Defined an array but it is treated like an ordinary string

Question

I defined an array that contains 4 integers. I accessed the 3rd element, modified the last element then accessed it. However instead of doing array operations latex simply replaces every instance of \array with the entire array . How should I modify my code so that it works as expected?

\documentclass{article}
\usepackage{tikz}
\begin{document}
\edef\array{5,12,13,14}
\array{2} \
\array{3}=5 \
\array{3}
\end{document}

Expected:

13
5

Reality:

5,12,13,142
5,12,13,143=5
5,12,13,143

Answer 1

You can do

\def\foo{5,12,13,14}

but this won't magically provide you some array. In some scripting languages, the value which a variable is assigned to can also type the same variable. But TeX has just macros and registers and the code above just defines a macro that takes no argument.

Note that I changed your \array to \foo because \array is predefined in LaTeX and is a really, really important command for math constructions.

If you type in \foo{2}, TeX will just typeset

5,12,13,142

because that's the way it works: the macro \foo is expanded and the replacement text is simply put in the input stream.

Just because something is done some way in Perl doesn't mean that the same would work in Python. TeX language is even funnier than Perl and Python to program in.

Do you want to emulate arrays in LaTeX? Allocating, setting and changing their values? That's surely possible.

\documentclass{article}
\ExplSyntaxOn
%%% User interface
\NewDocumentCommand{\newarray}{m}
 {
  \nver_array_new:n { #1 }
 }
\NewDocumentCommand{\setarray}{mom}
 {
  \IfNoValueTF { #2 }
   {
    \nver_array_set:nn { #1 } { #3 }
   }
   {
    \nver_array_change:nnn { #1 } { #2 } { #3 }
   }
 }
\NewExpandableDocumentCommand{\getfromarray}{mm}
 {
  \nver_array_get:nn { #1 } { #2 }
 }
\NewExpandableDocumentCommand{\lengthofarray}{m}
 {
  \nver_array_length:n { #1 }
 }
%%% Public functions
\cs_new_protected:Nn \nver_array_new:n
 {
  \seq_new:c { l__nver_array_#1_seq }
 }
\cs_new_protected:Nn \nver_array_set:nn
 {
  \seq_set_split:cnn { l__nver_array_#1_seq } { , } { #2 }
 }
\cs_generate_variant:Nn \seq_set_split:Nnn { c }
\cs_new_protected:Nn \nver_array_change:nnn
 {
  \int_compare:nTF { \seq_count:c { l__nver_array_#1_seq } < #2 }
   {% we need to add empty items
    __nver_array_append:nnn { #1 } { #2 } { #3 }
   }
   {% we need to change a value
    __nver_array_change:nnn { #1 } { #2 } { #3 }
   }
 }
\cs_new:Nn \nver_array_get:nn
 {
  \seq_item:cn { l__nver_array_#1_seq } { #2 }
 }
\cs_new:Nn \nver_array_length:n
 {
  \seq_count:c { l__nver_array_#1_seq }
 }
%%% Private functions and variables
\seq_new:N \l__nver_array_in_seq
\seq_new:N \l__nver_array_out_seq
\cs_new_protected:Nn __nver_array_append:nnn
 {
  \int_step_inline:nnn
   { \seq_count:c { l__nver_array_#1_seq } + 1 } % start from the end
   { #2 - 1 } % just before the index
   { \seq_put_right:cn { l__nver_array_#1_seq } {} } % add empty items
  % now we can add the new item
  \seq_put_right:cn { l__nver_array_#1_seq } { #3 }
 }
\cs_new_protected:Nn __nver_array_change:nnn
 {% we need to change a value
  \seq_set_eq:Nc \l__nver_array_in_seq { l__nver_array_#1_seq }
  \seq_clear:N \l__nver_array_out_seq
  \int_step_inline:nn { \seq_count:N \l__nver_array_in_seq }
   {
    \int_compare:nTF { ##1 = #2 }
     {% the item we want to change
      \seq_put_right:Nn \l__nver_array_out_seq { #3 }
     }
     {% copy the existing value
      \seq_put_right:Nx \l__nver_array_out_seq { \seq_item:Nn \l__nver_array_in_seq { ##1 } }
     }
   }
  \seq_set_eq:cN { l__nver_array_#1_seq } \l__nver_array_out_seq
 }
\ExplSyntaxOff
\begin{document}
\newarray{foo}
\setarray{foo}{3,12,13,14}
Second item in \texttt{foo} is ``\getfromarray{foo}{2}''
\setarray{foo}[2]{5} % change the value of the second item
Second item in \texttt{foo} is ``\getfromarray{foo}{2}''
The array \texttt{foo} has \lengthofarray{foo} items
\setarray{foo}[10]{Tenth item!}
Tenth item in \texttt{foo} is ``\getfromarray{foo}{10}''
The array \texttt{foo} has \lengthofarray{foo} items
\end{document}

The optional argument in \setarray tells TeX that an item has to be changed. Additional empty items are inserted if the index is greater than the length of the array at the time of the call.

Answer 2

If you want to use a TiKZ array, you should define it with two braces. Once defined you can acces to the elements for reading, but I don't know how to change its value:

Note: egreg advertised that using \array is a dangerous idea, so I've changed to \myarray. Also Ulrike Fischer advertised about def and it was changed to \newcommand

\documentclass{article}
\usepackage{tikz}
\begin{document}
\newcommand\myarray{{5,12,13,14}}
\pgfmathparse{\myarray[2]}\pgfmathresult \
%\array[3]=5 \
\pgfmathparse{\myarray[0]}\pgfmathresult \
\end{document}