comparing and cross product

Question

Oday Shahadh 2020-6-8

0
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https://ww2.mathworks.cn/matlabcentral/answers/544754-comparing-and-cross-product

回答： Walter Roberson 2020-6-8

采纳的回答： Walter Roberson

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hi guys,

why c in below return with one coulmn, it supposed to return (x,y,z,) vector?

regards

c=zeros[];
for i=1:length(p)
    if (L1(i,3)==p(i,3))
        c=[c,cross(L1(i,:),p(i,:))];
    end
end

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James Tursa 2020-6-8

Besides the fact that zeros[] is not valid MATLAB syntax, the cross product returns a vector and it sounds like that is what you are getting as a result. Can you describe the problem in more detail? Maybe with example inputs and desired output?

Oday Shahadh 2020-6-8

编辑：Walter Roberson 2020-6-8

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I two arrays A(x1,y2,z1), and B(x2,y2,z2).

I need to cross product between one row from A with all rows in B which have same z value.

for instance

a(1,2,3)

-------

b(2,4,5)

b(2,2,3)

b(7,5,9)

b(5,8,3)

cross(a,b) in case a(i,3)=b(i,3)

---------------------------------------------------------------

script:

clear
close all
format long
x=[];
y=[];
z=[];
for L=-2:0.5:2
    for r=0:.5:2
        for theta=0:45:360;
            x=[x;r*cos(theta*pi/180)];
            y=[y;r*sin(theta*pi/180)];
            z=[z,L];
            L1=[x-x,x-x,z'];
        end
    end
end
lx=L1(:,1);ly=L1(:,2);lz=L1(:,3);
z=z';
p=[x,y,z];
b=cross(L1(1,:),p(1,:));
c=zeros[];
for i=1:length(p)
    if (L1(i,3)==p(i,3))
        c=[c,cross(L1(i,:),p(i,:))];
    end
end
figure(1)
plot3(x,y,z);hold on
plot3(lx,ly,lz,'r');hold on

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Answer 1

Walter Roberson 2020-6-8

0
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Your first column of L1 is derived from something minus itself, so the first column is going to be all 0.

Your second column of L1 is derived from something minus itself, so the second column is going to be all 0.

Your third-column is generally non-zero.

Cross-product of [0; 0; something] and [0; 0; something_else] is always going to be 0

>> syms x1 x2 y1 y2
>> cross([x1;y1;z1],[x2;y2;z2])
 
ans =
 
 y1*z2 - y2*z1
 x2*z1 - x1*z2
 x1*y2 - x2*y1
 

but your x1 and x2 and y1 and y2 are all 0, and every term involves one of those variables, so every term is going to come out 0. Therefore your cross-product will come out 0 .