Count number of references using biblatex

Question

I am creating an annotated bibliography. Is there a way of obtaining a count of the number of references in a bibliography meeting certain criteria and inserting that into a document?

For example using biblatex I create one section using:

\nocite{*} ;; include all references in .bib file
\printbibliography[keyword=MyProject,type=report,title=My project reports]

and another using:

\printbibliography[keyword=AnotherProject,type=report,title=Another project's reports]

Somewhere else in the document I would like to be able to put a table with counts of number of number of reports by project, or include these counts in text. So for example:

"This year, My Project produced XX reports, while Another Project produced YY reports."

Any ideas on how to get started on that?

Update:

Based on Maieul's excellent and quick answer, I have the following MWE:

\documentclass[oneside,oldfontcommands,9pt,a4paper]{memoir}
\usepackage{url}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=authoryear,natbib=true,backend=biber,url=false,doi=false,eprint=false,isbn=false,maxbibnames=98]{biblatex}
\addbibresource{mybiblio.bib}
\begin{document}
\nocite{*}
\chapter{Introduction}
My project produced \thearticles peer-reviewed articles and Another Project produced \theotherarticles peer-reviewed articles. \par 
Note - no numbers here.
\newcounter{articles}
\renewbibmacro*{finentry}{\stepcounter{articles}\finentry}

\printbibliography[keyword=MyProject,title=My project peer-reviewed articles,type=article]
\newcounter{otherarticles}
\renewbibmacro*{finentry}{\stepcounter{otherarticles}\finentry}
\printbibliography[keyword=AnotherProject,title=Another project peer-reviewed articles,type=article]
My project produced \thearticles peer-reviewed articles and Another Project produced \theotherarticles peer-reviewed articles. \par

This works

\end{document}

Where mybiblio.bib looks like:

@ARTICLE{Other2000, 
author = { Other, A N},
title = { my article},
journal = { Some journal },
keywords = {MyProject},
year = {2000},
number={2},
volume = {343},
pages = {230-8}
}
@article{Author1993,
author = {Author, E and Author, M},
journal = {Some Journal},
keywords = {MyProject},
number = {4},
pages = {424--428},
title = {{Another article}},
volume = {6},
year = {1993}
}
@article{Author1997a,
author = {Author, F and Author, T },
journal = {Another journal},
keywords = {AnotherProject},
title = {{A third article}},
year = {1997}
}

However, as you will see I cannot refer to the counters earlier in the document than the \printbibliography commands. Is there any way to be able to refer to the counters early on?

Answer 1

Ok, so in your preamble, put

\newcounter{refs}
\makeatletter
\defbibenvironment{counter}
  {\setcounter{refs}{0}
  \renewcommand{\blx@driver}[1]{}
  }
  {We have \therefs references}
  {\stepcounter{refs}}
\makeatother

And use \printbibliography[env=counter,options for selection]

Explanation: you create a new type of bibliography, called "counter". It defined by \defbibenvironment.

1. Type of bibliography, here "counter"
2. Secund argument : what is doing at the beginning of bibliography. In your case it's: a. Reset counter b. Say to print nothing when a entry is tested.
3. What is printing at the end of bibliography.
4. What is printing / executed at each entry.

Sorry for English. If ok, say to me. I will write a french article in my weblog.

Answer 2

You can count references in \AtDataInput. To avoid specifying the counting/filtering criteria twice, you can create categories.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[american]{babel}
\usepackage{csquotes}
\usepackage[backend=biber,style=authoryear]{biblatex}

\DeclareBibliographyCategory{primary}
\DeclareBibliographyCategory{secondary}
\defbibheading{primary}{\subsection*{Primary sources}}
\defbibheading{secondary}{\subsection*{Secondary sources}}

\newcounter{primary}
\newcounter{secondary}
\AtDataInput{%
  \ifboolexpr{ test {\ifkeyword{primary}} and not test {\ifentrytype{misc}} }
    {\addtocategory{primary}{\thefield{entrykey}}%
     \stepcounter{primary}}
    {}%
  \ifboolexpr{ test {\ifkeyword{secondary}} and not test {\iffieldundef{year}}
               and test {\ifnumless{\thefield{year}+0}{1980}} }
    {\addtocategory{secondary}{\thefield{entrykey}}%
     \stepcounter{secondary}}
    {}}

\addbibresource{biblatex-examples.bib}
\begin{document}
In this document, I cite \arabic{primary} primary works and \arabic{secondary}
secondary works.
\nocite{*}
% Filler text \parencite{aristotle:poetics,aristotle:rhetoric,nussbaum,hyman,pines}.
\printbibheading
\bibbycategory
% \bibbycategory is a shorthand for:
% \printbibliography[heading=primary,category=primary]
% \printbibliography[heading=secondary,category=secondary]
\end{document}

This approach also works with BibTeX as the backend. \ifboolexpr provides a flexible way to combine individual tests. Further details can be found in the biblatex manual.

A limitation is that \AtDataInput doesn't distinguish between reference sections. If your document uses these (via the refsection option or the refsection environment), you can make the counters section-specific using some commands from etoolbox.

\newcounter{primary}
\def\theprimary{\csuse{primary:\therefsection}}
\newcounter{secondary}
\def\thesecondary{\csuse{secondary:\therefsection}}

\AtBeginEnvironment{refsection}{%
  \csnumgdef{primary:\therefsection}{0}%
  \csnumgdef{secondary:\therefsection}{0}}

\AtDataInput{%
  \ifboolexpr{ test {\ifkeyword{primary}} and not test {\ifentrytype{misc}} }
    {\addtocategory{primary}{\thefield{entrykey}}%
     \csnumgdef{primary:\therefsection}{\csuse{primary:\therefsection}+1}}
    {}%
  \ifboolexpr{ test {\ifkeyword{secondary}} and not test {\iffieldundef{year}}
               and test {\ifnumless{\thefield{year}+0}{1980}} }
    {\addtocategory{secondary}{\thefield{entrykey}}%
     \csnumgdef{secondary:\therefsection}{\csuse{secondary:\therefsection}+1}}
    {}}

Here you should only access the counter values with \theprimary and \thesecondary.

Answer 3

I think, you could try this :

\newcounter{reports}
\renewbibmacro*{finentry}{\sttepcounter{reports}\finentry}
\printbibliography[keyword=MyProject,type=report,title=My project reports]
\newcounter{reports2}
\renewbibmacro*{finentry}{\stepcounter{reports2}\finentry}
\printbibliography[keyword=MyProject,type=report,title=My project reports]
This year, My Project produced \thereports reports, while Another Project produced \thereports2 reports.

L.1 and L. 4 : creating counters L.2 and L. 6 : increments counters at each entry of bibliography. L. 7 : prints counter

see also http://geekographie.maieul.net/Une-bibliographie-commentee