5.15.2 Expression input

GiNaC provides no way to directly read an expression from a stream because you will usually want the user to be able to enter something like ‘2*x+sin(y)’ and have the ‘x’ and ‘y’ correspond to the symbols x and y you defined in your program and there is no way to specify the desired symbols to the >> stream input operator.

Instead, GiNaC lets you construct an expression from a string, specifying the list of symbols to be used:

{
    symbol x("x"), y("y");
    ex e("2*x+sin(y)", lst(x, y));
}

The input syntax is the same as that used by ginsh and the stream output operator <<. The symbols in the string are matched by name to the symbols in the list and if GiNaC encounters a symbol not specified in the list it will throw an exception.

With this constructor, it's also easy to implement interactive GiNaC programs:

#include <iostream>
#include <string>
#include <stdexcept>
#include <ginac/ginac.h>
using namespace std;
using namespace GiNaC;

int main()
{
    symbol x("x");
    string s;

    cout << "Enter an expression containing 'x': ";
    getline(cin, s);

    try {
        ex e(s, lst(x));
        cout << "The derivative of " << e << " with respect to x is ";
        cout << e.diff(x) << ".\n";
    } catch (exception &p) {
        cerr << p.what() << endl;
    }
}