Finding frequency of values in an array

Question

Ahmad Alenezi 2019-7-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/473048-finding-frequency-of-values-in-an-array

评论： Dyuman Joshi 2023-7-26

Hi there

I have a 128x128x16 double array. Firstly, I would like to get the total number of values < zero in this array. Then, i would like to get the sum of the frequency of occurance of each value in this array. The array name is Phase.

thanks

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Answer 1

Star Strider 2019-7-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/473048-finding-frequency-of-values-in-an-array#answer_384381

One approach:

negPhase = nnz(Phase < 0);                          % Number Of Values < 0
[uPhase,ia,ic] = unique(Phase);
tally = accumarray(ic, 1);
freqOccurrence = [uPhase, tally];                   % Frequency Of Occurrenct Of Each Value

If you do not want the results sorted (since the unique function does this by default), use:

[uPhase,ia,ic] = unique(Phase, 'stable');

instead. The rest of the code is unchanged. (Note that this will conform to the MATLAB linear matrix indexing convention, so it will be a column vector, not a matrix.)

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Leyla Elyasizad 2023-7-26

Hey Guys

Thanks for your helpful comments but I tried both unique & tabulate for double values with 4 decimal places (0.0202, 0.0031,.....) and with both functions I get two 0.0031 with differenct occurence!

Do you have any idea why this happens?

Dyuman Joshi 2023-7-26

@Leyla Elyasizad, because both numbers do not appear to be the same. See below -

%Assign 0.3
y = 0.3
y = 0.3000
%Value st   
sprintf('%0.42f', y)
ans = '0.299999999999999988897769753748434595763683'

Read more here - FAQ: Why is 0.3 - 0.2 - 0.1 not equal to zero?

When trying to compare floating point numbers, it is better to use a tolerance.

y = 0.12230455
y = 0.1223
sprintf('%0.42f', y)
ans = '0.122304549999999997988986422114976448938251'
%Set tolerance of 10^-6
tol = 1e-6;
%%Comparison
%Using equality
isequal(y,0.12330455)
ans = logical
   0
%Using a tolerance
abs(y-0.12230455)<tol
ans = logical
   1