How to convert a string to an integer in C++?

Question

Here is my code so far:

 #include "stdafx.h"
 #include <iostream>
 #include <string>
 using namespace std;

 int main()
 {
 string exp;
 cout << "Enter a number and raise it to a power" << endl;
 cin >> exp;
 int num = exp[0];
 int pow = exp[2];

 cin.get();
 cin.ignore(256,'\n');
 }

Basically, I'm trying to make a program where you can enter something like "2^5" and it will solve it for you. So far, I have taken the first and third values of the string and called them "num" and "pow". (Number, Power) If you try something like "cout << num;" it will give you the decimal Ascii value. How do I convert it to a decimal?

Answer 1

You can read from cin directly to integer variables:

int n;
std::cin >> n;

but you cannot enter naturally looking expressions that way.

To read 2^5 you can use std::stringstream:

int pos = exp.find('^');
int n;
std::stringstream ss;
if(pos != std::npos){
    ss << exp.substr(0, pos);
    ss >> n;
}

and similar for the second variable.

This method is implemented in Boost as boost::lexical_cast.

More complicated expressions require building a parser and I suggest, that you read more about this subject.

Answer 2

strtol is very good at this. It reads as many numeric digits as possible, returns the number, and also gives you a pointer to the character that caused it to stop (in your case that would be the '^').

Answer 3

    int num;
    char op;
    int pow;
    if ((std::cin >> num >> op >> pow) && op == '^') {
            // do anything with num and pow
    }

Answer 4

It seems that all your numbers are inferior to 10, in this case exp[0]-'0' and exp[1]-'0' are enough.