Editing Enable large arrays: Build octave such that it can use arrays larger than 2Gb.

{{Note|The following only applies to systems that have 64-bit pointers (64bit architecture).}}

Starting with Octave 4.4.0, 64-bit indexing is the default for targets with 64-bit pointers. You can override that default by specifying <code>--disable-64</code> when configuring Octave.

However, if the configure script determines that the BLAS library uses 32-bit integers, then operations using the following libraries are limited to arrays with dimensions that are smaller than 2^31 elements:
* BLAS
* LAPACK
* QRUPDATE
* SuiteSparse
* ARPACK

Additionally, the following libraries use "int" internally, so maximum problem sizes are always limited:
* glpk
* Qhull

Useful information and projects are listed below in the [[#See also|See also]] section.

To determine the integer size of the BLAS library used by Octave, the following code can be executed:

<syntaxhighlight lang="Octave">
clear all;
N = 2^31;
## The following line requires about 8 GB of RAM!
a = b = ones (N, 1, "single");
c = a' * b
</syntaxhighlight>

If the BLAS library uses '''32-bit integers''', an error will be thrown:

 error: integer dimension or index out of range for Fortran INTEGER type

Otherwise, if the BLAS library uses '''64-bit integers''', the result is:

 c = 2^31 = 2147483648

Note that the test case above usually requires twice the memory, if <code>a</code> and <code>b</code> are not assigned by <code>a = b = ...</code>.
Note further, that the data type "single" has a precision of about 23 binary bits.
In this particular example no rounding errors occur.

===Versions prior to Octave 4.4===
On previous versions of Octave, the default is that the size of a single Octave array cannot have more than approximately 2^31 elements, even on systems that use 64-bit pointers.  This is because array indices were limited to 32-bit signed integers by default.  Trying to create one will produce the following error:

<pre>
octave:1> a = zeros(1024,1024,512);
error: memory exhausted or requested size too large for range of Octave's index type -- trying to return to prompt
</pre>

You will obtain this error even if your system has enough RAM to create this array (3 GB in the above case).

To use arrays with more than (approximately) 2 billion elements, Octave has to be configured with the option --enable-64. This option is experimental and you are (as always) encouraged to submit bug reports if you find a problem. 
With this option, Octave will use internally 64-bit integers for array dimensions and indexing. However, '''all numerical libraries''' used by Octave will need to use also 64-bit integers for array dimensions and indexing, and in most cases they need to be compiled from source.

===See also===

* [https://octave.org/doc/interpreter/Compiling-Octave-with-64_002dbit-Indexing.html GNU Octave manual] -- Details on how to compile some of Octave's library dependencies for 64-bit indices.
* [[MXE]] (M Cross Environment) which takes care to compile Octave's library dependencies for 64-bit indices.

Two more lightweight solutions compared to [[MXE]] to compile Octave's library dependencies for 64-bit indices.
* https://gitlab.com/mtmiller/octave-blas64-builder
* https://github.com/octave-de/GNU-Octave-enable-64

[[Category:Building]]