Sorting multiple citations with natbib

Question

I'm using natbib and an "apacite" bibliography style, in a report document class. I can easily do multiple cites. For example:

\citep{hilary2019, james2013}

Gives me:

(Hilary et al., 2019; D. A. James, 2013)

My question: I have many multiple citations like the above throughout my document, and I would like to order them by date (oldest to newest). I am not talking about the actual biblography, I am pointing to the example at the above. So for example, the above becomes:

\citep{hilary2019,james2013}

Should give me:

(D. A. James, 2013; Hilary et al., 2019)

Is there a way to automatise this? Rather than me having to go through each command and order my citations.

Any help would be great. Thank you!

Answer 1

natbib has a sort option that allows you to sort \citep/\citet calls in the order the entries appear in the bibliography.

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[sort]{natbib}
\bibliographystyle{plainnat}
\begin{filecontents}{\jobname.bib}
@book{aardvark,
  author  = {Anne Aardvark},
  title   = {Ardvarking},
  year    = {2020},
}
@book{zebra,
  author  = {Zoe Zebra},
  title   = {Zebraing},
  year    = {2010},
}
\end{filecontents}
\begin{document}
\citep{zebra,aardvark}
\citep{aardvark,zebra}
\bibliography{\jobname}
\end{document}

---

If you want to sort citations differently than the bibliography, some additional work is needed. I have demonstrated this for apalike in In-text citation sorted according to publication year with apalike bibliography style.

First we need to modify the .bst file in a way to share the expected citation sort order with us. That can be done by adding

FUNCTION {sort.again}
{
  year field.or.null sortify
  "   "
  * 
  sort.label
  *
  'sort.key$ :=
}
ITERATE {sort.again}
SORT
EXECUTE {newline$}
FUNCTION {produce.sortlist}
{
  "\sortentry{"
  cite$



"}"



write$
  newline$
}
ITERATE {produce.sortlist}

to the end of a renamed copy of the .bst file.

For plainnat.bst the modified plainnat-sortcites.bst would look like https://gist.github.com/moewew/783c43bd536de4198d5f5166c886fb61. The exact details can vary from style to style.

Then we need document-level code that can extract the sorting information from the .bbl file and use it for sorting. The following code is only slightly modified code from natbib.sty.

