Finding strings using an index - MATLAB

Question

I have a character array list and wish to tally the number of substring occurrences against an index held in a numerical vector chr:

list =
CCNNCCCNNNCNNCN

chr =

     1
     1
     1
     1
     2
     2
     2
     2
     2
     2
     2
     2
     2
     2
     2

Ordinarily, I am searching for adjacent string pairs i.e. 'NN' and utilise this method:

Count(:,1) = accumarray(chr(intersect([strfind(list,'CC')],find(~diff(chr)))),1);

Using ~diff(chr) to ensure the pattern matching does not cross index boundaries.

However, now I want to match single letter strings i.e. 'N' - how can I accomplish this? The above method means the last letter in each index is missed and not counted.

The desired result for the above example would be a two column matrix detailing the number of 'C's and 'N's within each index:

C     N
2     2
5     6

i.e. there are 2C's and 2N's within index '1' (stored in chr) - the count then restarts from 0 for the next '2' - where there are 5C's and 6N's.

Luis Mendo · Accepted Answer · 2015-08-16 21:52:47Z

3

[u, ~, v] = unique(list);          %// get unique labels for list in variable v
result = full(sparse(chr, v, 1));  %// accumulate combinations of chr and v

This works for an arbitrary number of letters in list, an arbitrary number of indices in chr, and chr not necessarily sorted.

In your example

list = 'CCNNCCCNNNCNNCN';
chr = [1 1 1 1 2 2 2 2 2 2 2 2 2 2 2].';

which produces

result =
     2     2
     5     6

The letter associated with each column of result is given by u:

u =
CN

answered Aug 16, 2015 at 21:52

Luis Mendo

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2 Comments

AnnaSchumann Over a year ago

Very concise +1. What is the ~ doing within [u,~,v]?

Luis Mendo Over a year ago

~ is just a way of discarding the second output of unique. It would be functionally equivalent to [u, x, v] = unique(list); clear x

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