Finding zero's groups that are bounded in a matrix and there amounts

Hi, i would to find "islands of zeros" in a matrix. for example, if i have this matrix i will get a vector that says [4] because there is just one group of bounded zeros and it is 4 elements. The size of the vector is the number of groups.

1 2 3 4 5

1 1 0 1 2

7 0 0 0 5

1 5 6 7 8

Is there an efficient way to do it?(build in functions).

thanks

Answer 1

[Walter Roberson]

A = [
     1     2     3     4     5
     1     1     0     1     2
     7     0     0     0     5
     1     5     6     7     8
     4     0     2     0     0
     5     1     3     8     2
     ]

A = 6×5

1 2 3 4 5 1 1 0 1 2 7 0 0 0 5 1 5 6 7 8 4 0 2 0 0 5 1 3 8 2

p = regionprops(~A, 'area');
results = [p.Area]

results = 1×3

4 1 2

[Guy Sivan]

is there also a way to detect islands of variables that are smaller then a threshold and not exactly zero?

[Walter Roberson]

thresh = 2;
A = [
     1     2     3     4     5
     1     1     0     1     2
     7     0     0     0     5
     1     5     6     7     8
     4     0     2     0     3
     0     1     3     8     2
     ]

A = 6×5

1 2 3 4 5 1 1 0 1 2 7 0 0 0 5 1 5 6 7 8 4 0 2 0 3 0 1 3 8 2

BW = ~A;
BWfilt = bwareafilt(BW, [1 thresh]);
p = regionprops(BWfilt, 'area');
results = [p.Area]

results = 1×2

2 1

BWfilt

BWfilt = 6×5 logical array
   0   0   0   0   0
   0   0   0   0   0
   0   0   0   0   0
   0   0   0   0   0
   0   1   0   1   0
   1   0   0   0   0

[Guy Sivan]

I don't think we talk about the same thing with threshold.

for input

1 2 3 4 5

1 1 6 1 2

7 0 6 0 5

7 5 6 7 8

and threshold 2

i accept to get all the islands that are smaller then 2:

Therefore: [4,2].

Thanks!

[Walter Roberson]

If you want islands that are smaller than 2, then why are you expecting a 4 outcome?

Does the "2" refer to the number of islands you want? As in you want the two largest islands?

[Guy Sivan]

2 is the threshold. there are two islands of numbers (not only zero) that are smaller then 2. One is sized 4 (1,1,1,0). And the other one is 2 (0,0)

[Walter Roberson]

A = [
     1     2     3     4     5
     1     1     6     1     2
     7     0     6     0     5
     7     5     6     7     8]

A = 4×5

1 2 3 4 5 1 1 6 1 2 7 0 6 0 5 7 5 6 7 8

threshold = 2;
BWfilt = A <= threshold & A ~= 0

BWfilt = 4×5 logical array
   1   1   0   0   0
   1   1   0   1   1
   0   0   0   0   0
   0   0   0   0   0

p = regionprops(BWfilt, 'area')

p = 2×1 struct array with fields:
    Area

[p.Area]

ans = 1×2

4 2