Arsthbt

Trying to implement a handy way to check if a TikZ point is right or left of another one, I came up with the following MWE.

documentclass[border=1mm, tikz]standalone

makeatletter
% prints 1 if #1.center is right than #2.center, 0 otherwise
newcommandisRight[2]
 %pgfpointdiffab gives b-a
 pgfpointdiffpgfpointanchor#2centerpgfpointanchor#1center
 pgfmathparsegreater(pgf@x,0)pgfmathresult


% 1 if #1.center is left than #2.center, 0 otherwise -> result in pgfmathresult
newcommandcheckIfLeft[2]
 %pgfpointdiffab gives b-a
 pgfpointdiffpgfpointanchor#2centerpgfpointanchor#1center%
 pgfmathparseless(pgf@x,0)

makeatother

begindocument

 begintikzpicture
 node[draw] (A) at (0,0) A;
 node[draw] (B) at (1,0) B;

 checkIfLeftAB
 ifnumpgfmathresult=1
 node at (0.5,1)A is left of B;
 fi

 %ifnumisRightBA=1
 % node at (0.5,-1)B is right of A;
 %fi
 endtikzpicture
enddocument

but I am quite unsatisfied with it. What I would like to have is something that may be used in a ifnum ... fi construct, as in the commented code (and I would like not to use packages beyond what pgf loads).

Here my questions:

1. In the commented code, what is exactly happening in the ifnum expansion, which makes the compilation fail?


2. How can I fix the isRight command in order to be able to use it together with ifnum?


3. If question 2 is tricky, how can I affect the ifnum expansion to achieve what I wish?


4. Is there in pgf a smarter/more straightforward way to check if a TikZ point is left/right of another point? My final goal would be to draw something only if a point is right/left of another.

Bonus question:

5. How (and where) is pdfstrcmp implemented, since it is harmless to use it in a ifnum ... fi construct?

How about

documentclass[border=1mm, tikz]standalone
usetikzlibrarycalc
tikzsetif left/.style n args=4insert path=%
let p1=($#1-#2$) in ifdimx1<0pt
#3 
else 
#4 
fi

begindocument

 begintikzpicture
 node[draw] (A) at (0,0) A;
 node[draw] (B) at (1,0) B;
 path[if left=(A)(B)(0.5,1) node A is left of B];
 path[if left=(B)(A)(0.5,-1) node B is left of A];
 endtikzpicture
enddocument

Here is some low level code based on this answer that do not use calc.

documentclass[tikz,border=7pt]standalone
newififleft
makeatletter
defisleft(#1)of(#2)?%
 tikz@scan@one@pointpgfutil@firstofone(#1)relax%
 pgf@xa=pgf@x%
 tikz@scan@one@pointpgfutil@firstofone(#2)relax%
 ifdimpgf@xa<pgf@xrelaxlefttrueelseleftfalsefi

makeatother

begindocument
 begintikzpicture[nodes=circle]
 node[draw] (A) at (0,0) A;
 node[draw] (B) at (2,1) right of A;
 isleft(A)of(B)?
 path (A) ifleft edge[latex-] (B) else edge[-latex] (B) fi ;
 foreach~in1,...,70
 path[ultra thin] (180*rand:1) coordinate(N)
 pgfextraisleft(N)of(A)?
 ifleft [red] else [blue] fi node[scale=2].
 (N) ifleft edge[latex-] (A) else edge[-latex] (A) fi;
 endtikzpicture
enddocument