afterEach
Run function after each function finishes running in the background
Description
B = afterEach(A,fcn,n)fcn automatically after each element in the
Future array A finishes and returns a
Future object B.
MATLAB® runs the function fcn using the outputs from each
element in Future array A. If
Future array A has M
elements, MATLAB runs the function M times. When the scheduled
function fcn finishes for the
Mth time, the
Future object B finishes.
For more information about using afterEach to run functions
after they finish running on a parallel pool, see afterEach (Parallel Computing Toolbox).
If the computations for any elements in A result in an error,
by default, afterEach does not run fcn
on the elements that failed.
Examples
This example shows how to use afterEach to schedule a callback function to run after a function finishes running in the background.
Use parfeval to run the function rand(1) and retrieve one output. Specify backgroundPool as the first argument to run the function in the background. Repeat 10 times to create 10 Future objects.
for i = 1:10 f(i) = parfeval(backgroundPool,@rand, 1, 1); end
After each Future finishes, display the value using the disp function. The input arguments for disp are the output arguments from each Future. Specify the third argument to the afterEach function as 0 to return no outputs from the callback.
afterEach(f,@disp,0);
This example shows how to use afterEach to handle errors from a function that runs in the background.
When computations for Future objects result in an error, by default, afterEach does not evaluate its function on the elements that failed. If you want to handle any errors, for example, when you have a user interface that you want to update, you can use the PassFuture argument. When you set PassFuture to true, MATLAB passes the Future object to the callback function. You can call fetchOutputs on it, process the outputs, and handle any possible errors.
Send a computation to the background using parfeval. The computation results in an error. You can view the error message using the Error property of the future.
errorFuture = parfeval(backgroundPool, ...
@(n,k) factorial(n)/(factorial(k)*factorial(n-k)),1,4,8);
wait(errorFuture);
errorFuture.Errorans =
ParallelException with properties:
identifier: 'MATLAB:factorial:NNegativeInt'
message: 'N must be an array of real non-negative integers.'
cause: {}
remotecause: {[1×1 MException]}
stack: [2×1 struct]
Correction: []
If you use afterEach on a future that results in an error, the callback function is not evaluated. In the code below, MATLAB does not execute the displayResult function because the future errors.
afterEach(errorFuture,@displayResult,0);
Write a callback function to handle futures that result in errors. If the error property of the future variable is empty, the handleError helper function calls fetchOutputs on the future variable, and process its outputs. If the error property of the future variable is not empty, the handleError helper function displays an error dialog box.
function handleError(f) fig = uifigure(Position=[100 100 425 275]); if isempty(f.Error) output = fetchOutputs(f); message = strcat("Computation complete: Result = ",num2str(output)); uialert(fig,message,"Sucess",Icon="success") else message = "Computation failed!"; uialert(fig,message,"Error",Icon="error") end end
Call afterEach with the handleError function and set PassFuture to true.
afterEach(errorFuture,@handleError,0,PassFuture=true);
function displayResult(output) fig = uifigure(Position=[100 100 425 275]); message = strcat("Computation complete: Result = ",num2str(output)); uialert(fig,message,"Sucess",Icon="success") end
This example shows how to use afterEach to update a wait bar with the progress of functions running in the background.
Create a wait bar, w.
w = waitbar(0,'Please wait ...');
Set the number of iterations for your for-loop, N. Store the current number of completed iterations, 0, and the total number of iterations, N, in the UserData property of the wait bar.
N = 
20;
w.UserData = [0 N];Run a for-loop with N iterations. In each iteration, use parfeval and backgroundPool to run pause in the background for a random number of seconds. Store each Future object in an array.
for i = 1:N delay = rand; f(i) = parfeval(backgroundPool,@pause,0,delay); end
Use the helper function updateWaitbar to update the waitbar after each Future finishes.
afterEach(f,@(~)updateWaitbar(w),0);
Use delete to close the wait bar after all the Future objects finish.
afterAll(f,@(~)delete(w),0);
Define Helper Function
Define the helper function updateWaitbar. The function increments the first element of the UserData property, then uses the vector to calculate the progress.
function updateWaitbar(w) % Update a waitbar using the UserData property. % Check if the waitbar is a reference to a deleted object if isvalid(w) % Increment the number of completed iterations w.UserData(1) = w.UserData(1) + 1; % Calculate the progress progress = w.UserData(1) / w.UserData(2); % Update the waitbar waitbar(progress,w); end end
Input Arguments
Output Arguments
Version History
Introduced in R2018a
