{"id":278,"date":"2020-08-05T12:24:23","date_gmt":"2020-08-05T11:24:23","guid":{"rendered":"https:\/\/blog.yryo.net\/en\/?p=278"},"modified":"2021-01-18T23:00:34","modified_gmt":"2021-01-18T23:00:34","slug":"enforcing-latex-macro-expansion-orders-and-unwritten-rules-about-formatting-a-macro","status":"publish","type":"post","link":"https:\/\/blog.yryo.net\/en\/?p=278","title":{"rendered":"Enforcing LaTeX macro expansion orders and unwritten rules about formatting a macro"},"content":{"rendered":"\n

LaTeX document is actually a program, with text to be formatted. However they are not particularly friendly to program on. At least it takes a lot of trial and error… Here is how I tackled a problem with working with macro in LaTeX.<\/p>\n\n\n\n\n\n\n\n

Problem:<\/h2>\n\n\n\n

I recently had to generate a rather complex table with colouring rules. (essentially a ‘heatmap’ table) To do this, I have used several packages: \\colortbl<\/code>, \\color<\/code> and \\fp<\/code>. <\/p>\n\n\n\n

colortbl<\/code> and color<\/code> was used to set the colour of the cell itself using \\cellcolor{}<\/code> command. In addition, fp<\/code> was needed to compute a colour based on the value by simply doing ‘1 – decimal value’ to get the ratio between the two colours in RGB. While LaTeX document is a program, is really not designed for us, endusers, to write a program that requires computation. By default, decimal point calculation is a nightmare, thus fp<\/code> was imported to make it less painful. <\/p>\n\n\n\n

One of the troublesome aspect is how LaTeX documents are compiled. The gist of this is basically a series of token expansion. It expands top to bottom, left to right, through a series of commands. And in the context of macros, something like this becomes a nightmare:<\/p>\n\n\n\n

  \\cellcolor[rgb]{\\mycommand{#1},0,0}  <\/pre>\n\n\n\n
Something like this will probably cause an error indicating this is an invalid argument for \\cellcolor{}<\/code> as they expect three values in the range of 0.0-1.0 representing red, green, and blue. 
To solve this, you need to prevent LaTeX compiler to attempt expanding  \\cellcolor{}<\/code> before  \\mycommand <\/code>. My initial attempt was to insert \\expandafter<\/code> to make LaTeX expand the later tokens to expand first. However, that did not solve this.<\/p>\n\n\n\n
Solution:<\/h2>\n\n\n\n
The solution ended up being rather hacky but almost similar to C macro:<\/p>\n\n\n\n
\\edef\\tmp{\\noexpand\\cellcolor[rgb]{\\mycommand{#1},0,0}}%\n\\tmp% <\/pre>\n\n\n\n
UPDATE: The solution (I believe) does the following:<\/p>\n\n\n\n