Using an array as Subscript Indice to access elements in a matrix

Question

I'd like to create an array from elements in an 3D-matrix using z-coordinates stored in an another array. The simplest way would be:

X=2;
Y=3;
lastZ=10000000
for i=1:lastZ
    new_array=matrix(X,Y,Z(i));
end

But I'm looking for a "vectorized" way using only matrices instead of for-loop. I've tried the following code but I get an error message "Subscript indices must either be real positive integers or logicals":

new_array=matrix(X,Y,Z);

I understand that Z is an array and cannot be put with X and Y which are integers. Is there a better way to create such array in one-liner code?

H.Muster · Accepted Answer · 2012-07-12 16:24:13Z

3

You can try:

new_array=squeeze(matrix(X,Y,1:Z));

answered Jul 12, 2012 at 16:24

H.Muster

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4 Comments

workson Over a year ago

That's clear and clean! I've spent hours trying to understand bsxfun and arrayfun but squeeze... But I still wonder why new_array=matrix(X,Y,Z) doesn't work while the only difference in your code is the usage of squeeze to flatten the new_array? I mean is there a problem in my indexing.

H.Muster Over a year ago

If you index any dimension beyond the second, you do not get a "normal" vector. Look at it yourself by displaying matrix(X,Y,1:10). Actually, I had lots of trouble too before I stumbled upon squeeze.

workson Over a year ago

Apparently, using squeeze is much slower than a straight for loop !!! I'm on Matlab 2012a. I've read a lot about the improvement in for loops performance . It appears like my code gets worse in term of running time with every vectorization step. Maybe because of functions like find, sum or squeeze as I want to avoid any for loops.

H.Muster Over a year ago

On my system with Matlab 2009a, the squeeze version takes about a third of the time the loop version takes.

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