MATLAB MATRIX cell index in integer format, not row and column format

Question

In the MATLAB, a matrix cell is numbered by its row and column position. I wanted to index by the integer number.

Consider a (3,4) matrix

for i=1:length(3)
  for j =1:length(4)
    fprint(i,j)
  end
end
1,1
1,2
.
.
3,4

However, the output I am expecting when iterating through each cell is given by

for i=1:length(3)
  for j =1:length(4)
    fprint(i+j+something)
  end
end
1
2
3
4
.
.
12

Does this answer your question? [Linear indexing, logical indexing, and all that](https://stackoverflow.com/questions/32379805/linear-indexing-logical-indexing-and-all-that) — HansHirse, Sep 18 '20 at 06:31

Cris Luengo · Accepted Answer · 2021-04-09T13:07:02.230

1

This is called linear indexing. You can use the function sub2ind to convert from row and column number to linear index, and ind2sub to go the other way.

index = sub2ind(size(M),i,j);
M(i,j) == M(index)

The formula applied by sub2ind for a 2D matrix is index = i + (j-1) * size(M,1). That is, numbers increase downward along the first column, then the second column, etc.

edited Apr 09 '21 at 13:07

answered Sep 18 '20 at 02:00

Cris Luengo

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I have question. A simple code is troubling me a lot. Here is my code: `for i=1:length(3) for j=1:length(3) n = sub2ind(size(Rld),i,j); fprintf('%i\n',n) end end ` For unknown reasons , it prints only `1`, not `1..9`. Any idea what could be wrong here. – Mainland Sep 18 '20 at 02:14
1

@Mainland: `length(3)` is 1. You want to do `for i=1:3`. – Cris Luengo Sep 18 '20 at 02:15
oho.. i got it now. This is a simple yet big mistake. THanks a ton. – Mainland Sep 18 '20 at 02:16
The index is actually set to `index = i + (j - 1) * size(M,1)` – JorenV Apr 09 '21 at 10:49
@JorenV: Thanks for pointing out the typo. – Cris Luengo Apr 09 '21 at 13:07

MATLAB MATRIX cell index in integer format, not row and column format

1 Answers1