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Math Problem(Integrate a read in definite function and calculate it by

 
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Math Problem(Integrate a read in definite function and calculate it by
jackoblee
31 May, 2008 - 08:29 PM
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I can do the part of calculating Reiman sum, but i cannot think of the way to read in a function and then integrate it.
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KYA
RE: Math Problem(Integrate A Read In Definite Function And Calculate It By
31 May, 2008 - 08:39 PM
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#include <nerd.h>
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Show some code. All you'd have to is send the function as a parameter to the integration function. The more appropriate question: do you know how to code an integrating function? The math needs to come before the code.
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jackoblee
RE: Math Problem(Integrate A Read In Definite Function And Calculate It By
31 May, 2008 - 08:44 PM
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I don't know how to integrate a function in programming.
Btw, is it very difficult?
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joske
RE: Math Problem(Integrate A Read In Definite Function And Calculate It By
1 Jun, 2008 - 01:18 AM
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You have two issues here:
  • For reading a custom function (for example from a file or user input), you need an expression parser, which is able to interpret and evaluate a string containing an expression, for example "x^2 - 5". C++ does not have such an expression parser built in. Some interpreted programming languages like Javascript and thinBasic do have a function eval("expression") built in, which is a powerful tool. If you can't use such a language for your project, you need to built an expression parser yourself, which is a lot of work. You can search on the internet for a "C++ expression parser".
  • Second issue is integrating the function. Have a look here. It is very easy to do a numeric integration. If you want to integrate a function 2*x^3 from x_start to x_end, with a stepsize dx, you just do:
    cpp

    double x_start = 0.0;
    double x_end = 2.0;
    double dx = 0.01;     // step size (making this smaller will gives more accurate result
    double x = x_start + dx / 2.0; // start at x_start plus a half stepsize!
    double integral = 0.0;
    while (x < x_end)
    {
        double y = 2.0 * pow(x, 3);
        integral += y * dx;
        x += dx;
    }
    printf("Integral = %g \n", integral);  // exact solution would be 8.0 for this function

    There are other (more complex) methods for integration too. You can have a look into that if the method above is not fast/accurate enough.
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