how to find where a value falls in a list

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steve 2023-12-3

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编辑： Voss 2023-12-4

I have a 2 x 1000 array (“A”) as shown in the first 2 columns. (I only include the first 10 rows). The values are in order.

Then I have another array(“B”) as shown in the third column, which is also in order. I want to find a set of weights that identify the weights of corresponding values in the first two columns.

For example, .029 is in between .000 and .053 with respective weights 0.72 and 0.28. Also, .117 is between .105 and .157

How do i calculate the 4 columns of “C” shown below. If I know C(1.x), i can do the rest, but need to identify the Index immediately before each of the values in B.

chart appears below

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Taylor 2023-12-4

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You could just use the find function. It would look something like this

idx = find(A(:,2) < B(1), 1)

In this case, the find function will return the index where the first value of the second column of A that is smaller than the first value of B occurs.

steve 2023-12-4

thank you

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Answer 1

Voss 2023-12-4

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在 MATLAB Online 中打开

One way to determine the index in A(:,2) of the first element greater than each element of B:

N = numel(B);
idx = zeros(N,1);
for ii = 1:N
    idx(ii) = find(A(:,2) > B(ii), 1);
end

Another way to do the same:

[~,idx] = max(A(:,2) > B(:).', [], 1);

Then the matrix C is:

C = [idx(:)-[1 0] [A(idx,2)-B B-A(idx-1,2)]./(A(idx,2)-A(idx-1,2))]
C = 9×4
0000    2.0000    0.7170    0.2830
0000    2.0000    0.4528    0.5472
0000    2.0000    0.1698    0.8302
0000    3.0000    0.8846    0.1154
0000    3.0000    0.6154    0.3846
0000    3.0000    0.3269    0.6731
0000    3.0000    0.0577    0.9423
0000    4.0000    0.7692    0.2308
0000    4.0000    0.4808    0.5192

Of course, if all you want to do with all this is to interpolate something, then you can let interp1 do the calculations of the weights and whatnot for you.

https://www.mathworks.com/help/matlab/ref/interp1.html