Select and plot value above a threshold

Question

I have a plot in which there are a few noise components. I am planning to select data from that plot preferably above a threshold in my case I am planning to keep it at 2.009 on the Y axis. And plot the lines going only above it. And if anything is below that i would want to plot it as 0. as we can see in the figure

enter image description here

t1=t(1:length(t)/5);  
t2=t(length(t)/5+1:2*length(t)/5);
t3=t(2*length(t)/5+1:3*length(t)/5);
t4=t(3*length(t)/5+1:4*length(t)/5);
t5=t(4*length(t)/5+1:end);
X=(length(prcdata(:,4))/5);
a = U(1 : X);
b = U(X+1: 2*X);
c = U(2*X+1 : 3*X);
d = U(3*X+1 : 4*X);
e = U(4*X+1 : 5*X);
figure;
subplot (3,2,2)
plot(t1,a);
subplot (3,2,3)
plot(t2,b);   
subplot(3,2,4)
plot(t3,c);
subplot(3,2,5)
plot(t4,d);
subplot(3,2,6)
plot(t5,e);
subplot(3,2,1)
plot(t,prcdata(:,5));
figure;
A=a(a>2.009,:);
plot (t1,A);

This code splits the data (in the image into 5 every 2.8 seconds, I am planning to use the thresholding in first 2.8 seconds. Also I had another code but I am just not sure if it works as it took a long time to be analysed

figure;
A=a(a>2.009,:);
plot (t1,A);
for k=1:length(a)
    if a(k)>2.009
        plot(t1,a(k)), hold on
    else 
        plot(t1,0), hold on
    end
end
hold off

Durkee · Accepted Answer · 2017-09-13T17:35:37.057

The problem is that you are trying to plot potentially several thousand times and adding thousands of points onto a plot which causes severe memory and graphical issues on your computer. One thing you can do is pre process all of the information and then plot it all at once which will take significantly less time.

figure
threshold = 2.009;
A=a>threshold; %Finds all locations where the vector is above your threshold
plot_vals = a.*A; %multiplies by logical vector, this sets invalid values to 0 and leaves valid values untouched
plot(t1,plot_vals)

Because MATLAB is a highly vectorized language, this format will not only be faster to compute due to a lack of for loops, it is also much less intensive on your computer as the graphics engine does not need to process thousands of points individually.

The way MATLAB handles plots is with handles to each line. When you plot a vector, MATLAB is able to simply store the vector in one address and call it once when plotting. However, when each point is called individually, MATLAB has to store each point in a separate location in memory and call all of them individually and graphically handle each point completely separately.

Per request here is the edit plot(t1(A),plot_vals(A))

Thank you for this. Using this you mean to say it will be faster? I will try it out thank you. — Matlaber, Aug 16 '17 at 01:54
When I ran thsi code I got an error Error using .* Matrix dimensions must agree. Error in al (line 62) plot_vals = a.*A; %multiplies by logical vector, this sets invalid values to 0 and leaves valid values untouched — Matlaber, Aug 16 '17 at 01:54
Is there a way to eliminate the columns and rows which corresponds to 0 in this code? — Matlaber, Sep 08 '17 at 05:32

Select and plot value above a threshold

1 Answers1