Counting occurance of exact word from string array

Question

Tae Lim 2020-10-4

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https://ww2.mathworks.cn/matlabcentral/answers/604555-counting-occurance-of-exact-word-from-string-array

编辑： Stephen23 2020-10-4

Hi,

I am having trouble counting how many times a certain word appears in a string. I need to count the 'exact match' only, ignoring substrings. I tried 'count' and some other options but couldn't figure this out. The strings are chemical equations and the words are chemical species. Here's an example:

R1 = ["Ar* + Ar*  ","  Ar+ + Ar + e "]; % chemical reaction (string array; col1=reactants, col2=products)
S = {'Ar';'Ar*';'Ar+';'e'}; % species (cell array)

I want to get two variables as a result (below).

NumR = [0, 2, 0, 0]; % Number of occurance of each species as reactants
NumP = [1, 0, 1, 1]; % Number of occurance of each species as products
% each column represent number of occurance of Ar, Ar*, Ar+, e respectively

I tried 'count' function but this gives me a wrong result. For instance, if I try to count the number of occurance of 'Ar', Matlab also counts 'Ar+' and 'Ar*' as 'Ar' even though they are different species. How do I ignore substrings (i.e. 'Ar+' and 'Ar*' in this case)?

Thank you!

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Answer 1

Stephen23 2020-10-4

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https://ww2.mathworks.cn/matlabcentral/answers/604555-counting-occurance-of-exact-word-from-string-array#answer_504736

在 MATLAB Online 中打开

>> C = {'Ar* + Ar*  ','  Ar+ + Ar + e '};
>> S = {'Ar';'Ar*';'Ar+';'e'};
>> rgx = strcat('(?<=\s|^)',regexptranslate('escape',S),'(?=\s|$)');
>> NumR = cellfun(@numel,regexp(C{1},rgx))
NumR =
     0
     2
     0
     0
>> NumP = cellfun(@numel,regexp(C{2},rgx))
NumP =
     1
     0
     1
     1

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Tae Lim 2020-10-4

Thank you for your reponse. This works great! Thank you. Could you explain further how 'rgx' works? I do not follow the logic behind it.

Stephen23 2020-10-4

编辑：Stephen23 2020-10-4

在 MATLAB Online 中打开

"Could you explain further how 'rgx' works?"

rgx is a cell array of regular expressions, one for each corresponding cell in S.

A simple break-down of the regular expression, where XXX is the escaped string from S:

'(?<=\s|^)XXX(?=\s|$)'
'(?<=\s|^)           ' % lookbehind: must be space or start of string
'         XXX        ' % string from S (special characters are escaped first)  
'            (?=\s|$)' % lookahead: must be space or end of string

These regular expressions are used by regexp to search the strings in C. When a substring matches this regular expression, regexp returns the indices. Then cellfun is used to count those indices (i.e. how many matches).

https://www.mathworks.com/help/matlab/matlab_prog/regular-expressions.html

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Answer 2

KSSV 2020-10-4

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https://ww2.mathworks.cn/matlabcentral/answers/604555-counting-occurance-of-exact-word-from-string-array#answer_504733

在 MATLAB Online 中打开

R1 = ["Ar* + Ar*  ","  Ar+ + Ar + e "]; % chemical reaction (string array; col1=reactants, col2=products)
s = "Ar" ;
n = nnz(strfind(R1,s))

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Tae Lim 2020-10-4

Thank you for your response. I actually get an error saying that I have either missing or incorrect argument for nnz. The 'strfind(R1,s)' gives me 1x2 cell array. Is there a way to fix this?

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