How do I store row swap information in a vector?

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Sandeep 2012-10-22

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https://ww2.mathworks.cn/matlabcentral/answers/51487-how-do-i-store-row-swap-information-in-a-vector

Suppose I have 2 matrices A and B.. A is an NxN matrix and B is an Nx1 matrix. Now, I want to swap certain rows in A and store the information in an array P and use the array P to swap the rows in B. Is there a way to do this?

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Image Analyst 2012-10-22

I don't know what P is - is it A with "certain rows" swapped? What does "use the array P to swap the rows in B" mean. How, given only that P matrix, are you supposed to know what to do with B? If you know which rows in A were swapped and you wanted to swap the same elements in B, you wouldn't even need P at all. Please give a small example (say N = 4 or 5) showing A, P, and B.

Sandeep 2012-10-23

Ok, let me give an example.. Suppose I want to solve Ax=B using gaussian elimination with partial pivoting in 2 steps. First, I want to apply LU decomposition on A and then solve LUx=B. Now, I want to create 2 functions, one to solve LU decomposition and another one to solve LUx=B. The first function needs to store L and U in a single vector and create a P vector which stores the swap information. The second function takes L and U in a single matrix, P and B as input and then solves for x. My question is how to store the row swap information in P in function 1 and how to implement it in the second function on B.

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