How to use try/catch so that my program rerun until it correct ?

Dear Matlab users,

I need to process a large amount of data using parfor and I am running into a problem where my program can sometime success, sometime not success

parfor i = 1:100
	MyProgram
end

Is there anyway to utilize the try/catch statement so that everytime when my progragram run into some kind of error, the worker will

1/ Retry untill success

2/ Put a warning about the iteration i and then abandon the current job to move on

Thank you very much

Answer 1

[Walter Roberson]

niters = 100;
status = logical(niters, 1);
message = cell(niters, 1);
parfor i = 1 : niters
    blah blah blah
    try
        something
        status(i) = true;
        message{i} = [];
    catch ME
        status(i) = false;
        message{i} = ME;
    end
end

[Tuong Nguyen Minh]

Oh, So in this case if the worker encouter a error will it move on with the next job and output a message ?

Should I put MyProgram in the blah blah blah or something ?

(My guess is that I should put MyProgram in the something)

[Walter Roberson]

With that code, it will not display a message, but it will record information that you can examine after the entire parfor is finished.

You would replace blah blah blah with any "safe" initialization code that will not error. You would replace something with the code that might fail.

If you want to retry then,

maxretry = 42;
niters = 100;
retry_count = zeros(niters, 1);
parfor i = 1 : niters
    blah blah initialization
    while true
        blah blah anything that needs to be reset every retry
        try
            something
            break;
        catch ME
            retry_count(i) = retry_count(i) + 1;
            if retry_count(i) > maxretry
                break;
            end
        end
    end
end

[Tuong Nguyen Minh]

Wow this is more difficult than I expected but I appreciate the opportunity to learn your way of thinking.

However, for the second case (the code that support retry) what is the reason behind the usage of "break" in the try block ? I am quite worried that eventhough there is no error in "something" the program would break out of the parfor loop ?

Thank you very much !

[Walter Roberson]

The code assumes that after a while of retrying, that you are not going to succeed in a reasonable amount of time, and so it gives up. For example if you were randomly placing non-overlapping circles inside a square, after awhile you can be pretty sure that there just are no more empty gaps large enough and should give up.

The break is out of the "while true", not out of the parfor.

After the parfor you would check

failed = retry_count > maxretry;

and you would do whatever is appropriate to discount the results at locations that failed.

[Walter Roberson]

You do not have to keep track of retries; you could just let it retry indefinitely. That would make the code a bit more simple.

niters = 100;
parfor i = 1 : niters
    blah blah initialization
    while true
        blah blah anything that needs to be reset every retry
        try
            something
            break;   %you did not fail, so you can stop retrying.
        end
    end
end

Perhaps it might be more clear to you as

niters = 100;
parfor i = 1 : niters
    blah blah initialization
    failed = true;
    while failed
        blah blah anything that needs to be reset every retry
        try
            something
            failed = false;
        end
    end
end

[Tuong Nguyen Minh]

Oh thank you very much, I did not know that this try catch command would even be that flexible. Your explanation is excellent and the code organization is very thoughful !