Otdfbt

Fellows and folks!

I have been fighting to draw what's on the picture for a long time:

I cannot draw those 3 arrows that are pointing to the same comment. Plus, the other two arrows that are pointing to letters "i" and "j". Could you please give me a hand?

This is what I have done so far:

​`beginfigure[H]
centering
begintikzpicture
matrix (A) [matrix of math nodes, nodes in empty cells,
nodes=draw, minimum width=8mm, minimum height=5mm, outer sep=0pt, anchor=center,
row sep=-pgflinewidth, column sep=-pgflinewidth,
row 3/.style = nodes=minimum height=8mm,
row 5/.style = nodes=minimum height=8mm,
column 3/.style = nodes=minimum width=10mm,
column 4/.style = nodes=fill=lightgray,
column 6/.style = nodes=fill=lightgray,
column 7/.style = nodes=minimum width=10mm,]
R & & ;
foreach i [count=xi] in 1,2,dots,$i$, ,$j$,dots,$m-1$,$m$
node[above, font=scriptsize] at (A-1-xi.north) istrut;
foreach i [count=xi] in 1,2,vdots,$u$,vdots,$n-1$,$n$
node[left, font=scriptsize] at (A-xi-1.west) istrut;
foreach i in 2,4,7
draw[dashed] ([shift=(.5mm,-.5mm)]A-i-4.north west) rectangle ([shift=(-.5mm,.5mm)]A-i-6.south east);
endtikzpicture
captionitem based 
endfigure`

It does not look good at all. Plus I have done the other version, where I have to calculate the similarity between rows. All I have to do is to transpose the matrix:

beginfigure[H]
centering
begintikzpicture
matrix (A) [matrix of math nodes, nodes in empty cells,
nodes=draw, minimum width=8mm, minimum height=5mm, outer sep=0pt, anchor=center,
row sep=-pgflinewidth, column sep=-pgflinewidth,
row 3/.style = nodes=minimum height=8mm,
row 5/.style = nodes=minimum height=8mm,
column 3/.style = nodes=minimum width=10mm,
column 4/.style = nodes=fill=lightgray,
column 6/.style = nodes=fill=lightgray,
column 7/.style = nodes=minimum width=10mm,]
R & & ;
foreach i [count=xi] in 1,2,dots,$i$, ,$j$,dots,$m-1$,$m$
node[above, font=scriptsize] at (A-1-xi.north) istrut;
foreach i [count=xi] in 1,2,vdots,$u$,vdots,$n-1$,$n$
node[left, font=scriptsize] at (A-xi-1.west) istrut;
foreach i in 2,4,7
draw[dashed] ([shift=(.5mm,-.5mm)]A-i-4.north west) rectangle ([shift=(-.5mm,.5mm)]A-i-6.south east);
endtikzpicture
captionitem based 
endfigure

Here's one possible option, using the fit and positioning libraries, and relative coordinates for the arrows on top. See comments in the code, ask if I should expand on anything.

documentclass[border=5mm]standalone
usepackagetikz
usetikzlibrary
matrix,
positioning, % added
fit % added

begindocument
begintikzpicture
matrix (A) [matrix of math nodes, nodes in empty cells,
nodes=draw, minimum width=8mm, minimum height=5mm, outer sep=0pt, anchor=center,
row sep=-pgflinewidth, column sep=-pgflinewidth,
row 3/.style = nodes=minimum height=8mm,
row 5/.style = nodes=minimum height=8mm,
column 3/.style = nodes=minimum width=10mm,
row 4/.style = nodes=fill=lightgray,
row 6/.style = nodes=fill=lightgray,
column 7/.style = nodes=minimum width=10mm,]
[draw,fill=blue!20];

% draw the box below
node [draw,
 text width=3cm,
 below=of A,
 name=textbox
 ] Lorem ipsum dolor sit amet etc. etc. ad infinitum;


foreach i [count=xi] in 1,2,vdots,$i$, ,$j$,vdots,$m-1$,$m$
 node[left, font=scriptsize,name=l-xi] at (A-xi-1.west) istrut;
foreach i [count=xi] in 1,2,dots,$u$,dots,$n-1$,$n$
 node[above, font=scriptsize] at (A-1-xi.north) istrut;

foreach i in 1,3,7
 
 % use the fit library instead of drawing box manually, then you get a node you can refer to
 node [fit=(A-4-i)(A-6-i), inner sep=-0.5mm, draw, dashed, name=Ri] ;
 % draw arrow from textbox node
 draw [<-] (Ri.south) -- (textbox);
 


