Repeating something for each “thing” in a set

As another question (repeating something a given number of times) explains, TeX is not explicitly designed for “regular” programming operations. As a result, we must resort to arcane tricks to do the seemingly simple task of repeating an operation. This answer deals with repeating an operation for each of a given set of objects.

The etoolbox package provides iteration over a comma-separated list of items, in its \docsvlist and \forcsvlist commands; they are well-described in the package documentation. The datatool package manages « databases » (of its own definition) and you may iterate over entries in such a database using the command \DTLforeach.

The forarray package defines its own « list » and « array » structures, together with commands \ForEach and \ForArray which enable a command to be executed for each element in a list or array, respectively.

The dowith defines a pair of macros \DoWith and \StopDoing that process each « thing » between them; a trivial example of use is:

\DoWith\foo a{BBB}c\StopDoing

which produces terminal output:

a+ BBB+ c+

so, the macros have found 3 « things », including one with braces around it. (The interpolated spaces come from the primitive \message command.)

The only constraint is that all commands in the enclosed stuff are « expandable » (which means, for example, that you may not use commands with optional arguments).

From the same stable (as dowith) comes the package commado, that provides commands \DoWithCSL (apply a command to each element of a comma-separated-variable list) and \DoWithBasesExts (apply a command to each of a set of files, defined by base name and « extension »). Use of these \DoWith* macros is also expandable (if the command applied to the list elements is itself expandable).

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