# Why the mod function does not work

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Eliska Paulikova on 22 Nov 2022
Answered: Walter Roberson on 22 Nov 2022
My code started with 0 degree but when it gets only 180 and than it get 170, 160 and so on ... I would like it to 2pi (360 degrees)
for f=1:h
t=(f-1)*k/20;
m=f+1;
P0=[70 70]; %střed [x,y]
P1=[htabulka(1,1),htabulka(1,2)]; %XY1 [x,y]
P2=[htabulka(f,1),htabulka(f,2)]; %XY2 [x,y]
%angstn=wrapTo2Pi(atan2(abs((htabulka(1,1)-60)*(htabulka(f,2)-60)-(htabulka(f,1)-60)*(htabulka(1,2)-60)),(htabulka(1,1)-60)*(htabulka(f,1)-60)+(htabulka(1,2)-60)*(htabulka(f,2)-60)));
angstn =mod(atan2(norm(det([P2-P0;P1-P0])),dot(P2-P0,P1-P0)),2*pi);
%angst= angstn * (angstn >= 0) + (angstn + 2 * pi) * (angstn < 0);
ang=angstn*180/pi
cosf=cos(ang);
##### 2 CommentsShowHide 1 older comment
Torsten on 22 Nov 2022
Is it better what you get from this code ?
for f=1:h
t=(f-1)*k/20;
m=f+1;
P1=[70 70]; %střed [x,y]
P2=[htabulka(1,1),htabulka(1,2)]; %XY1 [x,y]
P3=[htabulka(f,1),htabulka(f,2)]; %XY2 [x,y]
angstn = atan2(P3(2) - P1(2), P3(1) - P1(1)) - atan2(P2(2) - P1(2), P2(1) - P1(1));
if angstn < 0
angstn = angstn + 2*pi;
end
ang=angstn*180/pi
cosf=cos(ang);

Walter Roberson on 22 Nov 2022
angstn =mod(atan2(norm(det([P2-P0;P1-P0])),dot(P2-P0,P1-P0)),2*pi);
norm() is always non-negative, so you are taking atan2() in the case where y is positive and x is potentially either positive or negative. That is always going to give you a result between 0 and pi, so the mod() against 2*pi is always going to just return the same value.

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