Magnitude of a matrix

Question

David Harra 2021-9-16

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1454844-magnitude-of-a-matrix

评论： Star Strider 2021-9-16

I have a 1000 x 3 matrix where each column is associated the the x,y and z-direction. For each row I want to find the magnitude such that I have a 1000 x 1 array returned.

x=randi(100,1000,3);
X=norm(x(1:1000,1:3))

I tried this but I am only only getting a single number rather than a 1000 x 3 array.

Thanks

Dave :)

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Answer 1

Star Strider 2021-9-16

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https://ww2.mathworks.cn/matlabcentral/answers/1454844-magnitude-of-a-matrix#answer_789104

在 MATLAB Online 中打开

See if the vecnorm funcion (introduced in R2017b) will do what you want —

x=randi(100,1000,3);
X=vecnorm(x,2,2)
X = 1000×1
5600
0290
2785
2734
9185
2693
6504
7316
1950
6002

.

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David Harra 2021-9-16

在 MATLAB Online 中打开

Thanks.

Could I just ask you about this line

X=vecnorm(x,2,2)

I can see this takes my matrix x as the first argument, but what does the 2,2 inside the bracket do?

Dave :)

Star Strider 2021-9-16

My pleasure!

The first 2 specifies the 2-norm (rather than others), and the second 2 specifies that the function is to take the norm across the rows (dimension 2) rather than down the columns (dimension 1, the default dimension) or other dimensions if the matrix had more than 2.

Specifying only the matrix would take the 2-norm of the columns.

.

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