Count number of specific values in matrix

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?

Answer 1

[Walter Roberson]

sum(m(:) == 4)

[Romy Wolstencroft]

This worked perfectly for me. Thank you

[MathWorks Support Team]

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.

Answer 2

[Kye Taylor]

Try this:

numberOfNonZeros = nnz(m==4);

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

[Walter Roberson]

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.

Answer 3

[Sean de Wolski]

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)

Answer 4

[ntsh kr]

>> 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

[Manoj Payani]

Many Thanks - It works perfect.

Answer 5

[dipanka tanu sarmah]

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

Answer 6

[vimal kumar chawda]

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?

[Walter Roberson]

nnz(isnan(a))     %count nan

For a count of each value, see https://www.mathworks.com/matlabcentral/answers/37196-count-number-of-specific-values-in-matrix#answer_46452