matlab plot does not update in if statement

Lussy04

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24-01-2025

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MATLAB's plotting capabilities are powerful, but sometimes updates within conditional statements like if can be tricky. This article addresses the common issue where a MATLAB plot fails to update inside an if statement, offering solutions and best practices. We'll explore why this happens and provide effective strategies to ensure your plots refresh correctly.

Understanding the Problem: Why Plots Don't Update in if Statements

The core issue often stems from how MATLAB handles plot updates and the scope of variables within conditional blocks. MATLAB doesn't automatically redraw the plot after every change within an if statement unless explicitly instructed to do so. This can lead to a plot that appears frozen or only partially updated.

Here's a breakdown of the common causes:

• Missing drawnow: The drawnow command forces MATLAB to execute all pending drawing commands. Without it, plot updates might be buffered and only displayed after the entire script completes. This is the most frequent cause of the problem.

• Incorrect figure handling: If you're creating new figures or axes within the if statement, the existing plot might not be updated. Make sure you're working with the correct figure and axes handles throughout your code.

• Variable Scope Issues: If you are modifying plot data within the if statement using variables declared outside the if block, ensure those variables are appropriately updated and that the plot uses those updated values.

• Unintentional figure blocking: Certain operations can block the figure's update process. Check for lengthy computations or operations that might hold up the rendering process.

Solutions and Best Practices: Getting Your Plots to Update

Let's explore practical solutions to get your plots updating correctly inside if statements.

1. Utilize the drawnow Command

The simplest and often most effective solution is to incorporate the drawnow command within your if statement. This command immediately updates the plot.

x = 1:10;
y = x.^2;

figure;
plot(x,y);

if some_condition
    y = y + 5;  % Modify the data
    plot(x,y);
    drawnow;      % Force the plot update
end

2. Handle Figure and Axes Properly

Ensure that you're working with the correct figure and axes handles. Explicitly specify the figure and axes handles when plotting. Using clf (clear figure) before plotting a new dataset can also resolve issues.

figure_handle = figure;
axes_handle = axes('Parent', figure_handle);
%... your plot code using the handles ...
if some_condition
    %Modify data and replot using the existing handles
    plot(axes_handle, x,y); %Plot on the existing axes
    drawnow;
end

3. Verify Variable Scope and Updates

Check that your data is being updated correctly within the scope of your if statement. If you're using variables declared outside the if block, ensure they are properly modified and reflected in your plotting commands.

4. Optimize for Performance

Long, intensive computations within the if block can prevent plot updates. Consider breaking down complex tasks into smaller, manageable chunks, allowing time for the plot to redraw.

5. Using hold on and hold off

If you're adding multiple plots to the same figure, use hold on before plotting new data and hold off afterward to avoid overwriting previous plots.

figure;
hold on;
if some_condition
    plot(x,y);
end
if another_condition
    plot(x,z);
end
hold off;
drawnow;

6. Check for Blocking Operations

Look for potentially blocking operations within the if statement. Anything that ties up MATLAB's main thread will prevent drawing.

Example Illustrating the Solution

Let's examine a complete example showing the problem and the solution:

x = 0:0.1:10;
y = sin(x);

h = figure;
plot(x,y);

for i = 1:10
    pause(0.5); % Simulate some processing time
    y = y + 0.1*cos(x); %Update data
    plot(x,y);       %Plot the new data
    if mod(i,2)==0    %only update every other time
        drawnow;      %Forces plot update
    end
end

Running this code without drawnow will show a plot that only updates intermittently. Including drawnow will ensure a smooth, continuous update.

By following these steps and incorporating drawnow strategically, you can reliably update your MATLAB plots even within complex conditional logic. Remember to always check your variable scope, handle your figure and axes correctly, and optimize for performance to ensure smooth plot updates.