The error "KaTeX parse error: Invalid size" occurred when using \\ [

[ZigZagK]

Describe the bug:

When using \\[ such as:

a+b=c\\
[f(x)]^2

The error KaTeX parse error: Invalid size: 'f(x)' at position 9: a+b=c\\ [̲f̲(̲x̲)̲]̲^2 occurred.

To Reproduce:
Can be reproduced at https://katex.org/.

Environment (please complete the following information):

• KaTeX Version: 0.16.3

[ronkok]

That is not a bug. KaTeX is acting in precisely the same way that LaTeX acts with this input. When you write \\[,KaTeX is expecting you to specify a line skip distance, e.g., \\[2em].

There are many work arounds. For instance

a+b=c\\
{}[f(x)]^2

[ZigZagK]

Thank you for replying.
However, I think it's a bit strange that \\[ is considered to be a line skip distance when \\ and [ are not even in the same line.
And it can be parsed in MathJax successfully, so I still think it should be improved.
Thanks again!

[edemaine]

@ronkok I thought the same, but then I tested the following in LaTeX:

\begin{align*}
a+b=c \\
[f(x)]^2
\end{align*}

In fact, \\ seems to detect the whitespace before the [, and not treat it as an optional argument in this case.

In fact, the following KaTeX code seems to be trying to implement exactly this:

KaTeX/src/environments/array.js

Lines 221 to 223 in 2c13d26

	if (parser.gullet.future().text !== " ") {
	size = parser.parseSizeGroup(true);
	}

In fact, the above example works in KaTeX! What doesn't work is \\ outside of an align environment. Technically that shouldn't work at all in LaTeX (can't write $a \\ b$ for example), but we've accepted it as a kind of extension (inspired by MathJax).

It looks like the culprit is src/functions/cr.js, which uses numOptionalArgs to fetch the optional argument. It does seem that \@ifnextchar consumes spaces in LaTeX, so the default processing of optional arguments is correct. AMSMath's definition of \math@cr is just special to avoid consuming spaces, via \new@ifnextchar.

[ronkok]

Resolved via PR #3746.

[KaTeX-bot]

🎉 This issue has been resolved in version 0.16.4 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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