How to read equations from text file ?

[rroshan]

I am relatively new to C++. I would appreciate if you guys could help me out.
Fundamentally, I am trying to bring a large number of equations from Mathematica into c++. I have used some string functions in mathematica to convert everything into a C++ format. But I am stuck at importing them into C++. I could bring them as a string into a matrix. But I am clueless on how to make these meaningful equations.
let me put it in a simple way.I have a text file with contents:
a+b;
x+y;
a-b;
x-y;

I have a C++ program with a,b,x,y defined as float variables and they have been assigned values:
float a,b,x,y;
a=1;
b=2;
x=3;
y=4;

I used getline() to read these 4 lines from the text file, as strings into a matrix(vector of vectors), defined as RSS. So what I need to do is:
RSS[1][1]=a+b;
RSS[1][2]=x+y;
RSS[2][1]=a-b;
RSS[2][2]=x-y;
But I need them (a,b,x,y) as variables. Is there a way to do it? Or if there are any other better approaches you can recommend? Any help would be much appreciated.

[JLBorges]

Something along these lines, perhaps:

#include <iostream>
#include <string>
#include <cmath>

// define the variables a, b, x, y
const double a = 1.2 ;
const double b = 2.3 ;
const double x = 3.4 ;
const double y = 4.5 ;

// given a variable name as a char, return its value
// for instance, if var is 'x', return the value of x
double value_of( char var ) // get the value of a variable
{
    switch(var)
    {
        case 'a' : return a ;
        case 'b' : return b ;
        case 'x' : return x ;
        case 'y' : return y ;
        // http://en.cppreference.com/w/cpp/numeric/math/NAN
        default : return NAN ; // invalid variable name; decide what you want to do here
    }
}

// evaluate expression of the form var1 op var2 eg "a+b"
double evaluate( std::string expression )
{
    if( expression.size() != 3 ) return NAN ; // badly formed expression

    const double first = value_of( expression[0] ) ;
    const char oper = expression[1] ;
    const double second = value_of( expression[2] ) ;

    switch(oper)
    {
        case '+' : return first + second ;
        case '-' : return first - second ;
        case '*' : return first * second ;
        case '/' : return first / second ;
        default: return NAN ; // invalid operation name; decide what you want to do here
    }
}

std::string clean( std::string expr ) // remove whitespace, convert to lower case
{
    std::string clean_str ;

    for( char c : expr ) // for each character in expr http://www.stroustrup.com/C++11FAQ.html#for
        if( !std::isspace(c) ) // if it is not a white space http://en.cppreference.com/w/cpp/string/byte/isspace
            clean_str += c ; // add it to the clean string

    return clean_str ;
}

int main()
{
    std::string input = "  b + x " ;
    // read input from the file

    std::cout << "a==" << a << ",  b==" << b << ",  x==" << x << ",  y==" << y << '\n' ;
    const std::string expression = clean(input) ;
    std::cout << expression << " == " << evaluate( expression ) << '\n' ;
}

http://coliru.stacked-crooked.com/a/86753f8f76a328dc

Take it up from there.

For something more elaborate, create a lookup table, where the key is the name of the variable, mapped to its value.
see: http://en.cppreference.com/w/cpp/container/map