double * p =(double *)malloc(sizeof(double)* a . length)

If you define an array of type int, such as int a [10];

His length is simpler, n = sizeof(a)/sizeof(int).

Assuming that the array is of int type, first apply for 10 elements;

int * a =(int *)malloc(sizeof(int)* 10)；

If there is another element, then you can.

a=(int *)realloc(a， 1 1 * sizeof(int))；

Find the number of elements int i, n =1;

for(I = 0； (a+i)！ = NULLi++)

{

}

n = I+ 1； //n is the number of elements.