Homework (Beginner's Function Creation) + Unknown Variable

This is a discussion on Homework (Beginner's Function Creation) + Unknown Variable within the C Programming forums, part of the General Programming Boards category; Code: /*Write a program which calculates the integral of the function f(x)=(A*x^m)/m! on the interval from a to b (a<b). ...

  1. #1
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    Homework (Beginner's Function Creation) + Unknown Variable

    Code:
    /*Write a program which calculates the integral of the function
    
                         f(x)=(A*x^m)/m!
    
    on the interval from a to b (a<b). In the main program, scanf
    a non-negative integer value of m and double values of A, a and b.
    Call the function Intgr() to evaluate the integral.
    
    Your main program should be followed by three functions:
    
    double Intgr(int m, double A, double a, double b)
    
    double func(double x, int m, double A)
    
    double mfact(int m)
    
    
    When writing the function Intgr(), use the program w5-10.c
    or the program from part B of your hw5, appropriately modified
    to integrate an arbitrary function f(x) from a to be (a is not
    necessarily equal to 0.0; also, let the program ask
    you for n_trap only once). Within Intgr() call another
    function func() to specify f(x). The return value of func() should
    be equal to A if m=0, or A*x^m/m! if m>0. To evaluate m! call the
    function mfact() that you will also write. To evaluate x^m call
    the function pow() already embedded in the math.h library.
    
    ................................................................
    
    Your output should look like this:
    
    Enter the exponent m in f(x)=A*x^m/m! : 5
    
    Enter the coefficient A in f(x)=A*x^m/m! : 2.5
    
    Enter the bounds for the integration interval, a < b : 1.25 3.75
    
    Integrate f(x) on [a,b]
    Enter the number of trapezoids: 1000
    
    The value of the integral is 9.64272 .
    
    */
    
    
    #include <stdio.h>
    #include <math.h>
    
    double Intgr(int m, double A, double a, double b);
    double func(double x, int m, double A);
    double mfact(int m);
    
    main()
    {
          int n_trap, m;
          double del_x, f, x, sum, A, a, b;
          printf("Enter the exponent m in f(x)=A*x^m/m! :");
          scanf("%d",&m);
          printf("Enter the coefficient A in f(x)=A*x^m/m! :");
          scanf("%lf",&A);
          printf("Enter the bounds for the integration interval, a < b :");
          scanf("%lf%lf",&a,&b);
          printf("Integrate f(x) on [a,b]\n");
          printf("Enter the number of trapezoids:");
          scanf("%d",&n_trap);
          printf("\nThe value of the integral is %.5f .\n",Intgr(m, A, a, b));
    }
    double mfact(int m)
    {
           int k,abs;
    printf("(mfact function) m is %d",m);
           abs=1;
           if(m!=0)
           {
                for(k=m;k>1;k--)
                abs*=k;
    printf("(mfact function) abs is %d",abs);
           }
           else if(m=0)
           {
                abs=1;
    printf("(mfact function) k is %d",k);
           }
           else if(m<0)
           {
                printf("Error: Cannot take factorial of negative integer\n");
                return;
           }
    return abs;
    }
    double func(double x, int m, double A)
    {
           if(m>=0)
           {
    printf("(in func) A is %f\n",A);
    printf("(in func) x is %f\n",x);
    printf("(in func) m is %d\n",m);
    printf("(in func) mfact(m) is %f\n",mfact(m));
    
               return A*pow(x,m)/mfact(m);
    printf("(in func) return is %f\n",A*pow(x,m)/mfact(m));
           }
           else
           {
               printf("Error: Cannot take factorial of negative number\n");
               return;
           }
    }
    double Intgr(int m, double A, double a, double b)
    {
           int n_trap,k; double del_x,x,f,sum;
           del_x=(b-a)/(double)n_trap;
           x=0.;
           f=func(x,m,A);
           sum=0.;
           for(k=0;k<=n_trap;k++) // Sum of x0 + x1 + x2 +...+x(n-1)+x(n)
           {
               x=((double)k*del_x)+a;
               sum+=f;
           }
           for(k=1;k<n_trap;k++) // Sum of x1 + x2 +...+ x(n-2)+x(n-1)
           {
               x=((double)k*del_x)+a;
               sum+=f;
           }
           sum*=0.5;
           sum*=del_x;
           return sum;
    }
    My problem (as of now) is that my 'x' is undefined. This isnt really a programming question, but what is x ( func() function)

    Thanks for any help.

  2. #2
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    and ignore the printf's along the way... I was just trying to see where my problem was.

  3. #3
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    you mean you are confused about the x in the f(x) = a*x^m/m! ? or are you confused about the x that is being declared by the program?

  4. #4
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    basically, yes. func() returns 0 and I cant set an x until the integral part.

  5. #5
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    firstly, you are missing a return type for your main function:

    Code:
    int main(){
    
         //some code.....
         return 0;
    
    }
    x is merely a placeholder for a value. In the case of evaluating an integral using the trapezoid method, the x (or delta x more appropriately) is the unit by which you increase the value used to evaluate the function, so you would start with f(a)+f(a+delta x)+....f(b)

    you always have x available to the function func() so long as you have read in the values for the boundary conditions on the integral before calling func().

    from reading the definitions, it seems to me that func() should return the first element in the summation. I think you might want to reconsider your definition of the function intgr(). It might also be prudent for you to look over your calculus text book and really try to throughly understand the trapezoid method....I don't get the impression from your code that you really understand it.
    Last edited by ExxNuker; 11-15-2005 at 03:55 PM.

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