Htydjtk

I made a grid with TikZ. I would like to add the cell number in each cell (in the middle of each cell),
I used this command

draw (-4,2.5) node[below] {$Z_1$};

But this solution is time consuming. Is there any efficient way to add cell identifier to each cell?

documentclass[tikz]{standalone}

usepackage{graphicx} % for example images



begin{document}

    begin{tikzpicture}[every label/.style={fill=white, inner sep=1mm}]



    % styles

    tikzstyle{myLabel}=[draw=black, circle, fill=white]

    tikzstyle{myLine}=[draw=blue,  double]



    draw (0,0) node[inner sep=0] {includegraphics[width=10cm]{example-image-a}};

    draw[step=1.2cm,blue,very thin] (-5,-2.5) grid (5,2.4);    

draw (-4,2.5) node[below] {$Z_1$};



end{tikzpicture}

end{document}

This what I want to draw:

For example:

documentclass[tikz]{standalone} % tikz loads graphicx



begin{document}

begin{tikzpicture}[

    declare function={boxW=1.2cm;},

    box/.style={minimum size=boxW,draw},

]   



foreach x in {0,...,15}

  {

  ifnumx=0

    node [box,fill=blue!20] at ({mod(x,4)*boxW}, {-ceil((x+1)/4)*boxW}) {$C_{x}$};

  else

    node [box] at ({mod(x,4)*boxW}, {-ceil((x+1)/4)*boxW}) {$C_{x}$};

  fi

  }



end{tikzpicture}

end{document}

Since your MWE includes an image, with a grid that is superimposed on it, I considered that you wanted to draw the grid over this image.

This is what I get, is that what you want?

documentclass[tikz]{standalone}

usepackage{graphicx} % for example images



begin{document}

    begin{tikzpicture}[every label/.style={fill=white, inner sep=1mm}]



    % styles

    tikzstyle{myLabel}=[draw=black, circle, fill=white]

    tikzstyle{myLine}=[draw=blue,  double]



    draw (0,0) node[inner sep=0] {includegraphics[width=10cm]{example-image-a}};

    draw[xstep=2.5cm,ystep=1.2,blue,very thin] (-5,-2.4) grid (5,2.4); 

    foreach x [evaluate=x as xx using x*2.5-3.75]in {0,...,3}{

        foreach y [evaluate =y as label using int(x-4*y),

        evaluate=y as yy using y*1.2+1.8]in {0,-1,...,-3}{



    draw (xx,yy) node[below] {$C_{label}$};

    } }  





end{tikzpicture}

end{document}

This proposal combines two answers by @percusse:

1. 
   This nice answer allows you to set up a matrix with foreach loops.


2. 
   This nice answer allows you to fill entries specified in a list.

documentclass[tikz,border=3.14mm]{standalone}

usepackage{etoolbox}

usetikzlibrary{matrix}

begin{document}

newcommand{setupmatrix}[3]{%

letmymatrixcontentempty

  foreach j in {1,...,#2}{

    foreach i [evaluate=i as k using {int(#3*(j-1)+i-1)}]

    in {1,...,#3} {%

      begingroupedefx{endgroup

         noexpandgapptonoexpandmymatrixcontent{ 

         #1_{k} &}}x

      }%

    gapptomymatrixcontent{\}%

  }

} % setupmatrix{<entry>}{<number of rows>}{<number of columns>}   

setupmatrix{Z}{4}{8}  

tikzset{myfill/.style args={#1|#2}{row #2 column #1/.style={nodes={fill=blue!30}}}} 

begin{tikzpicture}[every label/.style={fill=white, inner sep=1mm}]



    % styles

    draw (0,0) node[inner sep=0] {includegraphics[width=10cm]{example-image-a}};

    draw[step=1.2cm,blue,very thin] (-5,-2.5) grid (5,2.4);    

    matrix (a) [ampersand replacement=&,matrix of math nodes,

        nodes={minimum width=1.2cm,minimum height=1.2cm},

        myfill/.list={1|1,3|2,6|3}]{

        mymatrixcontent

    };



end{tikzpicture}

end{document}