Discouraging line break in an alignment

Question

In the question Generate table with dynamic number of columns the OP posed a question to which my initial reaction was "No".

However, now I've run into something interesting which, if I'm reading it right, would fit the bill. From the TeX by topic by Victor Eijkhout (25.2.3):

\noalign{\penalty100} for discouraging a page break (or line break) in between two rows (columns) of an \halign (\valign).

So if I understood the above correctly, for \valign that would read:

\noalign{\penalty100} for discouraging a line break in between two columns of an \valign.

I thought: "That's exactly what was asked for", but couldn't figure out how.

Answer 1

Just as \halign creates an alignment by specifying a prototype row, \valign creates an alignment by specifying a prototype column. Inside a \valign, & specifies the end of a row in a column, and \cr means "end-of-column"; each cell and column is typeset in (internal) vertical mode and the whole alignment is then passed to the paragraph builder (in horizontal mode).

In a \halign there's no way to slice off one column once the alignment has been built; with a \valign it's impossible to slice off a row.

morbusg example specifies an alignment in which there can be an arbitrary number of rows, each with the template \hbox{\strut#}; the alignment entries tell TeX to build a succession of three columns (each one with two rows). Columns are separated from the next by a \penalty10000 which inhibits a line break after each column (and before the first and after the last one, also).