Question

I want to extract tokens from a given fraction expression, which is passed as a string,

Fraction s("-5 * 7/27");

Where the extracted tokens hold either a single operator or a sequence of digits as an operand, using the stl container only.

May someone guide me to do so? I want to know how to extract the tokens and differentiate the operand from operator, thank you.

Answer 1

Assuming there are always spaces between the tokens, here’s how put them all in a queue:

#include <string>
#include <queue>
#include <sstream>
#include <iostream>

using namespace std;

int main()
{
    stringstream formula("-5 * 7/27");
    string a_token;
    queue<string> tokens; 

    // Use getline to extract the tokens, and then push them onto the queue.
    while (getline(formula, a_token, ' ')) {
         tokens.push( a_token );
    }

    // Print and pop each token.
    while (tokens.empty() == false) {
        cout << tokens.front() << endl;
        tokens.pop();
    }
}

Running the program will output:

-5
*
7/27

Now to determine which are operators, numbers, or fractions, you can do something like this in the loop:

    if (a_token == "+" || a_token == "-" || a_token == "*" || a_token == "/")
    {
        // It's an operator
        cout << "Operator: " << a_token << endl;
    }
    else
    {
        // Else it's a number or a fraction.

        // Now try find a slash '/'.
        size_t slash_pos = a_token.find('/');

        // If we found one, it's a fraction.
        if (slash_pos != string::npos) {
            // So break it into A / B parts.

            // From the start to before the slash.
            string A = a_token.substr(0, slash_pos);

            // From after the slash to the end.
            string B = a_token.substr(slash_pos + 1);

            cout << "Fraction: " << A << " over " << B << endl;
        }
        else
        {
            // Else it's just a number, not a fraction.
            cout << "Number: " << a_token << endl;
        }
    }

This website: http://www.cplusplus.com/reference/string/string/ will give you information on the string functions.

After modifying the code and running it again, you’ll get output like this:

Number: -5
Operator: *
Fraction: 7 over 27