#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main()
{
    int i = 0;
	char as[30];
    double A[1024];       
    while(1)
	{
		printf("\nEnter a, please:\n");
        gets(as);
        if (!strcmp(as, "END"))		
			break;        			
		A[i++] = strtod(as, NULL);
	}
	printf("Result values:\n");
	for(int j = 0; j < i; j++)
		printf("%f\t", A[j]);
}

Enter a, please:
123

Enter a, please:
asdfg

Enter a, please:
456

Enter a, please:
END
Result values:
123.000000      0.000000        456.000000