How to count the lines crossing the y axis?

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Maaz Madha 2019-3-29

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评论： Star Strider 2019-3-29

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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Jan 2019-3-29

The y-axis is not "y=0", but "x=0". What does "then compare the lines crossing the y axis" mean? Compare with what?

Maaz Madha 2019-3-29

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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Answer 1

Jan 2019-3-29

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编辑：Jan 2019-3-29

在 MATLAB Online 中打开

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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Answer 2

Star Strider 2019-3-29

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在 MATLAB Online 中打开

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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Maaz Madha 2019-3-29

What does nnz(prod(x,2) <= 0) mean? What does prod(x,2) do etc

for k = 1:size(x,1)

plot(x(k,:), y(k,:)) also can you elaborate on what this means plz

Star Strider 2019-3-29

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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