How to return result as NA rather than invalid line

1 次查看（过去 30 天）
Ismat Mohd Sulaiman
评论： Dave 2021-7-27
Given the simple data below, I want to extract the result from the previous rows (L1 = rows-1, and L2 = rows-2). But L2 does not exist and MATLAB return as invalid and will not proceed. How to I return L2 = NA and allow the script to proceed?
data = [1 2 3; 4 5 6; 7 8 9; 10 11 12]
[nrows, ncols] = size(data) %determine the number of rows and columns
for rows = 1:nrows %start with row 1 to nrows (number of rows in the table)
if data(rows,2) == 5 %and if that rows is equal to what we want
disp("Row " + rows + " has number 5")
L1 = data(rows-1, 2)
L2 = data(rows-2, 2)
% The L2 row doesn't exist. Running this will return invalid. How to show L2 = NA and not be invalid
else
disp("Row " + rows + " has no number 5")
end
end

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回答（1 个）

Dave B
Dave B 2021-7-26
How about something like this?
if rows-2 > 0 && rows-2 <= nrows
L2 = data(rows-2, 2);
else
L2 = nan;
end
  2 个评论
Ismat Mohd Sulaiman
Ismat Mohd Sulaiman 2021-7-27
Yup. Thanks. After I posted, I simply added another if else to the script, and it worked. 😊
Now I am thinking how to store all the results for L1 and L2 in an array 😁
Dave
Dave 2021-7-27
Not sure where you're headed with the result, but there are a lot of solutions here that would skip the for loop altogether:
%% Alternative 1
ind = data(:,2) == 5; % Logical index of second column == 5
hit = find(ind); % Row number where second column == 5
L1 = nan(sum(ind),1); % Initialize L1 to have an item for each hit
L2 = nan(sum(ind),1); % Initialize L2 to have an item for each hit
L1(hit>1) = data(hit(hit>1)-1, 2); % L1, where the hit row is bigger than one, should get the previous row, second column
L2(hit>2) = data(hit(hit>2)-2, 2); % L2, where the hit row is bigger than two, should get two rows up, second column
% display the rows that don't have a 5:
miss = find(~ind);
disp(compose("Row %d has no number 5 (in the second column)", miss))
%% Alternative 2
L1 = [nan; data(:,2)]; % Pad the second column of data with a NaN
L2 = [nan; nan; data(:,2)]; % Pad the second column of data with two NaNs
ind = data(:,2) == 5; % true where data's second column is a 5
L1 = L1(ind); % Now just keep the items that were relevant
L2 = L2(ind);
% disp for the misses is the same as above
% * your code checked whether the second column had aa 5, if you wanted to
% check if *any* item in the row is a 5, try any(data==5, 2) for ind

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