Help with function main and writing cross product function

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CalamityGoat 2015-4-27

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评论： CalamityGoat 2015-4-27

So I am trying to write a function to calculate the cross product of any two vectors. I am trying to write the code to work with some test vectors I have defined both vectors as 2 dimensional for my test but in eventuality I will want the code to accept either 2 dimensional or 3 dimensional vectors. So far I have:

function main
%  vectors a = 3i + 11j; b = 14i - 7.3j;
a = [3 11]; b = [14 -7.3];
vec_c = crosspro(a, b)
end
function w = crosspro(u, v)
%  crosspro calculates the crossproduct of u cross v
    if length(u) == 2
      u = [u 0 ];
    end
    if length(v) == 2
      v = [v 0 ];
    end
i = (u(2)*v(3)-u(3)*v(2));
j = (u(3)*v(1)-u(1)*v(3));
k = (u(1)*v(2)-u(2)*v(1));
result=[i j k];
end

I get an error if length(u) == 2, I was given them as a hint based on the code I had previously. Also it says output "w" (and maybe others) are not assigned during call main>crosspro.

Can someone help explain what is going on here and also how to fix my code to work from here? Thanks a bunch!

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Answer 1

Michael Haderlein 2015-4-27

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You have to decide if you want to name the output argument "w" or "result". If you change your second last line to

w=[i j k];

things will work. Just as a comment, if you want to make sure that also column vectors work, replace

u=[u 0];

with

u(3)=0;

The same with v of course. Also, I guess this is for educational purpose only - you know the function "cross", don't you?

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CalamityGoat 2015-4-27

Excellent! That worked perfectly, it makes sense that I should have named result w. I think I was trying to make result work like I remember return; working in c++. In either case thanks for the help and explanation.

Also I do know of the cross function but thank you for mentioning it as well.

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