Calculating distance between 3 points.

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gblmtc 2018-5-16

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https://ww2.mathworks.cn/matlabcentral/answers/401014-calculating-distance-between-3-points

回答： gblmtc 2018-5-17

Hi there!

I have 3 matrix, each one has the first column the X coordinate, the second column the y coordinate and the 3rd column is the z coordinate. What I have to obtain is the distances between P and D and the distance between P and A in order to make a plot and to present the differences.

P = [2 3 6  ; 6 94 20 ; 9 6 3 ; 6 3 2];
D = [5 6 9 ; 8 3 20 ; 9 8 7 ; 6 5 8];
A = [5 2 3 ; 8 9 4 ; 7 5 2 ; 9 8 5];
xi = P(:,1);
yi = P(:,2);
zi = P(:,3);
xj = D(:,1);
yj = D(:,2);
zj = D(:,3);
dist = sqrt((xi-xj).^2 + (yi-yj).^2 + (zi-zj).^2); % calculate dist between P and D
xj = A(:,1);
yj = A(:,2);
zj = A(:,3);
dist = sqrt((xi-xj).^2 + (yi-yj).^2 + (zi-zj).^2); % now it calculate dist between P and A

My teacher actually asked me to make a function which can calculate both distances and not to make 2 different formulas ( what I wrote in the code makes no sense :( ). I was thinking to use for loop in order to define xj,yj and zj because xi,yi and zi are always the same but I don't know how could I do that. Could you please give me a hint? Thanks!

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Answer 1

sloppydisk 2018-5-16

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https://ww2.mathworks.cn/matlabcentral/answers/401014-calculating-distance-between-3-points#answer_320426

在 MATLAB Online 中打开

You can just use vecnorm, right?

PD = vecnorm(P-D, 2, 2);
PA = vecnorm(P-A, 2, 2);

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gblmtc 2018-5-17

Thanks sloppydisk :) Yes, it works! The teacher said to use the formula for distance because that's what we were studying. I'll ask him anyway! Thanks again!

sloppydisk 2018-5-17

编辑：sloppydisk 2018-5-17

Ah I think he wants you to write a function like vecnorm yourself. Check out how to do that by reading this link. You can use code similar to what you wrote, except wrapped inside a function getting two nx3 matrices as input.

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Answer 2

gblmtc 2018-5-17

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https://ww2.mathworks.cn/matlabcentral/answers/401014-calculating-distance-between-3-points#answer_320570

Yes, he wants me to think a function that calculate the both distances. Actually he wants me to figure out how to define xj, yj and zj in order to be ones the D matrix and after that to be A matrix. Thanks for your help!