x value on y max

Hi! How do I get matlab to show me the x value on my max y value that I have in my function and in my graph. I have already tried to find it by using plot(x,y) but don't know how.

Would be really nice to get an answer on my simple question.

Sincerely Simon

Answer 1

[Image Analyst]

xIndex = find(y == max(y), 1, 'first');
maxXValue = x(xIndex);

Or

[maxYValue, indexAtMaxY] = max(y);
xValueAtMaxYValue = x(indexAtMaxY(1));

The (1) is there in case the max occurs at multiple places, it takes the first.

[Simon]

Thank you, the second one did it. But how do I get Matlab to show it in my plot figure?

[Image Analyst]

Try

hold on;
plot(xValueAtMaxYValue, maxYValue, 'r+', 'MarkerSize', 30);

[Simon]

That was not really what I were lookin for, it shows in a figure a red cross where the max value is, but it doesn't show it on the original plot line figure.

[Image Analyst]

I don't understand what the difference is. Did you use "hold on" so as to not blow away the original plot? Maybe you could upload a picture of your desired plot(s).

[Simon]

Oh, I totally missed hold on; Sorry about that, but thanks for the answer, it works now.

[Brett Ruben]

This is a very old thread but helped me find the max value, so thank you! Using this as a guide, am I able to find the x value at 10% of the max value? I have a proton Bragg peak and the range of a proton in water is determined at 10% of the max value after the max of the curve. Please let me know if you can assist. I tried simply dividing the "max(y)" by 10, but that did not work. I used the second of your two suggestions above to find the max. Thank you!

[Image Analyst]

Brett, try this:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 18;
fprintf('Beginning to run %s.m ...\n', mfilename);
echo off;

% Make some sample oscillating signal.
t = 1 : 20;
signal = abs(cos(t)) .* exp(-0.1*t);
plot(t, signal, 'b.-', 'LineWidth', 2, 'MarkerSize', 15);
title('Proton Bragg peak and the range of a proton in water', 'FontSize', 15, 'Interpreter', 'none');
xlabel('x', 'FontSize', 15);
ylabel('y', 'FontSize', 15);
grid on;

% Find max of the entire signal value and it's location.
[maxSignal, indexAtMaxSignal] = max(signal);
fprintf('Found the first max at t = %.1f, it has a value there of %f.\n', t(indexAtMaxSignal), maxSignal);
xline(t(indexAtMaxSignal), 'LineWidth', 2, 'Color', 'r');
% Find max signal value from indexAtMaxSignal to the end, and it's location.
[maxSignal2, indexAtMaxSignal2] = max(signal(indexAtMaxSignal+1:end));
% Add in the offset so we get the index from the original signal
indexAtMaxSignal2 = indexAtMaxSignal2 + indexAtMaxSignal;
fprintf('Found the second max at t = %.1f, it has a value there of %f.\n', t(indexAtMaxSignal2), maxSignal2);
xline(t(indexAtMaxSignal2), 'LineWidth', 2, 'Color', 'r');
% Find the value that is 10% of the SECOND max.
v10Percent = signal(indexAtMaxSignal2) * 0.10
yline(v10Percent, 'LineWidth', 2, 'Color', 'g');
% Find out where that occurs
% Make copy of signal
s2 = signal;
% Erase up to indexAtMaxSignal2
s2(1 : indexAtMaxSignal2) = inf;
t10 = find(s2 < v10Percent, 1, 'first')
xline(t10, 'LineWidth', 2, 'Color', 'g');
% Shade the range
shareColor = 'g';
xl = xlim();
yl = ylim();
yFill = [yl(1), yl(1), yl(2), yl(2), yl(1)];
x1 = t(indexAtMaxSignal2);
x2 = t(t10);
xFill = [x1, x2, x2, x1, x1];
hold on;
fillHandle = fill(xFill, yFill, shareColor, 'FaceAlpha', 0.1);

The first/left red line is the overall max. The second/right red line is the max AFTER the first max. The horizontal green line is 10% of that second max. The green vertical line is the first place in the data, after the second max, where the data falls below that 10% value. The range is tinted light green. Is this what you meant?

[Siddharth Singh Chauhan]

I have two data one for x and one for y.

I need to find max(y) and the corresponding value for x.

In the most simple way.

[Image Analyst]

Siddarth:

Try this

maxY = max(y);
indexes = find(y == maxY);
% Get the x values everywhere that y is maxY
maxX = x(indexes); % May be at more than 1 x location.

Of more compactly:

maxX = x(y == max(y)); % May be at more than 1 x location.

[O. T.]

Hi,

I have also very similar problem. I have a lot of text file in one folder and each has only two column data as x and y. I want to read each text file and find an x value for maximum y, and extract it as a seperate text file. So final text file would be a single column of only x value of maximum y values for about thousands of text file. Thank you very much in advanced.

[Image Analyst]

I don't know what "extract it as a seperate text file" means? Once you've read in x and y and gotten the x at the maximum y, it's already been put into a variable. There is no text file to extract.

for k = 1 : numFiles
    fileName = allFilenames{k}
    xy = readmatrix(fileName)
    [maxY(k), index(k)] = max(xy(:, 2));
    maxX(k) = xy(index(k), 1); % Get x value where y is max.
    % Now you have maxX(k).  There is no way to "extract" it to a file.  
    % You can "write" it if you want, but not extract it.
end

[O. T.]

Hello Image Analyst

Thank you very much for the answer. I have this error when I run this script. What can be the problem? Yes, What I want is to write it in seperate text file, exactly.

Undefined function or variable 'numFiles'.

Error in value_x_in_max_y_3 (line 2)

for k = 1 : numFiles

[khaled elbatawy]

Hello,

@Image Analyst i know that it is old answer but it is related somehow to my Problem.

The different is my Data (y) is oscilating.

I want for every Maxima/Minima of y to find the corresponding Value of x. But without using findpeaks(Data)because it is needed the Signal Processing Toolbox.

Thank you in advance !

[Image Analyst]

@khaled elbatawy, try islocalmax and islocalmin or imregionalmax and imregionalmin.