Hi everyone, I have a question, what does p by m by n matrix means,,,,as far as i know, m and n represents the order of matrix, number of rows and columns...what is the third one?

Hi everyone, I have a question, what does p by m by n matrix means,,,,as far as i know, m and n represents the order of matrix, number of rows and columns...what is the third one?

Answer 1

[Walter Roberson]

The first coordinate is always the number of rows, no matter what it is named.

The second coordinate is always the number of columns, no matter what it is named.

The third coordinate is always the number of planes, no matter what it is named.

So a p by m by n matrix has p rows, m columns, and n planes.

For example, 480 x 640 x 3 has 480 rows, 640 columns, and 3 planes (and probably represents an RGB image)

[Stephen23]

In the MATLAB documentation, the third dimension is named "pages" or "sheets":

https://www.mathworks.com/help/matlab/math/multidimensional-arrays.html

[Walter Roberson]

With my math / science background, I am more likely to refer to the higher dimensions as planes and hyperplanes. In computer science, "pages" usually refers to the system's fundamental unit of virtual memory allocation, which was historically 512 bytes but was increased to 4096 bytes by the early 1980s; some systems still use 4096 bytes, others use larger such as 16 kilobytes.

[Ayisha Nayyar]

Thanks a lot Walter,

thanks Stephen,

Now if i am particularly talking about the modalfit command,

fn = modalfit(frf,f,fs,mnum,Name,Value)

where frf is:

Frequency-response functions, specified as a vector, matrix, or 3-D array. frf has size p-by-m-by-n, where p is the number of frequency bins, m is the number of response signals, and n is the number of excitation signals used to estimate the transfer function.

in my case, I have 2500 frequency bins, 21 points response data (at all 2500 frequencies) and only one excitation signal, so my frf should be of the order;

frf=2500 x 21 x 1

is this alright??? can i write;

How to adjust this third dimension, which is 1?

[Ayisha Nayyar]

more precisely, I want to ask, how to make a 3D matrix??? should i use cat? if yes; in which syntax????

[Ayisha Nayyar]

Thanks a lot Torsten, really obliged.

[Torsten]

A = zeros(p,m,n)

[Walter Roberson]

2500 x 21 x 1 looks like it would be a valid size for frf.

Every 2D array is also a 3D array, 4D array, 5D array, and so on, so you do not need to somehow signal to MATLAB that your 2500 x 21 is "really" 3D.

For example look at the first example, https://www.mathworks.com/help/signal/ref/modalfit.html#bvkw6bh-4 . At the time of the modalfit, u is 1 x 3000, and u' is passed in the first parameter, so modalfit is being passed an frf which is 3000 x 1, also known as 3000 x 1 x 1.

[Ayisha Nayyar]

Really obliged Walter....