Get the value of custom (pseudo?) length

Question

\DeclareFloatSeparators{halfgap}{\hskip0.5\columnsep}

and use it on a float by float basis with

\thisfloatsetup{subfloatrowsep=halfgap}

but I also need to use such length in a \fpeval calculation, is it possible?

Sorry to have misled you! I comment instead of answer when I haven't tested the solution... — musarithmia, Jun 10 '21 at 12:32

score 3 · Accepted Answer · answered Jun 10 '21 at 07:47

You might say

\newcommand{\halfgap}{0.5\columnsep}
\DeclareFloatSeparators{halfgap}{\hspace{\halfgap}}

Then you can use \halfgap wherever you need to refer to the length.

Don't follow the suggestion to say

\newlength{\halfgap}
\setlength{\halfgap}{0.5\columnsep}

because this would fix the value as it is computed when processing the document preamble, whereas \columnsep might change later.

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