Find the first element in an array

459 views (last 30 days)
Miguel Albuquerque
Miguel Albuquerque on 16 Jul 2022
Commented: Voss on 16 Jul 2022
Hey guys, thanks in advance.
I want to find the first element and the last element, besides Nan in the matrix attached how can I do that?
This just gives me Nan
distance_matrix(1);
distance_matrix(2);
  7 Comments
Miguel Albuquerque
Miguel Albuquerque on 16 Jul 2022
Edited: Miguel Albuquerque on 16 Jul 2022
Hey I have done this way, based on your code
c=3e8;
However last line:
tshift(first_idx:last_idx)=((first_idx:last_idx)-idx2).^2./(R0.*c);
This doesn´t calculate well the tshif, I want thar idx2= difference between col and colofMin, basically a distance between the column where I am and the column corresponding to the minimum value of the matrix.
How can I do this calculation but just for between first_idx and last_idx
[val,idx] = min(distance_matrix);
for col = 1 : columns
thisColumn = distance_matrix(:, col);
idx2=col-idx;
tshift(:,col)=(idx2.^2)./ (val.*c);
The entire code:
[rows,columns]=size(distance_matrix);
[R0,ixR0]=min(distance_matrix(distance_matrix ~= 0));
first_idx = find(distance_matrix, 1, 'first');
last_idx = find(distance_matrix, 1, 'last');
[rowOfMin, colOfMin] = find(distance_matrix == R0); % Find row and col of min.
tshift=zeros(rows,columns);
for col = 1 : columns
thisColumn = distance_matrix(:, col);
idx2=col-colOfMin;
tshift(first_idx:last_idx)=((first_idx:last_idx)-idx2).^2./(R0.*c);
end
Voss
Voss on 16 Jul 2022
That line doesn't belong in a loop. Move it out of the loop like I had it, and it seems to do what you want.
Here it is again, using the variable names you have in the latest version of your code:
load distance_matrix
c = 3e8;
distance_matrix(distance_matrix==0) = NaN;
[rows,columns]=size(distance_matrix);
[R0,ixR0] = min(distance_matrix)
R0 = 84.9718
ixR0 = 184
notNaN = ~isnan(distance_matrix);
first_idx = find(notNaN, 1, 'first')
first_idx = 184
last_idx = find(notNaN, 1, 'last')
last_idx = 276
% colOfMin is the same as ixR0 (minimum value appears only once)
% so you can use either one below in expression for tshift
[rowOfMin, colOfMin] = find(distance_matrix == R0) % Find row and col of min.
rowOfMin = 1
colOfMin = 184
% initialize tshift to NaNs the size of distance_matrix:
tshift = NaN(rows,columns);
% overwrite the elements of tshift where distance_matrix is non-NaN:
tshift(first_idx:last_idx) = ((first_idx:last_idx)-ixR0).^2./(R0.*c);
% first_idx == ixR0, so tshift(first_idx) == 0, as expected:
tshift(first_idx)
ans = 0

Sign in to comment.

Answers (1)

Image Analyst
Image Analyst on 16 Jul 2022
A little simpler:
% Load sample data.
load('distance_matrix.mat')
% Find non-nan indexes.
goodIndexes = find(~isnan(distance_matrix));
% Get the value of distance_matrix at the first non-nan index:
v1 = distance_matrix(goodIndexes(1))
v1 = 84.9718
% Get the value of distance_matrix at the last non-nan index:
v2 = distance_matrix(goodIndexes(end))
v2 = 111.3272
  1 Comment
Miguel Albuquerque
Miguel Albuquerque on 16 Jul 2022
Edited: Miguel Albuquerque on 16 Jul 2022
I created this:
If I want to calculate this just for this columns, how could I do it:
c=3e8;
first_idx=184;
last_idx=276;
calculation just for this= tshift((first_idx:last_idx)
calculatation I want to do= (idx2.^2)./(R0.*c)
[rows,columns]=size(distance_matrix);
[R0,ixR0]=min(distance_matrix(distance_matrix ~= 0));
first_idx = find(distance_matrix, 1, 'first');
last_idx = find(distance_matrix, 1, 'last');
[rowOfMin, colOfMin] = find(distance_matrix == R0); % Find row and col of min.
tshift=zeros(rows,columns);
for col = 1 : columns
thisColumn = distance_matrix(:, col);
idx2=col-colOfMin;
tshift(first_idx:last_idx)=(idx2.^2)./(R0.*c);
end

Sign in to comment.

Categories

Find more on MATLAB in Help Center and File Exchange

Tags

Products

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!