TikZ-PGF – Confusion Regarding + and ++ Symbols

tikz-pgf

I can't seem to figure out the difference between + and ++ relative coordinates in TikZ.

I understand ++. ++ specifies delta coordinates that are added to the last current point, the current point is then set to the new absolute current point (correct?):

\draw (0,0) -- ++(1,0) -- ++(0,1) means:

set Current Point to (0,0)
add (1,0) to Current Point = (1,0)
set Current Point to (1,0)
add Current Point (1,0) to (0,1) = (1,1)
set Current Point to (1,1)

I can't seem to figure out that + does, when I try it out it looks like the coordinates in the +(x,y) are absolute coordinates rather than delta coordinates, but the manual says otherwise

Best Answer

Your example:

\draw (0,0) -- ++(1,0) -- ++(0,1) means:

the current point is (0,0).
draw a line from the current point (0,0) to (0,0)+(1,0) (vector addition) and move the current point to (0,0)+(1,0).
next draw a line from the current point (0,0)+(1,0) to (0,0)+(1,0)+(0,1) and move the current point to (0,0)+(1,0)+(0,1).

Other examples are given as follows.

enter image description here

\documentclass[tikz,border=12pt]{standalone}

\begin{document}
\begin{tikzpicture}
    \draw (-2,-2) grid (2,2);
    \fill (110:2) circle (1pt);
    \draw (110:2) -- +(0,-1);
\end{tikzpicture}
\end{document}

Remarks for TikZ:

\draw (110:2) -- +(0,-1); The current point is (110:2). It draws a line from point (110:2) to point (110:2)+(0,-1) (vector addition). The current point is still at (110:2).
\draw (110:2) -- ++(0,-1); The current point is (110:2). It draws a line from point (110:2) to point (110:2)+(0,-1) (vector addition). The current point is moved to (110:2)+(0,-1).
\draw (110:2) +(0,-1) -- +(0,1); The current point is (110:2). It draws a line from point (110:2)+(0,-1) to point (110:2)+(0,1). The current point is still at (110:2).
\draw (110:2) ++(0,-1) -- +(0,1); The current point is (110:2)+(0,-1). It draws a line from point (110:2)+(0,-1) to point (110:2)+(0,-1)+(0,1). The current point is still at (110:2)+(0,-1).
\draw (110:2) ++(0,-1) -- ++(0,1); The current point is (110:2)+(0,-1). It draws a line from point (110:2)+(0,-1) to point (110:2)+(0,-1)+(0,1). The current point is moved to (110:2)+(0,-1)+(0,1) (which is equal to (110:2)).

Edit:

\draw (0,0) -- +(1,1) -- +(2,0) means

the current point is (0,0).
the first segment connecting (0,0) and (0,0)+(1,1).
the current point is still (0,0).
the second segment connecting the previous point (0,0)+(1,1) and (0,0)+(2,0).

The key is A -- B connects A and B with a line no matter how A and B are defined.

Related Solutions

Move a TikZ Node Below in Relative Positioning – How to Position TikZ Nodes

Just to sum up the above comments and get future readers an overview about the discussed answers:

Prerequisites:

\usetikzlibrary{positioning}

Per node distance definition:

\node (id) [below left=<x-value> and <y-value> of <reference>] {<text>};

Global distance definition:

\begin{tikzpicture}[node distance=<x-value> and <y-value>]
 \node (id) [below left=of <reference>] {<text>};
 % ...

[Tex/LaTex] Line up nested tikz enviroments or how to get rid of them

As is explained in How do I draw shapes inside a tikz node? pics can be used for defining new objects. My main problem using pics is how to place where you want because they aren't nodes and positioning them is not so easy.

Following code shows how to define EDFA block.

    EDFA/.pic={
        \begin{scope}[scale=.5]
        \draw (-1,0) coordinate (in) --  (-1,1) -- (1,0) coordinate (out) --  (-1,-1) -- cycle;
        \node[anchor=north,inner sep=2pt] at (0,-1) {$1$};
        \end{scope}

In this case, coordinate (-1,0) will act as west anchor and 1,0 as east. Both point will have an special name for further reference. Every pic is placed according its own origin (0,0). You can use Claudio's answer to Anchoring TiKZ pics for better positioning.

As your example was simple, I'd prefer to star with EDFA and place Source and Sink after it.

\documentclass[]{article}

% tikz
\usepackage{tikz}
\usetikzlibrary{positioning} %relative positioning

\begin{document}

\tikzset{%
    EDFA/.pic={
        \begin{scope}[scale=.5]
        \draw (-1,0) coordinate (in) --  (-1,1) -- (1,0) coordinate (out) --  (-1,-1) -- cycle;
        \node[anchor=north,inner sep=2pt] at (0,-1) {$1$};
        \end{scope}
    }
}

\begin{tikzpicture}[
block/.style={draw},
]

\draw pic (edfa) {EDFA};

\node[block, left=of edfain] (source) {Source};

\node[block, right= of edfaout] (sink) {Sink};

\draw[->] (source) -- (edfain);
\draw[->] (edfaout) -- (sink);

\end{tikzpicture}

\end{document}

I understand that your components are more complex than EDFA because for this particular case an isosceles triangle node with a label will do the work and it can be used as a node and not as a pic:

\documentclass[]{article}

% tikz
\usepackage{tikz}
\usetikzlibrary{positioning} %relative positioning
\usetikzlibrary{shapes.geometric}

\begin{document}

\begin{tikzpicture}[
    block/.style={draw},
    edfa/.style={isosceles triangle, minimum width=1cm, 
         draw, anchor=west, isosceles triangle stretches, 
         minimum height=1cm, label=-80:#1}
]

\node[block] (source) {Source};

\node[edfa=1, right=of source] (edfa) {};

\node[block, right= of edfa] (sink) {Sink};

\draw[->] (source) -- (edfa);
\draw[->] (edfa) -- (sink);

\end{tikzpicture}

\end{document}