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# How to count the lines crossing the y axis?

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Maaz Madha on 29 Mar 2019
Commented: Star Strider on 29 Mar 2019
Hi.
In my project I have to plot 50 lines randomly(length 3 and random angle) and to count the number of lines crossing the the y axis(y=0) and then compare the lines crossing the y axis. I have used for loops and managed to plot the 50 lines with length 3 and various angles but am struggling to count the number of lines crossing the y axis.
Thank you for any help
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Maaz Madha on 29 Mar 2019
yes you are correct im a fool it is meant to be x=0. What I meant was that find the ratio of the lines crossing x=0 vs the total lines there are(50)

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### Answers (2)

Jan on 29 Mar 2019
Edited: Jan on 29 Mar 2019
If you want to identify if a line crosses the y axis, check if the x component of the start point has another sign than the end point.
sum(sgn(x1) == sgn(x2)) / numel(x1) % Ratio of lines crossing y axis
Note that this catchs also line touching the y axis. Here I assume, that x1 contains the x positions of the startpoint and x2 the end poisitions.
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Star Strider on 29 Mar 2019
Try this:
x = randi([-10 10], 50, 2); % X-Coordinates
y = randi([-10 10], 50, 2); % Y-Coordinates
crossYaxis = nnz(prod(x,2) <= 0) % Number Of Lines Crossing Y-Axis
figure
hold all
for k = 1:size(x,1)
plot(x(k,:), y(k,:))
end
grid
Experiment to get the result you want.
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Star Strider on 29 Mar 2019
The nnz function counts the number of non-zero values in an array.
The ‘prod(x,2)’ call takes the product of the ‘x’ matrix across the rows (the second dimension in MATLAB), multiplying the first column by the second column element-wise.
The plot call here in the loop plots the lines individually, rather than as one continuous line. The plot call is optional

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