Slashed zero, the “right” way?

Question

The digit 0 in our documents, such as this one (external PDF link), is slashed “the wrong way” in source code blocks (lstlisting).

How can we get it slashed “the right way” (from bottom-left to top-right) in the monospace font??

Bonus for also getting the zero in the regular (sans- and serif) text to slashed “the right way”.

Update: these are the commands used to define the style (once, context in line 168ff)…

\newcommand{\listingsinpython}{
 \definecolor{listinggray}{gray}{0.9}
 \definecolor{lbcolor}{rgb}{0.9,0.9,0.9}
 \lstset{
        backgroundcolor=\color{lbcolor},
        tabsize=4,
        rulecolor=,
        language=python,
        basicstyle=\ttfamily,
        upquote=true,
        aboveskip={1.5\baselineskip},
        columns=fixed,
        showstringspaces=false,
        extendedchars=true,
        breaklines=true,
        prebreak = \raisebox{0ex}[0ex][0ex]{\ensuremath{\hookleftarrow}},
        frame=single,
        showtabs=false,
        showspaces=false,
        showstringspaces=false,
        keywordstyle=\color[rgb]{0,0,1},
        commentstyle=\color[rgb]{0.133,0.545,0.133},
        stringstyle=\color[rgb]{0.627,0.126,0.941},
 }
}

% and use it in the document
\listingsinpython

… and get the output (context in line 52ff):

\begin{lstlisting}[language=c]
 task main() {
  SetSensorTouch(IN_1);
  OnFwd(OUT_AB, 75);
  while (Sensor(IN_1) == 0) {}
  Off(OUT_AB);
 }
 \end{lstlisting}

(Yes, the “listings in Python” and “language=c” is a bit confusing, sorry about that, organic growth of the code.)

Answer 1

The TUGboat article linked by Barbara Beeton provides an in-depth study of the ambiguous-zero problem. In LaTeX's default typeface, Computer Modern, the zero in the typewriter font is narrow and round, as opposed to a squarish capital "Oh". But you may have your pick of other monospaced typefaces.

The LaTeX Font Catalog provides a list of all the monospaced fonts that come with a full TeXLive distribution, so if you have LaTeX installed on any operating system, you can probably any of those fonts. Each page in the font catalog includes an image of the numerals.

In most cases, as the catalog shows, you simply call a font package in the preamble. Most of the typewriter font packages only redefine the default typewriter family and so do not affect the other fonts in use.

The document you linked to uses Nimbus Sans for the main body text (I might recommend TeX Gyre Heros, which is based on it). To match it with a monospaced font with the slashed zero from bottom left to top right, you could use the Anonymous Pro package like so. (I will not try to reproduce all the formatting of the original.)

\documentclass{article}

% Set the necessary font encoding and input encoding
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

% Set the default sans-serif family and make it the default for body text
\usepackage{tgheros} % or \usepackage[scaled]{helvet} for URW Nimbus Sans
\renewcommand*\familydefault{\sfdefault}

% Set the default monospaced (teletype) family
\usepackage[ttdefault=true]{AnonymousPro}

% Use a package to allow verbatim code listings
% These will be set in the default monospace family (\ttdefault)
\usepackage{fancyvrb}

\begin{document}

\section*{Programm}

Dieses Programm geht davon aus, dass Du die Motoren mit den Anschlüssen A und B und den Sensor mit dem Anschluss 1 verbindest. 
Wenn Du andere Anschlüsse verwenden möchtest, musst Du das Programm entsprechend anpassen (wenn Du magst, überlege Dir, wie).

\begin{Verbatim}
task main () {
    SetSensorTouch (IN_1);
    OnFwd (OUT_AB , 75);
    while ( Sensor (IN_1) == 0) {}
    Off( OUT_AB );
}
\end{Verbatim}

\end{document}

Answer 2

EDITED to better demonstrate the technique, using txtt as the ttfont.

