try catch block in Matlab

Question

So, I am reading hundreds of image files via imread('D:\pic1\foo.jpg') and some of them are like imread('D:\pic2\Thumbs.db'). After reading I am storing in a database like this, train(i) = imread('D:\pic1\foo.jpg'). The problem is with imread('D:\pic2\Thumbs.db'), the reading of such files gives error obviously . I wanted to mitigate this problem like this:

for i=1:N
  try
    train(i) = imread(link{i})

    %link{i} can be 'D:\pic2\Thumbs.db' or 'D:\pic1\foo.jpg'

  catch 
    disp('Error')
  end
end

The issue is in the try block here. There are two things happening one is reading the file another is assigning imread value to train(i). Now, this is important, only on a successful imread() should there be an assignment and on a failure there would an error. Matlab takes care of the error via catch block, there isn't a block to take care of the success condition where I can do the assignment, so that I can read and write without much hassle.

I want something like this:

j = 0;
for i=1:N

  try:
   imread(links{i})

  if success:
   train(j) = imread(links{i})
   j = j+1;

  if fail:
   error
  end

end

I only came up with try and catch while searching Matlab docs, I will really appreciate if there is anything which will help me to write the code succinctly.

Community · Accepted Answer · 2017-05-23 12:09:55Z

11

The solution by @gnovice is correct, but it can be written a bit more succinctly:

ct = 1;
for i=1:N

  try
   train(ct) = imread(links{i});
   ct = ct +1; %# if imread fails, this line is not executed

  catch me
    %# report the problematic image, and the reason for failure
    fprintf('image #%i (%s) cannot be opened: %s\n',i,links{i},me.message)
  end

end

edited May 23, 2017 at 12:09

CommunityBot

11 silver badge

answered Dec 23, 2013 at 8:53

Jonas

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Comments

gnovice · Accepted Answer · 2013-12-23 05:36:53Z

3

You can solve this problem with a continue statement and a temporary variable like so:

for i=1:N
  try
    tempVar = imread(link{i});
  catch 
    disp(['Error reading file ' link{i}]);
    continue
  end
  train(i) = tempVar;
end

When imread throws an error, control is passed to the catch block, which displays a message and then calls continue to pass control to the next iteration of the for loop, skipping any later commands. When imread is successful, the code after the try\catch block is run, assigning the output from imread to your variable train.

answered Dec 23, 2013 at 5:36

gnovice

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2 Comments

motiur Over a year ago

I think there is no need for putting train(i) = tempVar at the end, it is possible to put it in the try block, the catch block will display the error and go to the next loop without continue.I was hoping for some Matlab 'magic' here, anyhow I will also be satisfied that there is none.

gnovice Over a year ago

@whoknows: It looks like I had focused on fixing your first example, while @Jonas focused on fixing the second. They mainly differ in whether you want train to always be length N (mine) or whether you want it to only have length equal to the number of valid images (Jonas'). If Jonas' answer better met your needs, please feel free to give him the check. I won't mind. ;)

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