Recreating a \monthname macro from scratch using \ifcase

[Eclair_de_XII]

TL;DR Summary

Basically I want to recreate a \monthname macro from scratch using the (\ifcase,\or,\fi) set of macros. The problem is that my macro keeps implying that the month is April on the second input line, when TeX says that the month number is 3 from the first output line.

\def\themonth{%
\ifcase%
\month=1 {\the\month}%
\or \month=2 {\the\month}%
\or \month=3 {\the\month}%
\or \month=4 {\the\month}%
\or \month=5 {\the\month}%
\or \month=6 {\the\month}%
\or \month=7 {\the\month}%
\or \month=8 {\the\month}%
\or \month=9 {\the\month}%
\or \month=10 {\the\month}%
\or \month=11 {\the\month}%
\or \month=12 {\the\month}%
\fi%
}

This is Month \the\month\ of the Year \the\year.

This is Month \themonth{} of the Year \the\year.

What is the problem, here? Do the (\ifcase,\or) macros not work with numerical Boolean tests or something? Using the \month parameter in this macro seems to increment it by one for some reason, and adding a line to \advance\month by -1 within the macro definition does not seem to have any effect. Using the parameter in a generic macro (like \def\three{\the\month}) does not seem to affect the printed value of \month. Why is that?

Edit: Changed all month names to {\the\month} for easier trouble-shooting.

 

[DrClaude]

You are using ifcase incorrectly. You do not test for conditions, but test for the value after ifcase and it selects the corresponding case depending on its value. See https://tex.stackexchange.com/questions/17676/conditional-cases-expression

In your case, you are checking the value of \month, which is 3, so it executes the 4th condition (counting starts at 0), which is

\month=4 {\the\month}

so its sets month to 4 before returning its value.

Your code needs to be modified to something like

\def\themonth{%
\ifcase\month%
impossible%
\or January%
\or February%
...
\fi%
}

 

[Eclair_de_XII]

Oh, so it basically just chooses the case from the case list corresponding to the index \month.

I don't know how to explain this well. This is my best interpretation of it.

month=3
list_of_months=[None,1,2,3,4,5,6,7,8,9,10,11,12]

def ifcase(month):
    return list_of_months[month]