Hello, I am new to Matlab and want help to count number of rows in a matrix that belong to data subset.

For example consider a random Matrix of 12*2. I want to count the number of rows from 4th row to the 7th row using a code(that will be 4 rows). The 8th row will contain NaN or any other identifier for example -999 to show that the first data set ends at row 7 and the next dataset begins at row 9. It is also possible to include a distinguishing identifier on the row before the data subset begins. Therefore for example with respect to the following matrix, I would like to write a code that will count the number of rows starting from the first identifier i.e. NaN upto the next identifier i.e. -999 (the identifiers need not be as NaN or -999 and can be anything that helps me to distinguish between the dataset).

Thank you for the help

 % Example matrix
4 7
6 8
NaN NaN
0.2 0.5
0.3 0.9
0.1 0.7
0.6 0.4
-999 -999
1.6 1.9
2.8 6.4
5.1 8.3
2.3 4.7

Answer 1

[Jos (10584)]

Let A be your matrix, as above. I suggest you study the outcome of each step below:

tf1 = isnan(A)
tf2 = all(tf1,2)
i1 = find(tf2,1,'first')
tf3 = A ==-999
tf4 = all(tf3,2)
i2 = find(tf4,1,'first') 
ix = i1:i2
B = A(ix,:)

You can do all this is in a single step but you'll get confused ...

[Zaheer Shariff]

Thanks Jos. As you suggest I will study each of the steps and let you know if I still have any questions. Can you also kindly post how we can do it in a single step. As this a part of a larger code and it will be better if I can achieve this in one step. Thank you again.

[Jos (10584)]

You're welcome. The single step:

B = A(find(all(isnan(A),2),1,'first'):find(all(A==-999,2),1,'first'),:)

but I strongly recommend you split it up. The one-liner won't run faster but will cause headaches when debugging later on (for instance, because you (I) may have missed a bracket ...)

Answer 2

[Fangjun Jiang]

a=[4 7
6 8
NaN NaN
0.2 0.5
0.3 0.9
0.1 0.7
0.6 0.4
-999 -999
1.6 1.9
2.8 6.4
5.1 8.3
2.3 4.7];
StartRow=find(all(isnan(a),2));
EndRow=find(all(a==-999,2));
Rows=EndRow-StartRow-1