Otdfbt

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?

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

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

Just for future record, I want to share a tiny elaboration of what Kpym suggested in his answer. Just to avoid having the newif, it is possible to supply two more arguments to the macro. Of course, having the ifleft gives versatility in the usage (as nicely demonstrated in the Kpym's answer), but maybe this is not really needed (like in my case).

documentclass[border=1mm, tikz]standalone

makeatletter
longdefIfLeft(#1)of(#2)#3#4%
 tikz@scan@one@pointpgfutil@firstofone(#1)relax%
 pgf@xa=pgf@x%
 tikz@scan@one@pointpgfutil@firstofone(#2)relax%
 ifdimpgf@xa<pgf@xrelax#3else#4fi

makeatother

begindocument

 begintikzpicture
 node[draw] (A) at (0,0) A;
 node[draw] (B) at (1,0) B;
 node[draw] (C) at (2,0) C;

 IfLeft(A)of(B)%
 foreach n in 10,20,...,90
 fill[red!n!yellow] (0.5,0) ++(n/100,0.5) circle(1mm);
 
 %
 foreach n in 10,20,...,90
 fill[black!n!gray] (0.5,0) ++(n/100,-0.5) circle(1mm);
 
 

 IfLeft(C)of(B)%
 foreach n in 10,20,...,90
 fill[black!n!gray] (0.5,0) ++(n/100,0.5) circle(1mm);
 
 %
 foreach n in 10,20,...,90
 fill[blue!n!magenta] (0.5,0) ++(n/100,-0.5) circle(1mm);
 
 
 endtikzpicture
enddocument