how to find where a value falls in a list

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

[Catalytic]

It looks like a reinvention of linear interpolation. Are you sure you don't just want to use interp1?

[steve]

yes, i want to interpolate. But, i don't know how to find the endpoints to interpolate. For example, my first value in Col B is .015. How do I determine that the values i need for the interpolation are (1,0) and (2,.053). Once i know that, i can interpolate.

[Taylor]

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]

thank you

Answer 1

[Voss]

A = [
    1 0
    2 0.053
    3 0.105
    4 0.157
    5 0.208
    6 0.258
    7 0.306
    8 0.353
    9 0.399
    10 0.442
    ];

B = [
    0.015
    0.029
    0.044
    0.059
    0.073
    0.088
    0.102
    0.117
    0.132
    ];

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

1.0000 2.0000 0.7170 0.2830 1.0000 2.0000 0.4528 0.5472 1.0000 2.0000 0.1698 0.8302 2.0000 3.0000 0.8846 0.1154 2.0000 3.0000 0.6154 0.3846 2.0000 3.0000 0.3269 0.6731 2.0000 3.0000 0.0577 0.9423 3.0000 4.0000 0.7692 0.2308 3.0000 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

[steve]

thank you

[Voss]

You're welcome! Any questions, let me know.