Hi, I want to find the peak position of a XY data

I looked on some earlier posts but my problem didn't matches with other problems exactly, Actually I have a single column data of X but Y data contains 25 columns. Each column of Y data is taken at the same X column data, I want to find Ymax along with Its X value for each column of Y data, and try to plot Ymax with its X values

[Suraj Singh]

I have tried [peakValues, indexes] = findpeaks(y); but it is giving wrong index values (x),and at some column of y data its giving more than one peak and index values

Answer 1

[Star Strider]

If you only have one maximum in each column (no repeats), this works:

x = linspace(0, 1, 30)';                                % Create ‘x’ Vector
Y = rand(30, 25)*90;                                    % Create ‘Y’ Matrix

[ymax,idx] = max(Y);                                    % First (Or Only) Value Of Each Column, & Index

figure(1)
plot(x(idx), ymax, 'bp')                                % Plot ‘ymax’ With Corresponding ‘x’ Value
grid

It plots the first or only maximum ‘y’ value in each column with its corresponding ‘x’ value.

EDIT —

If you have more than one value of the maximum in any one column, it is necessary to use a different strategy:

[rowidx,colidx] = find(bsxfun(@minus, Y, max(Y)) == 0); % Row & Column Indices For All Maxima
linidx = sub2ind(size(Y),rowidx,colidx);

figure(2)
plot(x(rowidx), Y(linidx), 'bp')                                % Plot ‘ymax’ With Corresponding ‘x’ Value
grid

[Suraj Singh]

Thanks man ! Its working. In case of mine the data can't have more than one maxima, but may be more than one peak, and i have to choose the max of that peak. Then how it could be easy to plot?

[Star Strider]

My pleasure!

My code plots the maximum for each column against the ‘x’ value for that value in figure(1). If you want to plot them as a line instead of points, you have to sort them by the index variable ‘idx’ first, otherwise the line is just a jumble.

The code changes to:

[ymax,idx] = max(Y);                                    % First (Or Only) Value Of Each Column, & Index
[sidx,srtidx] = sort(idx);                              % Sort ‘idx’ Ascending —> ‘sidx’ For ‘x’ & ‘srtidx’ For ‘ymax’

figure(1)
plot(x(sidx), ymax(srtidx))
grid

The ‘sidx’ variable are the sorted values of ‘idx’, and ‘srtidx’ are the indices of the original positions of the ‘srtidx’ values. See the documentation for sort for details.

That should do what you want. The first code works also, it just plots points rather than a line. It is possible that the different column maxima occur at the same value of ‘x’, so there would be two blue stars or a vertical line at that ‘x’.

Answer 2

[Azzi Abdelmalek]

x=(0:9)'  
y=rand(10,25)  % 10x25 array
max_val=max(y,[],2)
plot(x,max_val)

[Suraj Singh]

Thanks for your response ! I just copy your matlab code and put it into command window, its generating 10by25 array but max command is not taking the peak value from each column, max_val should be a 25by1 array but its showing 10by1. And I want to plot max y with x at which peak appears in Y data Could you please look it into more details?

[Azzi Abdelmalek]

You can calculate the max of y for each column

x=(0:9)'  
y=rand(10,25)  % 10x25 array
max_val=max(y)

but now, x length is 10, and max_val length is 25. How will you plot (x,mx_val)?

[Suraj Singh]

Your are right, but In mine case the the elements in X column and each column of Y are same. And i don,t have to plot ymax with x data but ymax with the corresponding x values at which peak appears