How to skip an error inside a loop so the loop continues?

Question

sittmo 2014-2-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/118293-how-to-skip-an-error-inside-a-loop-so-the-loop-continues

评论： Image Analyst 2016-10-25

The following is my full code: (The part of the code that is causing me trouble is towards the end)

   clc;
          clear;
          P = xlsread('b3.xlsx', 'P');
          d = xlsread('b3.xlsx', 'd');
          CM = xlsread('b3.xlsx', 'Cov');
          Original_PD = P;                       %Store original PD
          LM_rows = size(P,1)+1;                           %Expected LM rows
          LM_columns = size(P,2);                 %Expected LM columns
          LM_FINAL = zeros(LM_rows,LM_columns);    %Dimensions of LM_FINAL
          % Start of the outside loop
          for k = 1:size(P,2)
          P = Original_PD(:,k);
          interval = cell(size(P,1)+2,1);
          for i = 1:size(P,1)
              interval{i,1} = NaN(size(P,1),2);
              interval{i,1}(:,1) = -Inf;
              interval{i,1}(:,2) = d;
              interval{i,1}(i,1) = d(i,1);
              interval{i,1}(i,2) = Inf;
          end
          interval{i+1,1} = [-Inf*ones(size(P,1),1) d];
          interval{i+2,1} = [d Inf*ones(size(P,1),1)];
          c = NaN(size(interval,1),1);
          for i = 1:size(c,1)
              c(i,1) = mvncdf(interval{i,1}(:,1),interval{i,1}(:,2),0,CM);
          end
          c0 = c(size(P,1)+1,1);
          f = c(size(P,1)+2,1);
          c = c(1:size(P,1),:);
          b0 = exp(1);
          b = exp(1)*P;
          syms x;
          eqn = f*x;
          for i = 1:size(P,1)
              eqn = eqn*(c0/c(i,1)*x + (b(i,1)-b0)/c(i,1));
          end
          eqn = c0*x^(size(P,1)+1) + eqn - b0*x^size(P,1);
          x0 = solve(eqn);
          for i = 1:size(x0)
              id(i,1) = isreal(x0(i,1));
          end
          x0 = x0(id,:);
          x0 = x0(x0 > 0,:);
          clear x;
          for i = 1:size(P,1)
              x(i,:) = (b(i,1) - b0)./(c(i,1)*x0) + c0/c(i,1);
          end
          x = [x0'; x];
          x = double(x);
          x = x(:,sum(x <= 0,1) == 0)
          lamda = -log(x);
          LM_FINAL(:,k) = lamda;
          end
      % end of the outside loop

The important part of the above loop is towards the end:

x = x(:,sum(x <= 0,1) == 0)

This condition is sometimes not satisfied and hence the variable x is empty, which means LM_FINAL(:,k) = lamda is also empty. When this happens, I get the error:

    x =
         Empty matrix: 43-by-0
          Improper assignment with rectangular empty matrix.
          Error in Solution (line 75)
          LM_FINAL(:,k) = lamda;

How can I skip this error so that the column for LM_FINAL remains as empty, but the loop continues (so that the rest of LM_FINAL's columns are filled) rather than terminating?

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Answer 1

the cyclist 2014-2-23

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You could wrap the code inside the line

if not(isempty(x))
  ...
end

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sittmo 2014-2-23

编辑：sittmo 2014-2-23

Thanks, so just to confirm, this leaves the column of LM_FINAL as a column of 0's (i.e., unchanged) if the error happens and then proceeds to fill in the next column (assuming the error doesn't occur)?

the cyclist 2014-2-23

在 MATLAB Online 中打开

Yes, that is what should happen. You have initialized the entire array to be zeros, which of course means that each column starts off as zero. If you do

if not(isempty(x))
  lamda = -log(x);
  LM_FINAL(:,k) = lamda;
end

then it is only going to run that code if x is not empty. If x is empty, then it is going to do nothing with LM_FINAL, which means that that column will continue to be all zeros.

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Answer 2

Image Analyst 2014-2-23

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You could wrap the known troublesome code in try catch:

try
  % code that generates an error
catch
  % Error goes here but don't do anything, just ignore it.
end

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Dean Ranmar 2016-10-13

编辑：Dean Ranmar 2016-10-13

Thanks for responding. (I have been "out of action" for a couple of weeks.) I have a set of M files. A file has Ni>0 matrices (0<i<M+1) and, generally, Ni does not equal Nj, for i not equal to j. I use the dir() function to get the complete list of non-empty .csv files. There is no way to know Ni when I begin to read file i. If I process a file at a time, there's no problem: once all the matrices have been read & processed, an error occurs when csvread tries to read the next matrix in the file (Ni+1), which doesn't exist, and the script terminates. (I have done my processing and saved the results so I don't lose anything.) The problem is, I want to process a whole batch of M files at once, in a loop. Because I don't know how to find the end of a csv file without crashing the script, I can never complete more than one loop iteration. My fundamental question then is really: how do I detect the end of a csv file without crashing the script. (I am able to read the last matrix; I just can't tell it's the last matrix in the file until I try to read another one.)

Dean Ranmar 2016-10-25

I have to aplogize and this is a perfect example of why it's necessary to include the actual code you're using when asking a question. I cannot do as you suggest because the software that creates these csv files includes a header line before each array (matrix) which is in hex format. csvread expects only integer or floating point numbers so, if I read these lines - including the case where I try to read the entire file, execution is terminated. I cannot directly control the format in which these files are created but, I will try to persuade the provider to alter them. Thanks and I'm sorry I wasted your time; your solution would obviously work, otherwise.

Image Analyst 2016-10-25

You could use importdata() which allows you to specify the number of headerlines. Alternatively you could preprocess the file to extract out each chunk and write each chunk to a separate file. Of course it's easier if you can get them to just supply you with individual files in the first place.

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