Execute for loop with time

Question

I have a for loop like this

for t = 0: 1: 60
    // my code
end

I want to execute my code in 1st, 2nd, 3rd, ..., 60th seconds. How to do this? Also how can I run my code at arbitrary times? For example in 1st, 3rd and 10th seconds?

Answer 1

What you can do is use the pause command and place how many seconds you want your code to pause for. Once you do that, you execute the code that you want. As an example:

times = 1:60;
for t = [times(1), diff(times)]
    pause(t); % // Pause for t seconds
    %// Place your code here...
    ...
    ...
end

As noted by @CST-Link, we should not take elapsed time into account, which is why we take the difference in neighbouring times of when you want to start your loop so that we can start your code as quickly as we can.

Also, if you want arbitrary times, place all of your times in an array, then loop through the array.

times = [1 3 10];
for t = [times(1), diff(times)]
    pause(t); %// Pause for t seconds
    %// Place your code here...
    ...
    ...
end 

Answer 2

Polling is bad, but Matlab is by default single-threaded, so...

For the first case:

tic;
for t = 1:60
    while toc < t, pause(0.01); end;
    % your code
end;

For the second case:

tic;
for t = [1,3,10]
    while toc < t, pause(0.01); end;
    % your code
end;

The pause calls were added following the judicious observation of Amro about busy waiting. 0.01 seconds sounds like a good trade between timing precision and "amount" of spinning...

Answer 3

while pause is most of the time good enough, if you want better accuracy use java.lang.Thread.sleep.

For example the code below will display the minutes and seconds of your computer clock, exactly on the second (the function clock is accurate to ~ 1 microsecond), you can add your code instead of the disp command, the java.lang.Thread.sleep is just to illustrate it's accuracy (see after the code for an explanation)

while true
    c=clock;
    if mod(c(6),1)<1e-6
        disp([c(5) c(6)])
        java.lang.Thread.sleep(100);  % Note: sleep() accepts [mSecs] duration
    end
end

To see the difference in accuracy you can replace the above with java.lang.Thread.sleep(999); vs pause(0.999) and see how you sometimes skip an iteration.

For more info see here.

EDIT:

you can use tic\ toc instead of clock, this is probably more accurate because they take less time...

Answer 4

You can use a timer object. Here's an example that prints the numbers from 1 to 10 with 1 second between consecutive numbers. The timer is started, and it stops itself when a predefined number of executions is reached:

n = 1;
timerFcn = 'disp(n), n=n+1; if n>10, stop(t), end'; %// timer's callback
t = timer('TimerFcn' ,timerFcn, 'period', 1, 'ExecutionMode', 'fixedRate');
start(t) %// start the timer. Note that it will stop itself (within the callback)

A better version, with thanks to @Amro: specify the number of executions directly as a timer's property. Don't forget to stop the timer when done. But don't stop it too soon or it will not get executed the expected number of times!

n = 1;
timerFcn = 'disp(n), n=n+1;'; %// this will be the timer's callback
t = timer('TimerFcn', timerFcn, 'period', 1, 'ExecutionMode', 'fixedRate', ...
    'TasksToExecute', 10);
start(t) %// start the timer.
%// do stuff. *This should last long enough* to avoid stopping the timer too soon
stop(t)