Count number of specific values in matrix

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Corey Bullard
Corey Bullard 2012년 5월 2일
댓글: Walter Roberson 2021년 4월 3일
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?

채택된 답변

Walter Roberson
Walter Roberson 2012년 5월 2일
sum(m(:) == 4)
  댓글 수: 2
MathWorks Support Team
MathWorks Support Team 2020년 9월 2일
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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추가 답변(6개)

Kye Taylor
Kye Taylor 2012년 5월 2일
Try this:
numberOfNonZeros = nnz(m==4);
Using nnz is more efficient than converting logicals to numeric, which is required to apply sum()
  댓글 수: 1
Walter Roberson
Walter Roberson 2019년 8월 22일
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
Sean de Wolski 2012년 5월 2일
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)

ntsh kr
ntsh kr 2017년 10월 12일
편집: ntsh kr 2017년 10월 12일
>> 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 =
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Manoj Payani
Manoj Payani 2018년 5월 16일
Many Thanks - It works perfect.

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

vimal kumar chawda
vimal kumar chawda 2020년 5월 18일
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?

Patrick Benz
Patrick Benz 2021년 4월 2일
How can I count the values in the second column of an array depending on the values in the column?
I've got an array that looks something like that:
400 0
396 0
392 1
400 0
396 1
400 1
and I want to know how often there is a "1" or a "0" next to a "400" or next to the other values.
but this only gives me the total numbers of "1" and "0" and how often there is a 392 in the first column.
  댓글 수: 4

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