Find next value above threshold

Hello,

I have a large volume of data, and i need to find the next value above a threshold but within a range of values.

ie

Data range 300000 to 340000 (the corresponding values will drop off in the middle and increase either side of centre, centre = 320000)

i need to set a datum point of 320000 (middle) then i need to find the closest value to the centre that is greater than >1500

Im fairly new to matlab, so sorry if this is begineers question, i cant seem to find relavant links

Thank you

Joe

Answer 1

[Image Analyst]

Try this.

% First make sample data because Joe forgot to give us his actual data.
period = 20000;
x = linspace(22000, 42000, 800);
y = 3000 * (0.5 + cos(2 * pi * (x - 32000) / period));
% Add some noise
y = y + 800 * rand(1, length(y));
plot(x, y, 'b-');
grid on;
xlabel('x', 'FontSize', 15);
ylabel('y', 'FontSize', 15);
% Now we have our sample x,y data and we can begin:

% Find max of y - it should be at an x value of around 32,000.
[yMax, indexOfMax] = max(y);
% Draw vertical red line there
hold on;
line([x(indexOfMax), x(indexOfMax)], [min(y), y(indexOfMax)], 'Color', 'r', 'LineWidth', 2);
% Also draw red line across the threshold of 1500
line(xlim, [1500, 1500], 'Color', 'r', 'LineWidth', 2);
% Find logical indexes for indexes where y is less than 1500
logicalIndexes = y < 1500;
% Find linear indexes:
linearIndexes = find(logicalIndexes);
% Note: the above two lines could be done in one line 
% but I wanted to demo both linear and logical indexes for you.
% Find distances of all indexes where y<1500 to the index of the max y.
indexDistances = abs(linearIndexes - indexOfMax);
% Find out which index is closest to indexOfMax
[indexDistance, indexOfThreshold] = min(indexDistances)
% Draw red line there, at the closest threshold.
hold on;
line([x(indexOfThreshold), x(indexOfThreshold)], [min(y), y(indexOfThreshold)], 'Color', 'r', 'LineWidth', 2);

The index of the point you want is indexOfThreshold, and the x value is x(indexOfThreshold) and the y value is y(indexOfThreshold).

You can see in this example, that the closest point to the max where the data first goes above 1500 is 26500.

[Joe Gee]

Thank you so much for this, it was a great help.

I have implemented the code below, i have a few hundred variables, so please excuse messy variable names.

I cant seem to get the output quite right. The centre line is a known value in time, so ive made it constant with AAAM.

I get the resultant graph and the smaller red line should be where the value goes above 300 (data has changed slighly since first question).

I am very new to matlab and its been a huge learning curve, so forgive me if the changes ive made to the code are messy.

Thank you

Joe

AAA = Input165(89094:91060,:);
TAA= TimeSeries(89094:91060,:);
AAAM = TimeSeries(90077,:);
plot(TAA,AAA)
ylim([0 1000]);
hold on;
plot([AAAM AAAM],[0 1500])
line(xlim, [300, 300], 'Color', 'r', 'LineWidth', 2);
logicalIndexes = AAA > 300;
linearIndexes = find(logicalIndexes);
indexDistances = abs(linearIndexes - AAAM);

[indexDistance, indexOfThreshold] = min(indexDistances)
hold on;
line([TAA(indexOfThreshold), TAA(indexOfThreshold)], [min(AAA), AAA(indexOfThreshold)], 'Color', 'r', 'LineWidth', 2);

[Image Analyst]

You forgot to attach your data so I can't do anything (yet).

[Joe Gee]

please find attached

[Image Analyst]

Joe:

I misunderstood what you want. Now that we've wasted 2 days, I think you realize how important it is to give plots right away. If we had had your plots right at the very start, at the top, you'd have your answer 2 days ago. I'll have to work on a totally new and different program for you.

[Image Analyst]

Joe! These x and y ranges are totally different than what you originally stated. What gives?

[Image Analyst]

Joe:

Try this:

% Load mat files into structures.
s1 = load('TAA.mat')
s2 = load('AAA.mat')
s3 = load('AAAM.mat')
AAA = s2.AAA;
TAA = s1.TAA;
AAAM = s3.AAAM;

% AAA = Input165(89094:91060,:);
% TAA= TimeSeries(89094:91060,:);
% AAAM = TimeSeries(90077,:);

middleIndex = find(TAA < AAAM, 1, 'last')
plot(TAA,AAA, 'b-')
ylim([0 1000]);
hold on;
plot([AAAM AAAM],[0 1500], 'k-', 'LineWidth', 2)
grid on;

% Define threshold.
thresholdValue = 550;
% Draw line at threshold.
line(xlim, [thresholdValue, thresholdValue], 'Color', 'g', 'LineWidth', 2);

% Find left index
leftIndex = middleIndex;
for k = middleIndex : -1 : 1
	if AAA(k) > thresholdValue
		leftIndex = k
		TAALeft = TAA(k)
		% Draw vertical red line there.
		hold on;
		line([TAALeft, TAALeft], ylim, 'Color', 'r', 'LineWidth', 2);
		% Draw horizontal line
		line([TAA(1), TAA(leftIndex)], [AAA(leftIndex), AAA(leftIndex)], 'Color', 'r', 'LineWidth', 2);
		break;
	end
end

% Find right index
thresholdValue = 550;
rightIndex = middleIndex;
for k = middleIndex : length(TAA)
	if AAA(k) > thresholdValue
		rightIndex = k
		TAARight = TAA(k)
		% Draw vertical red line there.
		hold on;
		line([TAARight, TAARight], ylim, 'Color', 'm', 'LineWidth', 2);	
		% Draw horizontal line
		line([TAA(1), TAA(rightIndex)], [AAA(rightIndex), AAA(rightIndex)], 'Color', 'm', 'LineWidth', 2);
		break;
	end
end