem

276

answered Aug 01 '14 at 20:17

anon

556

2

Welcome to TeX.SX! – jub0bs Aug 01 '14 at 20:26

Bernard · Answer 2 · 2020-06-30T12:23:44.393

29

Unfortunately, with $:=$ , the colon is not centred on the math axis. A solution comes from the mathtools package, which defines \vcentcolon and \coloneqq commands; these lead to two different results, as regards the horizontal spacing between the colon and the equality sign. Compare:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\begin{document}
Let  $ a:  = b $
Let  $ a \vcentcolon  = b $
Let  $ a \coloneqq  b $
\end{document}

enter image description here

The \coloneq(one q!) symbol exists in mathabx, Mnsymbol and fdsymbol (at least). It's named \coloneqqin kpfonts and mdsymbol.

edited Jun 30 '20 at 12:23

answered Aug 01 '14 at 20:29

Bernard

271,350

thanks for this -- What is the use of \usepackage[utf8]{inputenc} here? – wonderich Feb 26 '23 at 17:43
Note, as of Summer 2022, the names for this symbol is now called \coloneq (one q) in mathtools too. With note "As is listed in the manual for newtxmath/newpxmath" in the mathtools docs – postylem Mar 09 '24 at 16:31

score -8 · Answer 3 · answered Aug 01 '14 at 20:09

-8

It is not necessary to do anything, LaTeX does it for you.

\documentclass{article}
\begin{document}
Here
\[X:=Y+Z
\]
\end{document}

answered Aug 01 '14 at 20:09

skpblack

8,904
3
31
42

12

Yes, it does, but incorrectly; in most fonts, the colon is not vertically centred (it sits on the baseline, like a regular . does). This is of course wrong since the = is vertically centred and this assymetry is seriously ugly. – yo' Aug 01 '14 at 20:49

Symbol for definition :=

3 Answers3