Using the if Statement in a for Loop in MATLAB - matlab

Question

I am working on a project and my aim is to color and 20 randomly generated lines of all fixed length, then count all lines crossing y=0 and color them green else color them blue.
I have come up with the code below but it doesn't work well in the if statement.
Can someone please have at look? Thank you if you can help!
Question:
How do I correct the if statement to display all the lines and count those lines crossing y = 0?
clear
clc
L = 1.5;
a = -5;
b = 5;
GLines = 0:5:5;
m = 0;
for i = 1:20
X1 = rand(1,i)*(b-a)+a;
Y1 = rand(1,i)*(b-a)+a;
Angle = rand(1,i)*360;
X2 = L*cosd(Angle) + X1;
Y2 = L*sind(Angle) + X2;
if X1(i) < L/2* sind(Angle)
m = m + 1;
plot([X1(i); X2(i)],[Y1(i); Y2(i)], '-g');
else
plot([X1(i); X2(i)],[Y1(i); Y2(i)], '-b');
end
for j = 1:length(GLines)
axis square
ylim([-5 5]);
xlim([-5 5]);
y = yline(GLines(j));
end
end
disp(m)

Answer 1

If a line crosses zero, the sign of Y1 and Y2 will be opposite, so you can do the following:
clear; clc
L = 1.5;
a = -5;
b = 5;
GLines = 0:5:5;
m = 0;
figure;
hold all;
for i = 1:20
X1 = rand*(b-a)+a;
Y1 = rand*(b-a)+a;
Angle = rand*360;
X2 = L*cosd(Angle) + X1;
Y2 = L*sind(Angle) + Y1;
if Y1*Y2 < 0 % if line crosses zero
m = m + 1;
c = 'g'; % color = green
else
c = 'b'; % color = blue
end
plot([X1; X2],[Y1; Y2],'color',c);
end
axis equal
disp(m)
which gives the follwing plot
and correctly outputs m = 2.