Picking ints from strings

[Toothkiller]

*Not asking for code just point me in the right direction*

So I've written a program to handle strings that look like mathematics. Ex: it knows that sin(cos(x)) is first cos(x)=y then sin(y)=answer.

What I need now is advice on how to read a string and pull integers out of it. If I have "x + 2", I can make it realize it needs to add but not sure how to make it realize that the 2 is a double 2.

Again, I don't need a code answer. I am new to programming so I just need to know where to look.

[Zhuge]

I think your problem is a bit more general than that. You should look up information on (specifically, calculator) parsers. I'm sure there is a description of an algorithm for an infix calculator or something similar.

[Toothkiller]

What i'm doing requires me to pass strings though. The program is (eventually) going to be able to derive functions, then compute with them. It allows me to use a bit more advanced mathematics in simulations.

[Zhuge]

I would read it in as a string too. I don't really see how that makes looking for an algorithm any less relevant.

In the simple case of something like "3 + 2", you read the first part of the string (up to a space), call that your lhs. Read the next part for your operator, and the last for your rhs. Then perform the operation based on your operator. Naturally, it is a bit more complex with things like trigonometric functions, parentheses, variables, etc. but I hope you see what I'm trying to mean.

[Galik]

I'm not sure I get what you are asking but if you want to convert a string represention of a number into a double then this works:

#include <sstream>
#include <string>

std::string str = "23.56";

double dd;

// convert the string into an input stream and
// read a double from it
std::istringstream(str) >> dd; // dd now has value 23.56