I wanna implement "1234"+"2345" = "3579" but It displays nothing.Code:void sumString(char *a, char *b, int len) { int i; for (i = 0; i < len; i++) a[i] = (char)(((int)a[i]-48 + (int)b[i]-48) % 10); for (i = 0; i < len; i++) printf("%c", a[i]); printf("\n"); }