How to include specific rows and columns of matrices into a zero matrix

Question

Stefano Russo 2023-7-12

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https://ww2.mathworks.cn/matlabcentral/answers/1994873-how-to-include-specific-rows-and-columns-of-matrices-into-a-zero-matrix

编辑： Stephen23 2023-7-12

采纳的回答： Dirk Engel

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Hi everyone,

I have a vector V (1x8) that include the number ID of some columns of a zero matrix A=[2x30000].

For the specific columns included in the vector V, I have a matrix M (2x8) that includes the values corresponding to that specific column.

Since I need this for an iterative process, is there a way to use the vector V to identify the corresponding column of the A, and then use it as a reference to insert at those specific columns the numbers of M?

Thank you in advance.

As an example:

A=zeros(2,8);
V=[1 4 7];
M=[1 2 3; 4 5 6];
ResultingA= [1 0 0 2 0 0 3 0;4 0 0 5 0 0 6 0];

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Answer 1

Dirk Engel 2023-7-12

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https://ww2.mathworks.cn/matlabcentral/answers/1994873-how-to-include-specific-rows-and-columns-of-matrices-into-a-zero-matrix#answer_1271898

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A(:, V) = M

inserts M into A at columns V.

This works if M and A have the same number of rows, and if M has as many columns as there are indices in V. Otherwise, you wil get some kind of indexing error.

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Stefano Russo 2023-7-12

Works perfectly. Thank you so much to both @Dirk Engel and @Malay Agarwal.

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Answer 2

Malay Agarwal 2023-7-12

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https://ww2.mathworks.cn/matlabcentral/answers/1994873-how-to-include-specific-rows-and-columns-of-matrices-into-a-zero-matrix#answer_1271808

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Hi @Stefano Russo,

You can do this as shown:

A=zeros(2,8);
V=[1 4 7];
M=[1 2 3 4 5 6 7 8; 9 10 11 12 13 14 15 16];
A(:, V) = M(:, V);
A
A = 2×8
     1     0     0     4     0     0     7     0
     9     0     0    12     0     0    15     0

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Stephen23 2023-7-12

编辑：Stephen23 2023-7-12

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variables.mat

"I'm attaching my variables, as I cannot make it work. It gives out this error: Index in position 2 exceeds array bounds. Index must not exceed 176."

The code given in this answer is buggy. The correct approach does not index into the RHS, as Dirk Engel showed here:

https://www.mathworks.com/matlabcentral/answers/1994873-how-to-include-specific-rows-and-columns-of-matrices-into-a-zero-matrix?s_tid=srchtitle#answer_1271898

Using your data:

S = load('variables.mat')
S = struct with fields:
     N: [2×176 double]
    nN: [1 8791 2 3 8042 4 5 7683 306 9234 305 8732 304 7851 303 9236 302 7266 301 7272 300 7774 299 7807 298 7219 297 7843 296 9079 295 9044 294 9239 293 9347 292 9382 291 9834 290 8817 289 … ]
A = zeros(2,30000);
A(:,S.nN) = S.N
A = 2×30000
   -0.0210   -0.0200    0.0200    0.0210    0.0210   -0.0210   -0.0200    0.0200         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0
    0.0100    0.0100    0.0100    0.0100   -0.0100   -0.0100   -0.0100   -0.0100         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0         0