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Need command help for a specific matrix operation

Asked by Surjya padhi on 3 May 2012


I need the command help to do the following. Situation.

A = [2 3 4;7 8 1;3 2 7;1 9 3];

I want to square this matrix and then want to save the 2nd and 3rd row of the result matrix(4x3) matrix to another matrix of 2x3 dimension. I can do like this

B = A.^2; C = B(2:3,:); now C contains my required result.

But I want to avoid the intermediate step of first saving it in matrix B above. I want the C matrix directly from A. I need a single line command for above two statements.

I tried like following C = (A.^2)(2:3,:); But it is showing syntax error.

Please help me in this.


Surjya padhi


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3 Answers

Answer by Richard Brown on 3 May 2012

EDIT Needed to strain my eyes harder to figure out that the matrix in question had 4 rows

C = [0 1 0 0; 0 0 1 0] * A.^2;


Walter Roberson on 3 May 2012

As A is 4 x 3, and the desired result is 2 x 3, the pre-multiplier for matrix multiplication would have to be 2 x 4, not 2 x 3.

Geoff on 3 May 2012

Don't blame you for mistaking that. The use of whitespace in computer languages is just as important as in human languages, even if it's not strictly necessary. I love languages like Python that recognise this and actually make whitespace part of the syntax!

Richard Brown
Answer by Andrei Bobrov on 3 May 2012


Andrei Bobrov
Answer by Walter Roberson on 3 May 2012

This is the official mechanism for the kind of indexing of results that you want to do:

subsref(A.^2, struct('type', '()', 'subs', {{2:3, ':'}}))

You can wrap the functionality into anonymous functions to make it look more compact.


Walter Roberson

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