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different parts of a matrix

Asked by Guido Pastore on 20 Mar 2019
Latest activity Commented on by KSSV
on 20 Mar 2019
Accepted Answer by KSSV
Given a matrix of 900 rows and 15 columns.
- How can I take the entire first column from line 3?
- Always from the first column how do I take portions of the column (for example that goes from line 33 to line 58) ??

  3 Comments

madhan ravi
on 20 Mar 2019
Illustrate with a short example.
Guillaume
on 20 Mar 2019
This sounds like homework on the basics of matlab indexing. This is covered in the Getting started tutorial and in any introductory textbook on matlab.
If you are not able to figure the answer to these on your own, I'm afraid you will struggle a lot with matlab.
I Know that if i want to get the first column of my matrix M i must do:
column_1 = M(:,1); ok
But, now i want to get another column equal to column_1 but without the first two rows.
How can i do?

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2 Answers

KSSV
Answer by KSSV
on 20 Mar 2019
 Accepted Answer

A = rand(900,15) ;
A(3,1) % entire first column from line 3
A(33:58,1) % first column 33 to line 58
Read about MATLAB indexing.

  2 Comments

thank you
KSSV
on 20 Mar 2019
This is a very basic question....we strongly advice you to read basics of MATLAB.

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Answer by Andrea Monfredini on 20 Mar 2019

A = rand(900,15);
first_col = A(3:end,1);
portion = A(33:58,1);
always remember that when you are indexing inside a matrix, the first argument refers to the rows while the second argument refers to columns. inside these arguments you can pass vectors that contain the keys for rows and columns elements you are interested in (like the number and letter of a labeled seat in a Cinema).
so in this excercize, you want the first column, so the second argument will be always a vector containing only the lement "1", while for the rows you build:
  • a vector [3 4 5 6 ... end] to take the last n_rows - 2 elements
  • a vector [33 34 35 ... 58] to take the portion of interest.

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