How can I count the number of elements of a given value in a matrix?

I have a matrix such as.

44.147231 ; 24.358619

44.461415 ; 24.118248

44.258173 ; 23.185014

43.166729 ; 24.100443

43.159002 ; 26.708013

42.954059 ; 27.338612

44.139759 ; 23.927882

44.424679 ; 25.301393

42.957588 ; 25.530434

44.615223 ; 24.200719

44.384409 ; 25.486735

43.965701 ; 28.454219

43.400971 ; 21.492123

42.068687 ; 22.303336

43.245372 ; 28.932286

44.593751 ; 23.635179

The first column is longitude , the second column is latitude.

So, I want to count the number of region.

In the above matrix,

But I don't know how make the code.

Please help me...

Answer 1

[Guillaume]

Note: when posting examples, it' s much easier for us if you use proper matlab syntax. As it is, I had to paste your example in a text editor to remove the ; between the numbers.

As often the case in matlab, the most efficient solution is to avoid loops. The simplest way to solve your problem is to convert your latitude and longitude into matrix coordinates (by rounding down the numbers, subtracting the min of each column, and adding 1) and use these coordinates with accumarray to compute the histogram:

 %this can be pasted straight into matlab, your example can't:
 longlat = [44.147231   24.358619
            44.461415   24.118248
            44.258173   23.185014
            43.166729   24.100443
            43.159002   26.708013
            42.954059   27.338612
            44.139759   23.927882
            44.424679   25.301393
            42.957588   25.530434
            44.615223   24.200719
            44.384409   25.486735
            43.965701   28.454219
            43.400971   21.492123
            42.068687   22.303336
            43.245372   28.932286
            44.593751   23.635179]; 

gridcoord = floor(longlat); %get integer by rounding down
gridcoord = 1 + bsxfun(@minus, gridcoord, min(gridcoord)); %shift everything so offsets start at 1
gridpop = accumarray(gridcoord, 1)

gridpop is the population of each region. Note that longitude is the vertical coordinate, increasing down and latitude is the horizontal coordinate, increasing right. Like matrices in matlab. If you want it in the same format as in your example:

gridpop = flipud(gridpop')

[MIHYUN]

Thank you for helping me!!

It is now possible to reduce the time thanks to you.

Answer 2

[Yona]

first use floor to round it.

after this run double loop, one on latitude and the second on longitude, and by using length(find(A(:,1)==i & A(:,2)==j)) you will get the number of each combination.

Answer 3

[puneet joshi]

well u can compare the complete matrix with the num u want to count .... and each time there is a match u can increase the count

Answer 4

[Andrei Bobrov]

n = floor(min(a));
m = ceil(max(a));
[~,~,lo] = histcounts(a(:,1),n(1):m(1));
[~,~,la] = histcounts(a(:,2),n(2):m(2));
out = flip(accumarray([la,lo],1),1)

[Jos (10584)]

Did you mean

[~, lo] = histc(..)

[Guillaume]

histc has been deprecated in R2014b. The replacement is histcounts which uses a different syntax and of course behaves slightly differently while still not solving all the issues of histc.

The syntax for histcounts is indeed the one Andrei used in his answer.