你的程序里，变量 "h" 没有初始值，导致后面的结果为 0，
不过现在可以编译和运行了，检查一下是不是那些变量都设置对了。

good luck

程序如下
------------------------
#include <stdio.h>
#include <math.h>
#define NT double

NT f(NT x, NT y)
{
return x+y;
}

NT rkstep (NT x, NT y, NT h, NT *err_i)
{
NT k, y12, y1, k12;

k = f(x,y);

y12 = y + k*h/2;

k12 = f(x, y12);

y1 = y + k12*h;

*err_i = fabs(k-k12)*h/2;

return y1;
}

int main ()
{
int i = 0; // for loop index
NT x = 1;
NT h = 1; // what is the value of "h" ?

// dynamic sized arrays, (actually these are pointers)
NT *y;
NT *err;
NT *result;

int bound = 4; // "bound" is the size of your array

x=1;

// allocate memory for pointers
y = (NT*) malloc(bound*sizeof(NT));
err = (NT*) malloc(bound*sizeof(NT));
result = (NT*) malloc(bound*sizeof(NT));

// ensure memory allocation is successful
if(y == NULL || err == NULL || result == NULL)
{
printf("malloc() error: y, error or result!");
return 0;
}

y[0]=10;

for (i=0; i<bound; i++)
{
result[i]=rkstep(x, y[i], h, &err[i]);

printf("i=%d\n", i);
printf("result=%lf\n\n", result[i]);
}

y[0]=10;

for (i=0; i<bound; i++)
{
result[i]=rkstep(x, y[i], h, &err[i]);

printf("i=%d\n", i);
printf("result=%lf\n\n", result[i]);
}

// free the allocated memory space
free(y);
free(err);
free(result);

return 0;
}