Select graph section using ginput

I would like to find the maximum and minimum on a graph for a section of time defined by ginput. I would also like to save this max min y-value in the variables peak_max and peak_min.

I am having trouble writing this correctly in loop form. This is what I have:

 for i=(1:8)
         y=P(:,i);
         plot(time,y)
         [x,y]=ginput(2);
         pt1=x(1);
         pt2=x(2);

         peak_max(i)=max(y(pt1:pt2)); 
         peak_min(i)=min(y(pt1:pt2));
  end

Basically I want to find the max and min's of each column of data for a specific range and store it. Warnings occur with the storage part.

Any help is appreciated. Thanks.

Answer 1

[Evan]

It looks like you're reassigning y in your code when you call ginput. y should be the ith column of P, but it gets overwritten as the y-coordinate of your click.

Does changing the variable name work? Or, better yet, since you're never using the y value that you click, just discard it:

 for i=(1:8)
    y=P(:,i);
    plot(time,y)
    [x,~]=ginput(2);
    pt1=x(1);
    pt2=x(2);
    peak_max(i)=max(y(pt1:pt2)); 
    peak_min(i)=min(y(pt1:pt2));
 end

[Camilla]

I'm still getting this Warning: Integer operands are required for colon operator when used as index.

Additionally I ran my entire script and received this at the end:

Attempted to access x(1); index out of bounds because numel(x)=0.

Error in testmuscle (line 150) pt1=x(1);

Could you explain any of these? Thanks a lot.

[Evan]

The warning occurs because you have to index your variables with integer values, but the points you click might not be rounded numbers. You should round them to the nearest whole number.

pt1 = round(x(1));
pt2 = round(x(2));

As for the error, can you place a breakpoint in your script just after the ginput line and see what the value of x is?

[Kelly Kearney]

The values clicked on by the user are unlikely to fall exactly on top of one of your points. Instead, use a logical mask to analyze the points located within the ginput range:

y=P(:,i);
plot(time,y)
[x,~]=ginput(2);
isin = time >= x(1) & time <= x(2);
peak_max(i)=max(y(isin)); 
peak_min(i)=min(y(isin));