FIX for lstlisting and \verb (and \texttt and \ttfamily) given in FOLLOW UP at end:

Here, I show how a simple \scalebox can be used to invert the slash through a defined macro \0. I also show how making the 0 active allows one to use the reversed version of the zero inside verbatim environments.

Obviously, one would not want 0 active all the time, but turning it on for verbatim environments is a viable way to remedy the issue cited by the OP. Alternately, in normal text, one could just invoke the reflected image as \0.

\documentclass{article}
\renewcommand*\ttdefault{txtt}
\renewcommand*\familydefault{\ttdefault} %% Only if the base font of the document is to be typewriter style
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\newcommand\0{\scalebox{-1}[1]{0}}
\begin{document}
Default look of zero: 0
\begin{verbatim}
\verbatim 0123456789
\end{verbatim}

Setting zero to its mirror image
\catcode`0=\active
\def0{\0}

Revised look of zero: 0
\begin{verbatim}
\verbatim 0123456789
\end{verbatim}

\catcode`0=12% RESET MEANING OF 0

0 restored to original, but\\
I can still call on \0 at will.
\end{document}

FOLLOW UP:

Using egreg's answer at email symbol while using package listings, I can automagically replace the reverse 0 with its mirror image in lstlisting (by using literate) and in \verb arguments (through a patch) as such.

EDITED to fix \ttfamily and \texttt, as well. Note, though, that use of ttfamily should be grouped, or the active 0 could break other things.

\documentclass{article}
\usepackage{marvosym,listings,etoolbox}
% THIS REPLACES 0 with \0 IN lstings
\lstset{literate={0}{\0}1{0\ }{\0\ }2}
% THIS REPLACES 0 WITH \0 IN verb
\patchcmd{\verb}{\dospecials}{\dospecials\atspecial}{}{}
\def\atspecial{\begingroup\lccode`~=`0%
  \lowercase{\endgroup\let~}\0%
  \catcode`0=\active}
%
\renewcommand*\ttdefault{txtt}
% COMMENT THE NEXT LINE TO TURN OFF texttt AS DEFAULT FONT FAMILY
\renewcommand*\familydefault{\ttdefault} % makes base font typewriter style
\usepackage[T1]{fontenc}
\usepackage{graphicx}
% DEFINE \0 AS MIRROR OF 0
\newcommand\0{\scalebox{-1}[1]{0}}
% FIX FOR \texttt AND \ttfamily
\let\svttfamily\ttfamily
\let\svtexttt\texttt
\catcode`0=\active
\def0{\0}
\renewcommand\ttfamily{\svttfamily\catcode`0=\active }
\renewcommand\texttt{\bgroup\ttfamily\texttthelp}
\def\texttthelp#1{#1\egroup}
\catcode`0=12 %
%
\begin{document}
Here is the font's normal 0 (which is reverse sense)\par
But it is fixed (un-reversed) in lstlisting:
\begin{lstlisting}[framexleftmargin=0mm,
                   basicstyle=\ttfamily\small,
                   breaklines,
                   columns=fullflexible]
' 0000 having 1=1--
' or 1 in (select @@version)--
' union all select @@version 0.0--
' OR 'unusual' = 'unusual'
\end{lstlisting}\par
as well as in verb:~\verb|declare @s0 varchar(8000)|

\noindent\hrulefill

%Now we try it in texttt: \texttt{foo0bar} or this {\ttfamily foo0bar},
texttt: \texttt{foo0bar}

or back to 0

ttfamily: {\ttfamily foo0bar}

or back to 0 again

\noindent\hrulefill

I didn't break verb, did I? \verb|declare @s0 varchar(8000)|
\end{document}

By commenting out the \familydefault redefinition, the result is basically what the OP is asking for: normal text unaltered in \rmfamily, but in listings, verbatim, ttfamily, and texttt, the reverse 0 is un-reversed.