How do you make a matrix out of vectors in eigen?

Question

I have four column vectors. I need to append them to make a four by four matrix. Is there a constructor or something for that?

Possible duplicate of [Initialise Eigen::vector with std::vector](https://stackoverflow.com/questions/17036818/initialise-eigenvector-with-stdvector) — Fantastic Mr Fox, Sep 16 '19 at 07:21
@FantasticMrFox Eigen has its own matrices and vectors. He is asking about them I guess, not using `std::vector`. — Hakan Demir, Oct 24 '21 at 20:03
@HakanDemir it was some time ago, but i guess what i was going for was that you could use a vector to initialise it. Anyway, not a dupe. — Fantastic Mr Fox, Oct 26 '21 at 06:35

ggael · Accepted Answer · 2019-03-27T22:05:14.597

30

You can also append them using the comma initializer syntax:

m << v1, v2, v3, v4;

The matrix m must have been properly resized first.

edited Mar 27 '19 at 22:05

answered May 10 '13 at 06:25

ggael

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Does this copy the vectors `v1`, `v2`, `v3`, and `v4`? Is it possibly to create the matrix `m` using the data in the vectors without copying them? maybe a const reference or something like that? – ad_ad Nov 18 '17 at 20:12
1

What if `v{i}` is row vectors? – mrgloom Mar 27 '19 at 12:48
1

Then they be stacked up row by row, as you would expect. – ggael Mar 27 '19 at 22:05
Does not seem to work with current eigen. – oarfish Sep 17 '22 at 09:38

score 11 · Answer 2 · answered May 09 '13 at 21:04

A quick check on the docs:

Vector4f v1(1,0,0,0);
Vector4f v2(0,1,0,0);
Vector4f v3(0,0,1,0);
Vector4f v4(0,0,0,1);
Matrix4f m;

m.row(0) = v1;
m.row(1) = v2;
m.row(2) = v3;
m.row(3) = v4;

std::cout << m << std::endl;

output:

1,0,0,0
0,1,0,0
0,0,1,0
0,0,0,1

How do you make a matrix out of vectors in eigen?

2 Answers2

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