Finding a specific rows?

I have this Matrix:

m = [0	0	0	0	0	0	0	0
    0	3.410	0	0	0	0	0	0
    10.83	11.87	7.240	3.470	1.610	1.610	0	0
    20.99	18.23	17.72	6.010	3.790	3.790	1.100	0
    25.26	20.40	22.70	7.590	6.990	6.990	1.550	0
    22.86	19.13	24.48	8.280	9.240	9.240	2.180	0
    14.92	15.73	23.19	8.280	11.61	11.61	2.690	0
    10.83	11.87	22.18	8.280	12.79	12.79	3.010	0
    9.050	10.69	22.70	8.490	14.91	14.91	3.500	0
    9.840	11.19	23.75	8.490	16.02	16.02	3.790	0
    9.840	11.54	24.48	8.840	16.43	16.43	4.100	0
    8.070	10.20	25.18	9.180	17.73	17.73	4.750	0
    5.230	8.350	24.48	9.180	19.28	19.28	5.800	0
    3.680	6.630	25.18	8.840	22.49	22.49	6.980	2.190
    0	3.190	22.70	7.840	28.06	28.06	8.280	3.320
    0	0	13.03	6.010	34.68	34.68	10.88	5
    0	0	0	3.470	32.87	32.87	13.86	5.300
    0	0	0	0	15.14	15.14	12.27	0
    0	0	0	0	1.240	1.240	1.450	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    0	0	0	0	0	0	0	0
    40.53	25	0	0	0	0	0	0]

% * Edited by Adam Danz to allow for copy-pasting into command window

I want to find the first row where all the columns have a value below 10. How to do this? I have made an if statement, but this seems to specific and I was wondering if there would be another way.

Answer 1

[Adam Danz]

This does not require a for-loop. In fact, the last line is the solution. The first few lines just creates an example matrix where rows 3 and 13 are all below 10.

% Create a 15x7 matrix of random integers 1:20
m = randi(20,15,7); 
% Rows 3 and 13 are all below 10 (maybe others, too)
m([3,13],:) = m([3,13],:)-11; 

%find the first row where all the columns have a value below 10.
rowNumber = find(all(m < 10,2),1);

% find the first row AFTER ROW N where all columns are < 10
N = 3;  %start at row N
rowNumber = find(all(m(N:end,:) < 10,2),1)+N-1;

[JamJan]

but how to find the first row in the middle, and not the two on the top first?

[Adam Danz]

I'm not sure what you mean. Your question is "I want to find the first row where all the columns have a value below 10". That's what this does. If you want to ignore certain rows, please explain the rule you'd like to add and I can help implement it.

[JamJan]

I see, I'm sorry I meant to find the switch in the middle of the matrix. Do you know how to do that?

[Adam Danz]

What's the switch? I'm lost.

Another problem is that I cannot copy-paste your matrix into matlab (an error occurs) which makes it difficult to follow.

[Adam Danz]

This command below gives you ALL row numbers whose values are all less than 10.

rowNumber = find(all(m < 10,2))

The command below gives you the FIRST row number STARTING AT ROW N whose values are all less than 10.

N = 3;  %start at row N
rowNumber = find(all(m(N:end,:) < 10,2),1)+N-1;

[JamJan]

I'm looking for the transition of the positive values into the zero's, but I want to build in a certain threshold (let's say values lower than 10). So that means that I want to find the following row in matrix m.

    0	0 0 0 1.240 1.240 1.450	0
  

[Adam Danz]

I see. Here' how to compute N (the first non-zero row).

midx = all(m < 10,2);
N = find(midx == 0,1); 
rowNumber = find(all(m(N:end,:) < 10,2),1)+N-1;

Answer 2

[Mario Chiappelli]

I would use a double nested For statement and a counter variable as follows:

for i = 1:31                        % Number of rows
    counter = 0;                    % Reset the counter
    for j = 1:8                     % Number of columns
        if myMatrix(i,j) < 10
            counter += 1;
        end
        if counter == 8             % In other words when all values are under 10
            disp("Row ");
            disp(i);
            disp(" only contains numbers less than 10");
        end
    end
end

If you want it to stop after the first row is detected just add a series of break statements to break out of the for loop in the if statement that checks if the counter is equal to 8

If your matrix changes sizes over the course of the program, you are going to have to assgin the dimensions of the matrix to variables and use those as your for statement parameters.

Hope this helps :)

[Adam Danz]

Have you tried to run this? There are several errors.

• counter += 1; I think you mean counter = counter + 1;
• if myMatrix(i,j) < 10 this might work for the example given but if the dimensions of myMatrix ever changes, this might break.
• Same with i = 1:31 and j = 1:8 and counter == 8

Even after correction those errors, the algorithm doesn't work for this example matrix:

myMatrix =
     0     0     0    10    10
    10    10     0     0     0
     0     0     0     0     0