MATLAB Answers


Find the index of given value in an array

Asked by Mirlan Karimov on 10 Apr 2019
Latest activity Commented on by Adam
on 12 Apr 2019
array = [ 1 2 3 4 5 6 ];
find(array == 3);
This is clear!
I want to find fractional index when array == 2.5 or any other intermediate value.


What exactly is a "fractional index" ?
Please show the expected output for your example values.
When you ask a question , make sure you give an example clearly instead of advising others how to answer the question.

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2 Answers

Answer by Stephen Cobeldick on 11 Apr 2019
Edited by Stephen Cobeldick on 11 Apr 2019
 Accepted Answer

Much simpler (and also works for multiple val values):
For example:
>> array = [2,4,5,7,8,9]; % a more interesting sequence.
>> val = 3.8;
>> interp1(array,1:numel(array),val)
ans = 1.9
And compared to the (very complex) accepted answer:
>> idxAboveVal = find( array >= val, 1 );
>> idxFract = idxAboveVal - ( array( idxAboveVal ) - val ) / ( array( idxAboveVal ) - array( idxAboveVal - 1 ) )
idxFract = 1.9
Note: this answer is based on the original answer by madhan ravi:

  1 Comment

Yeah, I was originally going to give an answer based on interp1 but had a brainfade on working out how to use it for this!

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Answer by Adam
on 10 Apr 2019
Edited by Adam
on 10 Apr 2019

val = 2.5;
idxAboveVal = find( array >= val, 1 );
idxFract = idxAboveVal - ( array( idxAboveVal ) - val ) / ( array( idxAboveVal ) - array( idxAboveVal - 1 ) );
I'm guessing this is what you mean.
Obviously it would need error checking if idxAboveVal is 1 or empty.
There's probably neater ways to do it too, or shorter, at least!


Thank you for your answer. This is a manual approach and probably would cause bugs in my case cuz the original code I am running is much more complicated in a numerical sense. Therefore I am looking for find() type function that would do the job.
The find function simply finds integer indices into an array that correspond to the logical expression you give it. It isn't magic. It can't find things that don't exist. Hence I used it to find the next value greater than the one you want and did the required maths from there.
You should always give an example that shows the full complexity of the question you are asking though if you want a useful answer.
Giving what people call a 'Minimum working example' is fine, but it needs to have the full complexity of what you actually want to know still, otherwise it's of no use.
A manual approach to things causes bugs if you get it wrong, not if you get it right.
I had a manual code but then realized yours is shorter and I will be using that one. Thanks!

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