In LyX, how do you remove the brackets around a matrix after they're already there?

Question

I used the GUI to make a matrix with round brackets, and now I want to remove the round brackets (or change them to square brackets), how do I do that?

Answer 1

A very dirty workaround.

Suppose you have myfile.lyx:

Save it as myfile2.lyx to have a backup in case of error.

Close LyX and open myfile.lyx with a text editor (like Notepad++ or similar), you'll have this:

#LyX 2.2 created this file. For more info see http://www.lyx.org/
\lyxformat 508
\begin_document
\begin_header
\save_transient_properties true
\origin unavailable
\textclass article
\use_default_options true
\maintain_unincluded_children false
\language english
\language_package default
\inputencoding auto
\fontencoding global
\font_roman "default" "default"
\font_sans "default" "default"
\font_typewriter "default" "default"
\font_math "auto" "auto"
\font_default_family default
\use_non_tex_fonts false
\font_sc false
\font_osf false
\font_sf_scale 100 100
\font_tt_scale 100 100
\graphics default
\default_output_format default
\output_sync 0
\bibtex_command default
\index_command default
\paperfontsize default
\use_hyperref false
\papersize default
\use_geometry false
\use_package amsmath 1
\use_package amssymb 1
\use_package cancel 1
\use_package esint 1
\use_package mathdots 1
\use_package mathtools 1
\use_package mhchem 1
\use_package stackrel 1
\use_package stmaryrd 1
\use_package undertilde 1
\cite_engine basic
\cite_engine_type default
\biblio_style plain
\use_bibtopic false
\use_indices false
\paperorientation portrait
\suppress_date false
\justification true
\use_refstyle 1
\index Index
\shortcut idx
\color #008000
\end_index
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\paragraph_indentation default
\quotes_language english
\papercolumns 1
\papersides 1
\paperpagestyle default
\tracking_changes false
\output_changes false
\html_math_output 0
\html_css_as_file 0
\html_be_strict false
\end_header

\begin_body

\begin_layout Standard
I want this without round brackets: 
\begin_inset Formula 
\[
\begin{pmatrix}a & b\\
c & d
\end{pmatrix}
\]

\end_inset


\end_layout

\begin_layout Standard
I want this with square brackets: 
\begin_inset Formula 
\[
\begin{pmatrix}a & b\\
c & d
\end{pmatrix}
\]

\end_inset


\end_layout

\end_body
\end_document

Now change pmatrix with array if you don't want brackets or pmatrix with bmatrix if you want square brackets, you'll get this:

#LyX 2.2 created this file. For more info see http://www.lyx.org/
\lyxformat 508
\begin_document
\begin_header
\save_transient_properties true
\origin unavailable
\textclass article
\use_default_options true
\maintain_unincluded_children false
\language english
\language_package default
\inputencoding auto
\fontencoding global
\font_roman "default" "default"
\font_sans "default" "default"
\font_typewriter "default" "default"
\font_math "auto" "auto"
\font_default_family default
\use_non_tex_fonts false
\font_sc false
\font_osf false
\font_sf_scale 100 100
\font_tt_scale 100 100
\graphics default
\default_output_format default
\output_sync 0
\bibtex_command default
\index_command default
\paperfontsize default
\use_hyperref false
\papersize default
\use_geometry false
\use_package amsmath 1
\use_package amssymb 1
\use_package cancel 1
\use_package esint 1
\use_package mathdots 1
\use_package mathtools 1
\use_package mhchem 1
\use_package stackrel 1
\use_package stmaryrd 1
\use_package undertilde 1
\cite_engine basic
\cite_engine_type default
\biblio_style plain
\use_bibtopic false
\use_indices false
\paperorientation portrait
\suppress_date false
\justification true
\use_refstyle 1
\index Index
\shortcut idx
\color #008000
\end_index
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\paragraph_indentation default
\quotes_language english
\papercolumns 1
\papersides 1
\paperpagestyle default
\tracking_changes false
\output_changes false
\html_math_output 0
\html_css_as_file 0
\html_be_strict false
\end_header

\begin_body

\begin_layout Standard
I want this without round brackets: 
\begin_inset Formula 
\[
\begin{array}a & b\\% <-- changed
c & d
\end{array}% <-- changed
\]

\end_inset


\end_layout

\begin_layout Standard
I want this with square brackets: 
\begin_inset Formula 
\[
\begin{bmatrix}a & b\\% <-- changed
c & d
\end{bmatrix}% <-- changed
\]

\end_inset


\end_layout

\end_body
\end_document

Save it and close the text editor.

Open it with LyX, et voilà:

Pay attention: do not change anything else, unless you know what you're doing, and always make a backup copy of your .lyx document before doing these things.

Answer 2

1. Highlight the matrix in the LyX GUI.
2. Copy and paste it into a text editor.
3. Edit "pmatrix" to "matrix" (no brackets) or "bmatrix" (square brackets),
4. Paste back into the LyX GUI.