Kdtyjb

I am new to plotting in TikZ and has come across the "Missing number error" in the following example on the line marked by %(**),

documentclass{book}

usepackage{tikz}

usetikzlibrary{calc,shapes.geometric,arrows,intersections} 

%shapes.geometric and arrows are used in other tikz graphics not included in this MCVE

begin{document}

begin{tikzpicture}[scale=0.3]

draw [name path=yaxis, ->,thick] (0,-2)--(0,12) node (yax) [above] {$Im E$};

draw [->,thick] (-20,0)--(12,0) node (xax) [right] {$Re E$};

path [name path=circular] (-15,0) arc (180:0:8);

path [name intersections={of=circular and yaxis}];

coordinate (c) at (intersection-1);

draw [->,thick] let

    p1=(c),

    n1={y1/8},              %(*)

    n2={asin(n1)}           %(**)

in (-15,0) arc (180:n2:8);   %(***)

draw let p1=(c) in (0,y1)--(7,y1);

end{tikzpicture}

end{document}

I have checked the existing questions on this site but has not yet found an answer, I tried adding braces in various ways around %(*) and %(**), with no success. I also tried to merge the two lets into one like n1={asin(y1/8)}, and tried variations of this with braces added. All of them report missing numbers error. If I replace n1in line %(**) with a number, or use n1 in place of n2 in %(***) (Not what I intended but I just want to check) there is also no errors.

Other observations,

• If I replace acos with ln, no error is reported


• If I replace y1/8 with y1, the error "Dimension too large" is reported

Edit

asin(y1/8)=acos(7/8), so I can plot what I want anyway, but I just don't understand where the error comes from.

I personally would compute the angle in a different way (assuming you do not want to do it analytically). You have a semicircle around (-7,0) of radius 8cm, of which you want to draw the path to the intersection of the y-axis. That is, we want the polar coordinates of (c)-(-7,0). The angle can be computed via atan2, which avoids having to divide by dimensions, and related errors that may occur when asking TikZ to compute the arcsin of a number larger than 1, as it occurs. atan2 is defined for any arguments, so this error cannot occur (and this way of computing the angle is arguably more intuitive). The horizontal stretch does not require calc (but of course there is nothing wrong with using calc here).

documentclass{book}

usepackage{tikz}

usetikzlibrary{calc,intersections} 

begin{document}

begin{tikzpicture}[scale=0.3]

draw [name path=yaxis, ->,thick] (0,-2)--(0,12) node (yax) [above] {$Im E$};

draw [->,thick] (-20,0)--(12,0) node (xax) [right] {$Re E$};

path [name path=circular] (-15,0) arc (180:0:8);

path [name intersections={of=circular and yaxis}];

coordinate (c) at (intersection-1);

draw [->,thick] let  p1=($(c)-(-7,0)$),

    n1={atan2(y1,x1)} in (-15,0) arc (180:n1:8);  

draw (c-|0,0) --  (c-|7,0);

end{tikzpicture}

end{document}

I found the answer in a closed question. To me that question is very clear what is being asked and I don't know why it was closed.

Following the 2nd comment in Tikz: Translate between pt and unit measurements, I changed n1={y1/8} to n1={y1/8cm}, and it worked as expected. This is because the internal coordinate system uses cm as the unit length but reports coordinates with unit pt when using the coordinate assignment.

Still, I don't understand why the error is "Missing number, treated as zero", but at least I know what is the correct thing to do in the future.