Drawing grid with random color in TikZ

Question

I'm trying to draw a 10x10 grid in TikZ like so:

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}[scale=1]
    \foreach \y in {0.05,0.15,...,0.95}
    \foreach \x in {0.05,0.15,...,0.95}
        \pgfmathparse{0.9*rnd+0.3}
        \definecolor{MyColor}{rgb}{\pgfmathresult,\pgfmathresult,\pgfmathresult}
        \node[fill=MyColor,minimum size=0.1] at (\x,\y) {}; 
    \draw[step=.1,help lines] (0,0) grid (1,1); 
    \draw[->] (0,1) -- (1.2,1);
    \draw[->] (0,1) -- (0,-0.2);
\end{tikzpicture}
\end{document}

Which is already quite close to what I had in mind, but the nodes don't line up properly. What am I doing wrong? Here is an image of my output:

.

Also, since I'm new to TikZ, feel free to comment on the code, in terms of readability, coding practices and so on.

Answer 1

You should not always use the node feature. Sometimes you need the coordinates of the node, if you don't it can often pay to do it "manually" (less code).

I have here compiled 2 methods, one where yours is corrected, and the other without the use of nodes. I prefer the one without nodes as it is more drawing clean, but everyone can prefer whatever style they want!

Without node

\begin{tikzpicture}[scale=1]
  \foreach \y in {0.0,0.1,...,0.9} {
      \foreach \x in {0.0,0.1,...,0.9} {
          \pgfmathparse{0.9*rnd+0.3}
          \definecolor{MyColor}{rgb}{\pgfmathresult,\pgfmathresult,\pgfmathresult}
          \path[fill=MyColor] (\x,\y) rectangle ++(0.1,0.1); 
      }
  }
  \draw[step=.1,help lines] (0,0) grid (1,1); 
  \draw[->] (0,1) -- (1.2,1);
  \draw[->] (0,1) -- (0,-0.2);
\end{tikzpicture}

As you can see you just draw a rectangle and fill it with the color of your choice.

Fixed code

What you need to ensure is correct placement, and correct size. A node usually has some space associated with the filling of the text. This space is to ensure not too tight a drawing compared to the text, compare the following two tikz drawings:

\tikz \node[draw,inner sep=0pt,outer sep=0pt] {H};
\tikz \node[draw] {H};

Therefore i have added that you do not want any outer sep, but kept an inner sep which has the size of your node. Next is your ability to place the node with respect to its size. Here anchor is your friend, it basically tells if the node should be placed at the coordinate anchoring the node at the coordinate within the node. Try for instance and change anchor=center to anchor=east. With anchor=center it takes the center coordinate in the node and place at the coordinate you specify. With anchor=east it takes the east point of the node and places at the coordinate. It is strictly not needed in the below example, but gives a good idea what it does (and it is VERY useful)

\begin{tikzpicture}[scale=1]
  \foreach \y in {0.1,0.2,...,.9} {
      \foreach \x in {0.1,0.2,...,.9} {
          \pgfmathparse{0.9*rnd+0.3}
          \definecolor{MyColor}{rgb}{\pgfmathresult,\pgfmathresult,\pgfmathresult}
          \node[fill=MyColor,inner sep=0.1cm,outer sep=0pt,anchor=center] at (\x,\y) {}; 
      }
  }
  \draw[step=.1,help lines] (0,0) grid (1,1); 
  \draw[->] (0,1) -- (1.2,1);
  \draw[->] (0,1) -- (0,-0.2);
\end{tikzpicture}

Hope this brings you in the correct direction.

Colors in tikz

Also when defining new colors you can skip a define color step:

\pgfmathparse{70*rnd+30}
\edef\tmp{\pgfmathresult}
\node[fill=white!\tmp!black] ... 

In this case you skip the \definecolor, and need not worry about that. In this case the number should be between 0 and 100. Further more you need the \edef step to keep that result. There is a lot of math going on in tikz so you need to save the math variable temporarily (which is why \edef is needed.)