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How to find the derivative without using a symbolic function in Matlab?

How can I count the number of items using the FIND function in MATLAB?

  • How can I use the function FIND to count the number of items of a given value instead of using a loop? For example, in the array item below there are 3 occurrences of the number 23, 2 occurrences of the number 22, and 2 occurrences of the number 20. .... for i=2:n if item(i-1)~=item(i) nItem21(i)=1; else nItem21(i)=nItem21(i-1)+1; end end item Num 23 2 23 4 23 6 22 3 22 1 20 6 20 8

  • Answer:

    You can do the following: identify where the value of item changes, then use diff to get the counts. item = [ 23 23 23 22 22 20 20]; % find the 'last' entries of each consecutive group of numbers chgRowNum = [find(item(1:end-1) ~= item(2:end);length(item)]; counts = diff([0;chgRowNum]); correspondingItems = item(chgRowNum);

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http://www.mathworks.com/access/helpdesk/help/techdoc/ref/find.html returns the indices of nonzero elements in an array. If you want a count of all the occurrences of your elements (assuming they're integers), you can use the http://www.mathworks.com/access/helpdesk/help/techdoc/ref/hist.html function. By assigning an output, it won't plot a histogram. Instead it will return an array of occurrences. x=[20 23 20 22 23 21 23 22]; bins=min(x):max(x); count=hist(x,bins); list=unique(x); Now count contains the number of occurrences, and list contains each unique array element. To get rid of zero count elements: idx=find(count); count=count(idx); or the one line option (without using find): count=count(count~=0);

Doresoom

Just for completeness I would use the histc function. item = [ 23 23 23 22 22 20 20]; %get the unique items [uni_items, minds, uinds] = unique(item); %count them counts = histc(uinds, 1:numel(uni_items)); %put them in the original order ocounts = counts(minds); This takes care of them not being in order or if they're not integers.

JudoWill

Another option for this situation is to use the function http://www.mathworks.com/access/helpdesk/help/techdoc/ref/accumarray.html, which wouldn't require that the list be sorted first. This is particularly useful if you were to have a set of numbers in item that span a range of 1:N, where N is any integer value. Here's how it would work for your example: item = [23; 23; 23; 22; 22; 20; 20]; %# A column vector of integers counts = accumarray(item,1); %# Collect counts of each item into %# a 23-by-1 array The array counts is a 23-by-1 array where the elements indexed by 23, 22, and 20 contain the counts 3, 2, and 2, respectively. All the other elements are 0 (i.e. there are no counts found for the numbers 1 through 19 or 21). If you want to get a list of the unique values in item and their corresponding counts, you can do this using the function http://www.mathworks.com/access/helpdesk/help/techdoc/ref/unique.html: >> uniqueValues = unique(item) %# Get the unique values in item uniqueValues = 20 22 23 >> counts = counts(uniqueValues) %# Get just the counts for each unique value counts = 2 2 3

gnovice

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