how to find the first element that satisfy a condition in a matrix?

Hallo,

I have a matrix describes the difference between secuencial elements Dwf (127*2398).

now I want to find out the first element (column-wise) that satisfy the condition < 0.01 then starting from this element I want to find the first element that is < 0 and make a new matrix limited with the elements bewteen this two conditions as in the following : ( first_element : last_element +1)

I have tried some coding but it didn't work out so far, I am still a beginner with MATLAB.

hope someone could help and thatnks in advance

for i=6:127 % the rows, I must start from the 6th row
    for j = 1:2700 % the columns
first_element(:,j) = find(Dwf(:,j)> 0.01,1,'first'); 
last_element(:,j) = find(Dwf(:,j) < 0 & j > first_element,1,'first') ;
    end
end

[Stephan]

Hi ,

you use a nested loop with i & j - no use of i - why?

Could you attach your data as a .mat-file?

Usually this can be solved without for loops.

[Matt J]

make a new matrix limited with the elements bewteen this two conditions as in the following : ( first_element : last_element +1)

A matrix must have columns that are all the same length. Are you planning to fill the elements outside the interval [first_element, last_element+1] with some value (e.g., zero, NaN) so as to keep all columns at their original length?

[Rima Habib]

@Stephan, the idea was to let the programm runs for all the columns to find the requested conditions

i explained wrong above. the columns represent the time in other matrix ( that means I have 2398 values regarding specific time data stored in other matrix) but in the Dwf matrix they are actually the differences not the rows! sorry!

so i need to find the first element that satisfy the condition < 0.01 in each column then starting from this element I want to find the first element that is < 0

i used the nested loop thinking it might be easier

[Rima Habib]

@Matt J, you are right, i forgot about this essencial condition in matlab

in general for each column i want to get a matrix that meets the explained conditions so at the end i get n matrx each one satisfies : ( first_element : last_element +1).

so it doesnt have to be in one matrix.

i dont know if that possible?

Answer 1

[Bruno Luong]

Try this

for j = 1:ny
    idx_start(:,j) = find(Dwf(:,j)> 0.01,1,'first') ;   
    idx_end(:,j) = find (Dwf(:,j) < 0 & (1:nx)' > idx_start(:,j) , 1, 'first'); 
end

[Rima Habib]

thank you Bruno Luong, it finally works!

Answer 2

[KSSV]

YOu may follow something like below:

load('dwf.mat')
[nx,ny] = size(Dwf) ;
iwant = NaN(size(Dwf)) ;

for i = 1:ny
    idx1 = find(Dwf(:,i)<0.01) ; idx1 = idx1(1) ;
    idx2 = find(Dwf(:,i)<0.) ; idx2 = idx2(1) ;
    iwant(idx1:idx2,i) = Dwf(idx1:idx2,i) ;
    
end
pcolor(iwant) ;
shading interp ;
colorbar

YOu have to check the conditions properly.

[Rima Habib]

Hallo KSSV, thanks for your help

the code works for the first condition at the first colum. how can I apply it for the entire set of columns?

regarding the second condition idx2 : i want to find the index starting from idx1 so i tried to write it as the colde below but it didnt wokr too

but i also want it to be applied for the entire columns not just the first one

for i = idx1:ny

    idx2 = find(Dwf(:,i) < 0) ; idx2 = idx2(1) ;
end 

[KSSV]

It works for the entire columns....see the loop.....it is across all the columns. I have edited the code for the second condition .

[Rima Habib]

I know i saw the loop, please bare with me, I am a beginner here

why it gives me one value idx=35 instead of a value for each column, which was i want

load('dwf.mat')

[nx,ny] = size(Dwf) ;

iwant = NaN(size(Dwf)) ;



for j = 1:ny

    idx_start(:,j) = find(Dwf(:,j)> 0.01,1,'first') ;   
    idx_end(:,j) = find (Dwf(:,j) < 0 & j > idx_start(:,j) , 1, 'first'); 

end

I tried this code, for the first condition it works but for the second, i couldnt find a solution yet.