Count number of specific values in matrix

Question

Corey Bullard 2012-5-2

4
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https://ww2.mathworks.cn/matlabcentral/answers/37196-count-number-of-specific-values-in-matrix

评论： Andrew Frane 2024-10-22

I have a large matrix, m, and am trying to count the number of a specific value (i.e. How many indexes are of the value 4?)

I tried using

val = sum(m == 4);

but I end up with val being a matrix/vector of numbers. I assume these numbers are from each column and should be added together for the total, so I tried another

num = sum(val == 4);

but then I just end up with another vector/matrix.

How can I do it?

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Dev 2023-10-18

What if the value is zero?, how do I count then?

Walter Roberson 2023-10-18

在 MATLAB Online 中打开

@Dev

num = sum(val == 0, 'all');   %r2018b or later
num = sum(val(:) == 0);       %any version
num = nnz(~val);              %any version

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Answer 1

Walter Roberson 2012-5-2

37
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sum(m(:) == 4)

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MathWorks Support Team 2020-9-2

An alternative syntax available in R2018b or later is sum(m==4,'all'). But for this simple problem colonizing the input with m(:) is likely to be faster.

Andrew Frane 2024-10-22

Using the colon probably won't be faster to execute. If you use m(:) then Matlab has to do the additional step of creating a new array m(:) before doing the logical evaluation. In my tests, using 'all' is consistently faster than using the colon to vectorize the array.

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Answer 2

Kye Taylor 2012-5-2

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在 MATLAB Online 中打开

Try this:

numberOfNonZeros = nnz(m==4);

Using nnz is more efficient than converting logicals to numeric, which is required to apply sum()

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Walter Roberson 2019-8-22

编辑：Walter Roberson 2019-8-22

In the test I just did, the timings of sum() vs nnz() could not consistently tell the two cases apart. nnz() might possibly have been slightly faster, but the range of timings showed so much overlap that no real conclusion could be reached. It would make sense that nnz() could be faster, but I can't prove it at the moment. sum() on a large enough array could be dispatched to LAPACK after all.

Andrew Frane 2024-10-22

I did a million iterations after defining m as a 1x1000000 vector, and nnz(m==4) beat sum(m==4) every time.

I did another million iterations after defining m as a 1000x1000 matrix, and nnz(m(:)==4) beat both sum(m(:)==4) and sum(m==4, 'all') every time.

So nnz does appear to be more efficient than sum.

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Answer 3

Sean de Wolski 2012-5-2

9
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在 MATLAB Online 中打开

This could be done easily with histc() and unique() to get the number of each value:

uv = unique(x);
n  = histc(x,uv);

Or with unique() and accumarray():

[uv,~,idx] = unique(x);
n = accumarray(idx(:),1)

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Royi Avital 2022-10-10

Pay attention that histcount() won't have the same result as histc() above for this case (Difference at the end).

Walter Roberson 2022-10-11

Royi is correct.

At the time the question was asked, histcounts did not exist.

The newer histcounts is recommended instead of histc()

In the where you pass in the bin edges, then histc() counts values that exactly match the upper limit separately, but histcounts counts them together with the previous bin.

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Answer 4

ntsh kr 2017-10-12

5
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https://ww2.mathworks.cn/matlabcentral/answers/37196-count-number-of-specific-values-in-matrix#answer_285403

编辑：ntsh kr 2017-10-12

在 MATLAB Online 中打开

>> a

a =

   5     5     5     5     5     5     6     9    96
   3     9     5     2     7     5     6     2     1
   3     6     9     8     7     5     1     6     9

>> ans1=sum(a==5)

ans1 =

2 1 1 2 1 1 3 0 0 0

>> b=sum(ans1)

b =

11

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Manoj Payani 2018-5-16

Many Thanks - It works perfect.

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Answer 5

dipanka tanu sarmah 2017-11-11

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along with this if you want to count the number of NaN ,(if there any) use nnz(isnan(m))

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Answer 6

vimal kumar chawda 2020-5-18

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https://ww2.mathworks.cn/matlabcentral/answers/37196-count-number-of-specific-values-in-matrix#answer_433073

But if we want ot do for NaN and any numeric value in large matrix then ?

ans1=sum(a==5) so at this my value is numerical (which is not same all time) and other is NaN which is common. But i need to count only numerical value at particular value of x.,x2,x3...............x7000 which is on y axis.

-How many times y appear on the at particular value of x?