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### 8.2 array

Synopsis:

\begin{array}{cols}
column 1 entry &column 2 entry ... &column n entry \\
...
\end{array}


or:

\begin{array}[pos]{cols}
column 1 entry &column 2 entry ... &column n entry \\
...
\end{array}


Produce a mathematical array. This environment can only be used in math mode, and normally appears within a displayed mathematics environment such as equation (see equation). Inside of each row the column entries are separated by an ampersand, (&). Rows are terminated with double-backslashes (see \\).

This example shows a three by three array.

\begin{equation*}
\chi(x) =
\left|              % vertical bar fence
\begin{array}{ccc}
x-a  &-b  &-c  \\
-d   &x-e &-f  \\
-g   &-h  &x-i
\end{array}
\right|
\end{equation*}


The required argument cols describes the number of columns, their alignment, and the formatting of the intercolumn regions. For instance, \begin{array}{rcl}...\end{array} gives three columns: the first flush right, the second centered, and the third flush left. See tabular for the complete description of cols and of the other common features of the two environments, including the optional pos argument.

There are two ways that array diverges from tabular. The first is that array entries are typeset in math mode, in textstyle (see Modes) except if the cols definition specifies the column with p{...}, which causes the entry to be typeset in text mode. The second is that, instead of tabular’s parameter \tabcolsep, LaTeX’s intercolumn space in an array is governed by \arraycolsep, which gives half the width between columns. The default for this is ‘5pt’ so that between two columns comes 10pt of space.

To obtain arrays with braces the standard is to use the amsmath package. It comes with environments pmatrix for an array surrounded by parentheses (...), bmatrix for an array surrounded by square brackets [...], Bmatrix for an array surrounded by curly braces {...}, vmatrix for an array surrounded by vertical bars |...|, and Vmatrix for an array surrounded by double vertical bars ||...||, along with a number of other array constructs.

The next example uses the amsmath package.

\usepackage{amsmath}  % in preamble

$$\begin{vmatrix}{cc} % array with vert lines a &b \\ c &d \end{vmatrix}=ad-bc$$


There are many packages concerning arrays. The array package has many useful extensions, including more column types. The dcolumn package adds a column type to center on a decimal point. For both see the documentation on CTAN.