How do I compute the number of times a character appears in a character array in MATLAB?

Question

I want to determine the number of times a character appears in a character array, excluding the time it appears at the last position.

How would I do this?

Answer 1

In Matlab computing environment, all variables are arrays, and strings are of type char (character arrays). So your Character Array is actually a string (Or in reality the other way around). Which means you can apply string methods on it to achieve your results. To find total count of occurrence of a character except on last place in a String/Character Array named yourStringVar you can do this:

YourSubString = yourStringVar(1:end-1)
//Now you have substring of main string in variable named YourSubString without the last character because you wanted to ignore it

numberOfOccurrence = length(find(YourSubString=='Character you want to count'))

It has been pointed out by Ray that length(find()) is not a good approach due to various reasons. Alternatively you could do:

numberOfOccurrence = nnz(YourSubString == 'Character you want to count')

numberOfOccurrence will give you your desired result.

Answer 2

What you can do is map each character into a unique integer ID, then determine the count of each character through histcounts. Use unique to complete the first step. The first output of unique will give you a list of all possible unique characters in your string. If you want to exclude the last time each character occurs in the string, just subtract 1 from the total count. Assuming S is your character array:

%// Get all unique characters and assign them to a unique ID
[unq,~,id] = unique(S); 

%// Count up how many times we see each character and subtract by 1
counts = histcounts(id) - 1;

%// Show table of occurrences with characters
T = table(cellstr(unq(:)), counts.', 'VariableNames', {'Character', 'Counts'});

The last piece of code displays everything in a nice table. We ensure that the unique characters are placed as individual cells in a cell array.

Example:

>> S = 'ABCDABABCDEFFGACEG';

Running the above code, we get:

>> T

T = 

    Character    Counts
    _________    ______

    'A'          3     
    'B'          2     
    'C'          2     
    'D'          1     
    'E'          1     
    'F'          1     
    'G'          1