Merge columns in LaTeX

Question

I would like to merge column in LaTeX. I can make that but this line is incomplete because it still open on the left. Perhaps you will not understand what I mean, but this is the code that I wrote

\documentclass{article}
\usepackage{array}

\usepackage[french]{babel}

\begin{document}    \begin{table}[!h]
\begin{center}
\begin{tabular}{|c|c|c|c|} 
%\begin{tabular}{|p{2cm}|p{2.1cm}|p{3.5cm}|p{3.3cm}|p{1.6cm}|p{1.7cm}|p{1.7cm}|}
  \hline
 \bf Id Page  & \bf Politique de Partage & \bf Nombre Amis & \bf Nombre Pages qui ont reçu la pub \\
  \hline
   78 & Amis Seulement & 23&23 \\
   81&Amis Seulement&25&25\\
   65&Amis Seulement&25&25\\
   59&Amis et leurs Amis&26&51\\
   16&Tout le Monde&27&100\\
   45&Amis et leurs Amis&27&71\\
   69&Amis Seulement&27&27\\
   74&Amis Seulement&27&27\\
   99&Amis Seulement &27&27\\
   30&Amis et leurs Amis&27&64\\


   \hline
   \multicolumn{3}{|c|}{Moyenne des pages qui ont reçu  }

    \\
    \hline  
\end{tabular}
\end{center}
\bf {\caption{Publication partagée avec une page ayant un nombre minimal d'amis dans réseau 1}}
\end{table}
\end{document}

Answer 1

Leaving out column separations & for columns that contain nothing doesn't always have the desired effect when you include vertical lines:

\documentclass{article}
\begin{document}
\begin{tabular}{|*{4}{c|}}
  One & Two & Three & Four \\
  \multicolumn{3}{|c|}{OneTwoThree} \\
  One & Two & Three & Four
\end{tabular}

\bigskip

\begin{tabular}{|*{4}{c|}}
  One & Two & Three & Four \\
  \multicolumn{3}{|c|}{OneTwoThree} & \\
  One & Two & Three & Four
\end{tabular}
\end{document}​

As such, include a single & (or as many as is required, like in the last tabular) to add the final vertical line, "closing" the row.

---

Here is an updated view of your minimal working example:

\documentclass{article}
%\usepackage{array}% http://ctan.org/pkg/array
%\usepackage[french]{babel}% http://ctan.org/pkg/babel
\begin{document}
\begin{table}[!h]
  \centering % \begin{center}
  \begin{tabular}{|c|c|c|c|} 
  %\begin{tabular}{|p{2cm}|p{2.1cm}|p{3.5cm}|p{3.3cm}|p{1.6cm}|p{1.7cm}|p{1.7cm}|}
    \hline
    \bfseries Id Page  & \bfseries Politique de Partage & \bfseries Nombre Amis & \bfseries Nombre Pages qui ont reçu la pub \\
    \hline
    78 & Amis Seulement     & 23 &  23 \\
    81 & Amis Seulement     & 25 &  25 \\
    65 & Amis Seulement     & 25 &  25 \\
    59 & Amis et leurs Amis & 26 &  51 \\
    16 & Tout le Monde      & 27 & 100\\
    45 & Amis et leurs Amis & 27 &  71 \\
    69 & Amis Seulement     & 27 &  27 \\
    74 & Amis Seulement     & 27 &  27 \\
    99 & Amis Seulement     & 27 &  27 \\
    30 & Amis et leurs Amis & 27 &  64 \\
    \hline
    \multicolumn{3}{|c|}{Moyenne des pages qui ont re\c{c}u  } & \\ % <---- inserted &
    \hline  
  \end{tabular}
  %\end{center}
  \caption{Publication partag\'ee avec une page ayant un nombre minimal d'amis dans r\'eseau 1}
\end{table}
\end{document}​

Some comments regarding your table construction:

• Don't use the center environment (\begin{center}...\end{center}). Rather use \centering. See Should I use center or centering for figures? - this holds for tables as well;
• Use \bfseries switch or \textbf{...} instead of \bf. See Will two-letter font style commands (\bf, \it, ...) ever be resurrected in LaTeX?;
• For bold formatting of captions, use the formatting options of the caption package;
• You table doesn't fit within the regular text block. In the above image output of your example, the caption is set within the text margins, giving an indication of how much the table extends beyond the margins. You may consider using the tabularx package to correct for this, or modify your layout all-together.