# What's wrong with my LaTeX3 Code?

Background: I'm trying to write an L3 function which returns the nth digit from a given number's binary expansion. Getting this to work, however, has been very frustrating.

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn

\tl_new:N \tl_bin % set up variable to store binary digits

\cs_new:Npn \get_bin_digit:nn #1#2 {
\tl_set:Nn \tl_bin {\int_to_bin:n {#1}} % store binary expansion in our variable
\tl_item:Nn \tl_bin {#2} % index into the variable storing the binary expansion
}

\get_bin_digit:nn{5}{2}

\ExplSyntaxOff
\end{document}


The expected behavior is that the function call should output "0". The second digit in the binary expansion of the number 5, 101, is 0. However, instead the result I get is ''5''. I have no idea what's going on here!

You have to expanded the result of \int_to_bin:n using \tl_set:Nx. If you do only

\tl_set:Nn \tl_bin {\int_to_bin:n {#1}}


the result in \tl_bin will literally be \int_to_bin:n {5}. If you count from the left then the first token is \int_to_bin:n and the second is 5 (braces are ignored).

Also, your function is performing an assignment and should therefore be protected.

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn

\tl_new:N \tl_bin % set up variable to store binary digits

\cs_new_protected:Npn \get_bin_digit:nn #1#2 {
\tl_set:Nx \tl_bin {\int_to_bin:n {#1}} % store binary expansion in our variable
\tl_item:Nn \tl_bin {#2} % index into the variable storing the binary expansion
}

\get_bin_digit:nn{5}{2}

\ExplSyntaxOff
\end{document}


To save some typing and the temporary variable you could also define a variant of \tl_item:nn which expands the first argument.

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn

\cs_generate_variant:Nn \tl_item:nn { xn }

\cs_new_protected:Nn \get_bin_digit:nn
{
\tl_item:xn { \int_to_bin:n { #1 } } { #2 }
}

\get_bin_digit:nn { 5 } { 2 }

\ExplSyntaxOff
\end{document}