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[sort]{natbib}
\bibliographystyle{plainnat-sortcites}
\usepackage{etoolbox}
\makeatletter
\newcommand{\sortentry}[1]{%
  \if@filesw
    \immediate\write@auxout{\string\scNAT@aux@sortentry{#1}}%
  \fi}
\newcommand{\scNAT@aux@sortentry}{%
  \listgadd{\scNAT@bibsortlist}}
\newcommand*{\scNAT@bibsortlist}{}
\newcommand*{\scNAT@citekeys}{}
\newcommand*{\scNAT@writetocitelistsort}[1]{%
  \ifinlist{#1}{\scNAT@citekeys}
    {\ifdefvoid{\NAT@cite@list}
       {\def\NAT@cite@list{#1}}
       {\expandafter\def\expandafter\NAT@cite@list\expandafter{\NAT@cite@list,#1}}%
     \listgadd{\scNAT@foundkeys}{#1}}
    {}}
% Let's be paranoid: we might have dropped an item that
% has not made it to the bibliography yet
\newcommand*{\scNAT@writetocitelistforgotten}[1]{%
  \ifinlist{#1}{\scNAT@foundkeys}
    {}
    {\ifdefvoid{\NAT@cite@list}
       {\def\NAT@cite@list{#1}}
       {\expandafter\def\expandafter\NAT@cite@list\expandafter{\NAT@cite@list,#1}}}}
\newcommand*{\scNAT@sortcites}[1]{%
  \let\NAT@cite@list@empty
  \let\scNAT@citekeys@empty
  \let\scNAT@foundkeys@empty
  \forcsvlist{\listadd{\scNAT@citekeys}}{#1}%
  \forlistloop{\scNAT@writetocitelistsort}{\scNAT@bibsortlist}%
  \forlistloop{\scNAT@writetocitelistforgotten}{\scNAT@citekeys}%
}
\def\NAT@citex%
  [#1][#2]#3{%
  \NAT@reset@parser
  \NAT@sort@cites{#3}%
  \scNAT@sortcites{#3}%<- this is new
  \NAT@reset@citea
  @cite{\let\NAT@nm@empty\let\NAT@year@empty
    @for@citeb:=\NAT@cite@list\do
    {@safe@activestrue
     \edef@citeb{\expandafter@firstofone@citeb@empty}%
     @safe@activesfalse
     @ifundefined{b@@citeb@extra@b@citeb}{@citea%
       {\reset@font\bfseries ?}\NAT@citeundefined
                 \PackageWarning{natbib}%
       {Citation `@citeb' on page \thepage \space undefined}\def\NAT@date{}}%
     {\let\NAT@last@nm=\NAT@nm\let\NAT@last@yr=\NAT@year
      \NAT@parse{@citeb}%
      \ifNAT@longnames@ifundefined{bv@@citeb@extra@b@citeb}{%
        \let\NAT@name=\NAT@all@names
        \global@namedef{bv@@citeb@extra@b@citeb}{}}{}%
      \fi
     \ifNAT@full\let\NAT@nm\NAT@all@names\else
       \let\NAT@nm\NAT@name\fi
     \ifNAT@swa\ifcase\NAT@ctype
       \if\relax\NAT@date\relax
         @citea\NAT@hyper@{\NAT@nmfmt{\NAT@nm}\NAT@date}%
       \else
         \ifx\NAT@last@nm\NAT@nm\NAT@yrsep
            \ifx\NAT@last@yr\NAT@year
              \def\NAT@temp{{?}}%
              \ifx\NAT@temp\NAT@exlab\PackageWarningNoLine{natbib}%
               {Multiple citation on page \thepage: same authors and
               year\MessageBreak without distinguishing extra
               letter,\MessageBreak appears as question mark}\fi
              \NAT@hyper@{\NAT@exlab}%
            \else\unskip\NAT@spacechar
              \NAT@hyper@{\NAT@date}%
            \fi
         \else
           @citea\NAT@hyper@{%
             \NAT@nmfmt{\NAT@nm}%
             \hyper@natlinkbreak{%
               \NAT@aysep\NAT@spacechar}{@citeb@extra@b@citeb
             }%
             \NAT@date
           }%
         \fi
       \fi
     \or@citea\NAT@hyper@{\NAT@nmfmt{\NAT@nm}}%
     \or@citea\NAT@hyper@{\NAT@date}%
     \or@citea\NAT@hyper@{\NAT@alias}%
     \fi \NAT@def@citea
     \else
       \ifcase\NAT@ctype
        \if\relax\NAT@date\relax
          @citea\NAT@hyper@{\NAT@nmfmt{\NAT@nm}}%
        \else
         \ifx\NAT@last@nm\NAT@nm\NAT@yrsep
            \ifx\NAT@last@yr\NAT@year
              \def\NAT@temp{{?}}%
              \ifx\NAT@temp\NAT@exlab\PackageWarningNoLine{natbib}%
               {Multiple citation on page \thepage: same authors and
               year\MessageBreak without distinguishing extra
               letter,\MessageBreak appears as question mark}\fi
              \NAT@hyper@{\NAT@exlab}%
            \else
              \unskip\NAT@spacechar
              \NAT@hyper@{\NAT@date}%
            \fi
         \else
           @citea\NAT@hyper@{%
             \NAT@nmfmt{\NAT@nm}%
             \hyper@natlinkbreak{\NAT@spacechar\NAT@@open\if#1\else#1\NAT@spacechar\fi}%
               {@citeb@extra@b@citeb}%
             \NAT@date
           }%
         \fi
        \fi
       \or@citea\NAT@hyper@{\NAT@nmfmt{\NAT@nm}}%
       \or@citea\NAT@hyper@{\NAT@date}%
       \or@citea\NAT@hyper@{\NAT@alias}%
       \fi
       \if\relax\NAT@date\relax
         \NAT@def@citea
       \else
         \NAT@def@citea@close
       \fi
     \fi
     }}\ifNAT@swa\else\if#2\else\NAT@cmt#2\fi
     \if\relax\NAT@date\relax\else\NAT@@close\fi\fi}{#1}{#2}}
\makeatother
\begin{filecontents}{\jobname.bib}
@book{aardvark,
  author  = {Anne Aardvark},
  title   = {Ardvarking},
  year    = {2020},
}
@book{zebra,
  author  = {Zoe Zebra},
  title   = {Zebraing},
  year    = {2010},
}
\end{filecontents}
\begin{document}
\citep{zebra,aardvark}
\citep{aardvark,zebra}
\bibliography{\jobname}
\end{document}