Convert mathematical operations from string to integer?

[TheBasix]

If I have input string s = "5+3"; and I have to calculate this and return INT which is equal to 8. How can I do that?

[MiiNiPaa]

You want to parse math expression. Here is example of algorithm using stacks to parse arithmetic expressions: http://en.wikipedia.org/wiki/Shunting-yard_algorithm
and its implementations:
http://www.smccd.net/accounts/hasson/C++2Notes/ArithmeticParsing.html
http://pastebin.com/gSsZjMh2
http://habrastorage.org/getpro/habr/comment_images/532/3b4/a47/5323b4a47b1039df5b46fb310f711720.png

If you want to add even more complex functions you might need to build an AST: http://en.wikipedia.org/wiki/Abstract_syntax_tree
http://stackoverflow.com/questions/1721553/how-to-construct-an-abstract-syntax-tree

Or you can use one of the existing parsers: https://www.google.ru/search?q=math+expression+parser+c%2B%2B

[TheBasix]

Thats seems a little to complicated for this task. But how do I take each number from that string?

[MiiNiPaa]

But how do I take each number from that string?

Either use stringstreams ( http://www.cplusplus.com/reference/sstream/istringstream/ ) and work with string like with normal stream (as std::cin), or extract substring containing number and apply stoi() function.