Tutorial :Is there a more elegant replacement for this MATLAB loop?



Question:

I want to get better at vectorizing my loops in MATLAB. At the moment, I'm trying to count the occurrences of values in a list of ints. My code is similar to this:

list = [1 2 2 3 1 3 2 2 2 1 5];  occurrence_list = zeros(1,max(list));    for x=list      occurrence_list(x) = occurrence_list(x) + 1;  end  

Is there a simple vectorized replacement for that for loop? (Or is there a built in MATLAB function that I'm missing?) I'm doing this on pretty small data sets, so time isn't an issue. I just want to improve my MATLAB coding style.


Solution:1

In addition to the HIST/HISTC functions, you can use the ACCUMARRAY to count occurrence (as well as a number of other aggregation operations)

counts = accumarray(list(:), 1)  %# same as: accumarray(list(:), ones(size(list(:))), [], @sum)  

Another way is to use TABULATE from the Statistics Toolbox (returns value,count,frequency):

t = tabulate(list)  t =              1            3       27.273              2            5       45.455              3            2       18.182              4            0            0              5            1       9.0909  

Note that in cases where the values don't start at 1m or in case there's large gaps between the min and the max, you will get a lot of zeros in-between the counts. Instead use:

list = [3 11 12 12 13 11 13 12 12 12 11 15];  v = unique(list);  table = [v ; histc(list,v)]'    table =       3     1      11     3      12     5      13     2      15     1  

representing the unique values and their counts (this will only list values with at least one occurrence)


Solution:2

You can use the hist function for that. Specify an output, and force the bins to be integers over the range of your array.

list = [1 2 2 3 1 3 2 2 2 1 5];   bins = min(list):max(list);  counts = hist(list,bins);  


Solution:3

So, this is basically a histogram. Off my memory - look for the HIST function.


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