Logical function not returning correct value for smaller numbers

Question

Ravi Rahul Kumar Shah 2020-12-23

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https://ww2.mathworks.cn/matlabcentral/answers/701047-logical-function-not-returning-correct-value-for-smaller-numbers

编辑： Mischa Kim 2020-12-23

I am writing a function which sould return back the index of the searched value. Because the numbers are smaller, the buildin find is not working.

Most of the times it works, but if the number is really small, it is not working. The condition Vektor(k) == x is false, although the numbers are exactly same.

function index_1 = find_Index(x,Vektor)
for k = 1:length(Vektor)
    if Vektor(k) == x  
        index_1 = k; 
        return; 
    end
end 
end

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Walter Roberson 2020-12-23

The values are probably not exactly the same.

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Answer 1

Mischa Kim 2020-12-23

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编辑：Mischa Kim 2020-12-23

在 MATLAB Online 中打开

Hi Ravi, try something like

if abs(Vektor(k) - x) < tol

and use tol to fine-tune your algorithm depending on how "different" your numbers are, e.g.

tol = 1e-4

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Answer 2

Walter Roberson 2020-12-23

2
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/701047-logical-function-not-returning-correct-value-for-smaller-numbers#answer_582837

在 MATLAB Online 中打开

function index_1 = find_Index(x, Vektor)
  index_1 = []; 
  [found, idx] = ismembertol(x, Vektor);
  if found; index_1 = idx; end

This returns empty if no match, and the index of an "almost-exact" match if found . For example if x was sqrt(13)/3*3 and Vektor contained sqrt(13) then there is not a bit-for-bit equality, but ismembertol() would be willing to overlook the 4.44089209850063e-16 difference in values.

Or perhaps you would prefer

  function index_1 = find_Index(x, Vektor)
    [~, index_1] = min( abs(Vektor - x) );

which returns the index of the nearest value, even if numerically it is a distance. For example, the input could be 73 and the match might be against 100