【C和C++】 指针求定积分

#include<stdio.h>
#include<math.h>

double f1(float b,double l,int i)
{
    double k;
    k=sin(b+l*i);
    return k; 
}

double f2(float b,double l,int i)
{ 
    double k;
    k=cos(b+l*i);
    return k; 
}
double f3(float b,double l,int i)
{
    double k;
    k=exp(b+l*i);
    return k; 
}

double f4(float b,double l,int i)
{ 
    double k;
    k=1+(b+i*l)*(b+i*l);
    return k; 
}

double f5(float b,double l,int i)
{ 
    double k;
    k=exp(b+l*i)+1;
    return k; 
}

double f6(float b,double l,int i)
{ 
    double k;
    k=2*(b+l*i)+3;
    return k; 
}

double f7(float b,double l,int i)
{ 
    double k;
    k=1+(b+l*i);
    return k; 
}

double f8(float b,double l,int i)
{ 
    double k;
    k=(b+i*l)*(b+i*l)*(b+i*l);
    return k; 
}

int main()
{ 
    int c;
    float a,b;
    double num;
    double (*p)(float,double,int); 
    int i,j;
    double result,h,l;
    double sum=0;
    puts("1 (sinx)\n2 (cosx)\n3 (e^x)\n4 (1+x^2)\n5 (e^x+1)\n6 (2*x+3)\n7 (1+x)\n8 (x^3)\n");
    to:
    printf("please enter a b\n");
    scanf("%f%f",&a,&b);
    printf("a=%f,b=%f\n",a,b);
    fflush(stdin);
    printf("choose a number to use function\n");
    scanf("%d",&c);
    switch(c)
    {
        case 1:p=f1;break;
        case 2:p=f2;break;
        case 3:p=f3;break;
        case 4:p=f4;break;
        case 5:p=f5;break;
        case 6:p=f6;break;
        case 7:p=f7;break;
        case 8:p=f8;break;
        default:printf("inlegal number please try again!\n"); 
    }
    l=(a-b)/1000;
    for(i=0;i<1000;i++)
    {
        h=(*p)(b,l,i);
        result=h*l;
        sum+=result; 
    } 
    printf("\n%.2lf",sum);
    printf("\nNow you can use another function\n");
    goto to;
    return 0; 
    }