# Count number of specific values in matrix

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Corey Bullard 2012 年 5 月 2 日
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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### 採用された回答

Walter Roberson 2012 年 5 月 2 日
sum(m(:) == 4)

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Romy Wolstencroft 2019 年 8 月 22 日
This worked perfectly for me. Thank you
MathWorks Support Team 2020 年 9 月 2 日
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.

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### その他の回答 (5 件)

Kye Taylor 2012 年 5 月 2 日
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 年 8 月 22 日
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.

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Sean de Wolski 2012 年 5 月 2 日
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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ntsh kr 2017 年 10 月 12 日

>> a
a =
5 5 5 5 5 5 5 6 9 96
5 3 9 5 2 7 5 6 2 1
8 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 年 5 月 16 日
Many Thanks - It works perfect.

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dipanka tanu sarmah 2017 年 11 月 11 日
along with this if you want to count the number of NaN ,(if there any) use nnz(isnan(m))

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vimal kumar chawda 2020 年 5 月 18 日
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?

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Walter Roberson 2020 年 5 月 18 日

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