MATLAB: How to get Y-values from X-values

carl witthoftjanstephan

Hi,

I have lot of data in excel file where column 1 denotes the X-value and column to denotes the Y-value. I tried to get Y-value to corresponind X-value using interp1 (X,Y, certainX) but the following error shows:

Error using griddedInterpolant

The grid vectors must contain unique points.

I can get values by first interpolating yq=(X,Y, xq) but it changes the graph position and does not give the exact value. Suppose I want Y value for a X=1600. When I see at the X-values from workspace, I can see there is a X value of 1601.35 but not 1600. As a result, it shows Y-value NAN.

Is there any way to use tolerance that will cover 1600+/- 2 and will give the corresponding Y value.

I would really appreciate your help.

Best Answer

if you just want the Y value for the nearest point:

[~,idx] = min(abs(X-certainX));
certainY = Y(idx);

I don't see quite that error. Is certainX just a number?

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Use interp1, interpolating on the y values, reversing the x and y arguments. Your data contain two identical y-values, so it is necessary to cheat a bit and add a very small value (I chose 1E-15) to make them monotonically increasing in y. I sorted them first to be certain I wasn’t missing any of the repeated values:

x = 0:0.5:10;
y = [0  0.383135237593798  0.762305633253970  1.12500381866696  1.47841592230188  1.82360426305799  2.16799972102249  2.50162151353647  2.83129834831420  3.15388959500014  3.47113990999891  3.77183336056194  4.06431023145887  4.35173946610087  4.63997932479654  4.92354827732318  5.20849294256939  5.49434987763603  5.77926587166893  5.77926587166893  6.28318530717959];
yq = [0 1.57079632679490 3.14159265358979 4.71238898038469];    % Query Points
xy = [x' y'];                                       % Create MAtrix For Sorting
[xys, idx] = sortrows(xy,2);                        % Sort
dup = find(diff([xys(:,2)]) == 0)+1;                % Find Non-Increasing Values
xys(dup,2) = xys(dup,2) + [1:length(dup)]*1E-15;    % Add ‘1E-15’ To Duplicates
xq = interp1(xys(:,2), xys(:,1), yq);               % Interpolate To Find ‘x’ Values
figure(1)
plot(x, y, 'g')
hold on
plot(xq, yq, 'mp')
hold off

The ‘xq’ variable holds the x values you want that correspond to the y values in ‘yq’. The plot shows them.

MATLAB: How to extract y-values related to x-values

   x = [1 2 3 4 5 6 7 8 9 10] ;
   y = [9 8 0 3 8 0 9 4 6 3] ;
   iwant = y(x >3 & x <8) ;

Best Answer

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MATLAB: How to extract y-values related to x-values

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