matlab plot does not update in while loop

Lussy04

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24-01-2025

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MATLAB's interactive plotting capabilities are powerful, but sometimes plots within a while loop refuse to update dynamically. This frustrating issue often stems from how MATLAB handles drawing and refreshing the figure. This guide will walk you through common causes and effective solutions.

Understanding the Problem

The core issue is that MATLAB doesn't automatically redraw the plot every iteration of the loop. It buffers commands until the loop completes, resulting in a single, final plot update instead of a continuous animation.

Common Causes and Solutions

Here's a breakdown of the most frequent culprits and how to address them:

1. Missing drawnow or pause

The simplest solution is often the most effective: adding drawnow or pause inside your loop. drawnow forces MATLAB to immediately update the figure, while pause introduces a short delay.

• drawnow: This command flushes the drawing queue, ensuring that the plot is updated immediately after each iteration. It's generally preferred for smoother animations, especially if updates are rapid.

• pause: Using pause(0.01) (or a similar small value) introduces a brief delay between iterations. This is a less elegant approach than drawnow, but it can sometimes be more effective if drawnow proves insufficient for slow updates. The delay allows the figure to fully refresh before the next iteration begins.

Example:

figure;
x = 0;
while x < 10
    plot(x, x^2);
    drawnow; % Or pause(0.1);
    x = x + 0.1;
end

2. hold on Misuse

If you're using hold on to add data to an existing plot, you might inadvertently prevent updates. Ensure that hold on is used correctly and released when appropriate using hold off to allow the plot to redraw cleanly.

Incorrect Usage (preventing updates):

hold on; % Incorrect placement, keeps plot locked
while ...
    plot(x, y);
end

Correct Usage:

figure;
hold on; % Correct usage: hold on before plotting starts
while ...
    plot(x, y); 
end
hold off;  % Release the hold after the loop

3. Figure Properties

Certain figure properties can hinder updates. Check these settings:

• Visible Property: Ensure the figure's visibility is set to 'on'. If it's 'off', MATLAB won't render the plot even if you use drawnow.

• NextPlot Property: The 'nextplot' property controls how new plots are added to the current axes. Setting it to 'add' will add to the existing plot, which might affect updates if not handled properly. Experiment with 'replace' or removing this line if having issues.

4. Complex Calculations

If your loop performs computationally intensive calculations before plotting, the plotting might lag. Try optimizing calculations, for example, by pre-allocating arrays.

5. Incorrect Plot Function

Rarely, the choice of plotting function can influence update behavior. If you're using more sophisticated plot functions, make sure they're compatible with real-time updates.

Advanced Techniques

• refreshdata: For plots linked to data sources (e.g., using plot(animatedline)), this function refreshes the data and redraws the plot.

• refresh: This function redraws the entire figure window and is useful for fixing more general rendering issues.

• GUI (Graphical User Interface): For complex, interactive plots, building a simple GUI using GUIDE or App Designer allows for better control and update management.

Debugging Strategies

1. Simplify: Remove unnecessary code within the loop to isolate the problem.

2. Step Through: Use the MATLAB debugger to examine the loop's execution step by step, and check variable values at each iteration.

3. Check Error Messages: Watch for any warnings or errors that might indicate issues with plot commands or data.

4. Profile: Use the MATLAB Profiler to identify bottlenecks in your code, highlighting areas for optimization.

By systematically addressing these potential issues, you'll likely resolve the problem and create smoothly updating plots within your MATLAB while loops. Remember to utilize debugging techniques to diagnose any persistent issues.