Interpolating the indexes of values in a Vector

Question

malik abdelli 2023-9-19

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https://ww2.mathworks.cn/matlabcentral/answers/2023067-interpolating-the-indexes-of-values-in-a-vector

评论： Star Strider 2023-9-19

hi

i have a Vector lets say a = [2 4 6 8 10 12 14 16]; and i have a value "b" that can change.

I want to write a code that compares the value "b" to every value in "a" starting from index 1 and going to index 8 in this example.

When the value "b" finds a value in "a" so that b >= a(:) and b < a( :+1) . The algorithm will give me the index "c" of that value wich can also be interpolated.

As an example lets say b = 6, then the algorithm will give me c = 3, because the index of 6 in "a" is 3.

but if b = 5 then the algorithm should give me c = 2.5. even tho 5 doesnt exist in "a" but we can know the index with interpolation

and if b = 4.5 then the algorithm should give me c = 2.25

if b = 14.3 then the algorithm should give me c = 7.15 etc...

How can i do this?

Thank you.

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Answer 1

Star Strider 2023-9-19

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Use interp1 for this —

a = [2 4 6 8 10 12 14 16];
k = 1:numel(a);
k = 1×8
     1     2     3     4     5     6     7     8
interpc = @(b) interp1(a,k,b);
bvector = [4.5 5 6 14.3];
c = interpc(bvector);
Result = [bvector; c]
Result = 2×4
    4.5000    5.0000    6.0000   14.3000
    2.2500    2.5000    3.0000    7.1500

.

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malik abdelli 2023-9-19

ok thank you

Star Strider 2023-9-19

As always, my pleasure!

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Answer 2

Konrad 2023-9-19

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Hi,

use interp1():

a = [2 4 6 8 10 12 14 16];
b = [6, 5, 4.5, 14.3];
interp1(a,1:numel(a),b,'linear') % 'linear' is also the default
ans = 1×4
    3.0000    2.5000    2.2500    7.1500