How to find a point within a curve?

Question

Angelavtc 2020-10-20

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https://ww2.mathworks.cn/matlabcentral/answers/620488-how-to-find-a-point-within-a-curve

评论： Angelavtc 2020-10-23

Hello to all,

How can I calculate a point (the red point) on the same curve but 500 units less than the point indicated in blue? (see graph). The pairs (x,y) of the curve are contained in a cell within an array. I tried with the code that I copy here, but I have problems with the “find" command, because I must be changing the absolute difference manually and since I have to automate the process, this is not an option for me. Thanks for your help!

w=1936 %This is an specific day and the graph shown here is the graph from this day
k = find(abs(Array_demand{w}.Volume- x_blue_point{w}+500) < 70,1)
%Find its correspondance within the curve for X (Volume)
x_red_point=Array_demand{w}.Volume(k);
%Find its correspondance within the curve for Y(Price)
y_red_point=Array_demand{w}.Price(k); 

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Answer 1

Image Analyst 2020-10-20

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https://ww2.mathworks.cn/matlabcentral/answers/620488-how-to-find-a-point-within-a-curve#answer_519548

在 MATLAB Online 中打开

Try this:

% Extract the curve for X (Volume)
x = Array_demand{w}.Volume;
% Extract the curve for Y (Price)
y = Array_demand{w}.Price;
% Define blue x
bluex = 43210;
% Define red x
redx = bluex - 500
% Find index of the red x
redIndex = find(x <= redx, 1, 'last');
% Get the y value there
redy = y(redIndex)

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Image Analyst 2020-10-23

在 MATLAB Online 中打开

answers.mat

You do not have any data in that range. 28,200 is below the lowest x value in your data. Try this:

s = load('answers.mat')
% Extract the curve for X (Volume)
x = s.x;
% Extract the curve for Y (Price)
y = s.y;
plot(x, y, 'k-', 'LineWidth', 2);
grid on;
xlabel('x', 'FontSize', 15);
ylabel('y', 'FontSize', 15);
% Define blue x
bluex = 28700;
xline(bluex, 'LineWidth', 2, 'Color', 'b');
% Define red x
redx = bluex - 500
% Determine if red x is in the range
if redx < min(x) || redx > max(x)
	message = sprintf('Error : red x is %.1f which is outside the range of data we have,\nwhich is %.1f - %.1f', redx, min(x), max(x))
	uiwait(errordlg(message));
	return;
end
% Find index of the red x
redIndex = find(x <= redx, 1, 'last')
% Get the y value there
redy = y(redIndex)

You'll see

message =

'Error : red x is 28200.0 which is outside the range of data we have,

which is 28263.0 - 35387.0'

You can pull it into the range that you DO have like this:

s = load('answers.mat')
% Extract the curve for X (Volume)
x = s.x;
% Extract the curve for Y (Price)
y = s.y;
plot(x, y, 'k-', 'LineWidth', 2);
grid on;
xlabel('x', 'FontSize', 15);
ylabel('y', 'FontSize', 15);
% Define blue x
bluex = 28700;
xline(bluex, 'LineWidth', 2, 'Color', 'b');
% Define red x
redx = bluex - 500
% Determine if red x is below the range
if redx < min(x)
	message = sprintf('Error : red x is %.1f which is outside the range of data we have,\nwhich is %.1f - %.1f. I will set it to %.1f', ...
		redx, min(x), max(x), min(x))
	uiwait(warndlg(message));
	% Set it equal to the min that we do have.
	redx = min(x);
end
% Determine if red x is above the range
if redx > max(x)
	message = sprintf('Error : red x is %.1f which is outside the range of data we have,\nwhich is %.1f - %.1f. I will set it to %.1f', ...
		redx, min(x), max(x), max(x))
	uiwait(warndlg(message));
	% Set it equal to the max that we do have.
	redx = max(x);
end
xline(redx, 'LineWidth', 2, 'Color', 'r');
% Find index of the red x
redIndex = find(x <= redx, 1, 'last')
% Get the y value there
redy = y(redIndex)