问题
I have expressions that can be provided by the user, such as:
a*sin(w*t)
a+b/c
x^2+y^2/2
And I would like to just get the list of variables there. I don't need to do any substitutions. So, for the first formula it's gonna be {a,w,t}. For the second one {a,b,c}, and for the last one {x,y}.
The expression is primarily written to be parsed with Sympy, but I need to be able to get the list of variables in C++ for some checks. I would like to:
- Avoid having to link the whole Python interpreter to my program
- Avoid reinventing the wheel, as I saw there are many parsing libraries available, such as
muparser, but I don't know if any of these provide this functionality
What's the easiest way to do this? How would you tackle this problem?
回答1:
Given an the input: const string input we can collect or variables into set<string> with a regex:
\b([a-zA-Z]\w*)(?:[^(a-zA-Z0-9_]|$)
You could use this in C++ as follows:
const regex re{ "\\b([a-zA-Z]\\w*)(?:[^(a-zA-Z0-9_]|$)" };
const set<string> output{ sregex_token_iterator(cbegin(input), cend(input), re, 1), sregex_token_iterator() };
Live Example
EDIT:
regex explanation:
\basserts a\Wcharacter or the beginning or end of the string([a-zA-Z]captures anything begining with an alphabetic charachter\w*)followed by any number of "word" characters(?:specifies the start of my non-capturing optional match[[^(a-zA-Z0-9_]the 1st option is a non-open-parenthesis\Wcharacter|$)the other option is that the end of the input has been reached
来源：https://stackoverflow.com/questions/40997788/c-extracting-symbols-variables-of-an-analytical-mathematical-expression