% draw arrows from the nodes using the names defined with the modification of the loop above
% and relative coordinates. Add helper coordinate on the first arrow 
draw [<-] (l-4) -- coordinate[pos=0.6] (s1) ++(-15mm,0);
draw [<-] (l-6) -- ++(-15mm,0);
% draw arrow between the helper coordinate s1 and the point that has the x-coordinate of t-6 and the y-coordinate of s1
draw [<->] (s1) -- node[fill=white,font=scriptsize,inner sep=1pt] $s_i,j=?$ (l-6 -| s1);

endtikzpicture
begintikzpicture
matrix (A) [matrix of math nodes, nodes in empty cells,
nodes=draw, minimum width=8mm, minimum height=5mm, outer sep=0pt, anchor=center,
row sep=-pgflinewidth, column sep=-pgflinewidth,
row 3/.style = nodes=minimum height=8mm,
row 5/.style = nodes=minimum height=8mm,
column 3/.style = nodes=minimum width=10mm,
column 4/.style = nodes=fill=lightgray,
column 6/.style = nodes=fill=lightgray,
column 7/.style = nodes=minimum width=10mm,]
R & & ;

% draw the box on the right
node [draw,
 text width=3cm,
 right=of A,
 name=textbox
 ] Lorem ipsum dolor sit amet etc. etc. ad infinitum;


foreach i [count=xi] in 1,2,dots,$i$, ,$j$,dots,$m-1$,$m$
 % added name=t-xi to the following
 node[above, font=scriptsize, name=t-xi] at (A-1-xi.north) istrut;
foreach i [count=xi] in 1,2,vdots,$u$,vdots,$n-1$,$n$
 node[left, font=scriptsize] at (A-xi-1.west) istrut;
foreach i in 2,4,7
 
 % use the fit library instead of drawing box manually, then you get a node you can refer to
 node [fit=(A-i-4)(A-i-6), inner sep=-0.5mm, draw, dashed, name=Ri] ;
 % draw arrow from textbox node
 draw [<-] (Ri.east) -- (textbox);
 


% draw arrows from the nodes using the names defined with the modification of the loop above
% and relative coordinates. Add helper coordinate on the first arrow 
draw [<-] (t-4) -- coordinate[pos=0.6] (s1) ++(0,15mm);
draw [<-] (t-6) -- ++(0,15mm);
% draw arrow between the helper coordinate s1 and the point that has the x-coordinate of t-6 and the y-coordinate of s1
draw [<->] (s1) -- node[fill=white,font=scriptsize,inner sep=1pt] $s_i,j=?$ (t-6 |- s1);

endtikzpicture

enddocument

Very similar to Torbjørn T's nice answer except that the border of the comment is as in your figure (and drawn via path picture, i.e. you can make it a style if you use such things more often).

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarymatrix,fit,positioning

begindocument
begintikzpicture
matrix (A) [matrix of math nodes, nodes in empty cells,
nodes=draw, minimum width=8mm, minimum height=5mm, outer sep=0pt, 
anchor=center,row sep=-pgflinewidth, column sep=-pgflinewidth,
row 3/.style = nodes=minimum height=8mm,
row 5/.style = nodes=minimum height=8mm,
column 3/.style = nodes=minimum width=10mm,
column 4/.style = nodes=fill=lightgray,
column 6/.style = nodes=fill=lightgray,
column 7/.style = nodes=minimum width=10mm,]
R & & ;
foreach i [count=xi] in 1,2,dots,$i$, ,$j$,dots,$m-1$,$m$
node[above, font=scriptsize] at (A-1-xi.north) istrut;
foreach i [count=xi] in 1,2,vdots,$u$,vdots,$n-1$,$n$
node[left, font=scriptsize] at (A-xi-1.west) istrut;
node[right=1cm of A,align=left,text width=4cm,path picture=
draw ([xshift=1em,yshift=-pgflinewidth]path picture bounding box.north west) -] (txt)%
Some very long text about ducks, koalas and marmots which is very long and deals
with ducks, koalas and marmots. dots; 
foreach i in 2,4,7
node[draw,rounded corners,fit=(A-i-4)(A-i-6),inner xsep=-1ex,
inner ysep=-0.3ex] (F-i);
draw[-latex] (txt) -- (F-i.east);


endtikzpicture
enddocument