For each positive integer $n$, let $S(n)$ be the sum of the digits of $n$. Determines the smallest positive integer $a$ such that there are infinite positive integers $n$ for which you have $S (n) -S (n + a) = 2018$.
1. **Lemma: \( S(n) \equiv n \pmod{9} \)**

   **Proof:**
   Consider a positive integer \( n \) expressed in its decimal form:
   \[
   n = 10^x a_1 + 10^{x-1} a_2 + \cdots + 10 a_{x-1} + a_x
   \]
   where \( a_i \) are the digits of \( n \) and \( 1 \leq a_i \leq 9 \). We can rewrite \( n \) as:
   \[
   n = (10^x - 1)a_1 + (10^{x-1} - 1)a_2 + \cdots + (10 - 1)a_{x-1} + a_1 + a_2 + \cdots + a_x
   \]
   Since \( 10^k \equiv 1 \pmod{9} \) for any integer \( k \), we have:
   \[
   n \equiv a_1 + a_2 + \cdots + a_x \pmod{9}
   \]
   Therefore, \( S(n) \equiv n \pmod{9} \), proving our lemma.

2. **Determine \( a \) such that \( S(n) - S(n + a) = 2018 \)**

   From the lemma, we know:
   \[
   S(n) \equiv n \pmod{9} \quad \text{and} \quad S(n + a) \equiv n + a \pmod{9}
   \]
   Therefore:
   \[
   S(n) - S(n + a) \equiv n - (n + a) \equiv -a \pmod{9}
   \]
   Given \( S(n) - S(n + a) = 2018 \), we have:
   \[
   2018 \equiv -a \pmod{9}
   \]
   Calculating \( 2018 \mod 9 \):
   \[
   2018 \div 9 = 224 \text{ remainder } 2 \quad \Rightarrow \quad 2018 \equiv 2 \pmod{9}
   \]
   Thus:
   \[
   -a \equiv 2 \pmod{9} \quad \Rightarrow \quad a \equiv -2 \pmod{9} \quad \Rightarrow \quad a \equiv 7 \pmod{9}
   \]
   The smallest positive integer \( a \) that satisfies \( a \equiv 7 \pmod{9} \) is \( a = 7 \).

3. **Verification:**

   Let \( n = 1\underbrace{000\cdots0}_{k} \underbrace{99\dots9}_{225} \), for any \( 1 \leq k \) with \( k \in \mathbb{Z} \), i.e., \( n = 10^{k+225} + 10^{226} - 1 \).

   Then:
   \[
   n + a = n + 7 = 1\underbrace{000\cdots0}_{k-1} 1\underbrace{000\dots0}_{224}6
   \]
   This can be written as:
   \[
   n + 7 = 10^{k+225} + 10^{226} + 6
   \]

   Calculating the sum of the digits:
   \[
   S(n) = 1 + 225 \cdot 9 = 226
   \]
   \[
   S(n + 7) = 8
   \]
   Therefore:
   \[
   S(n) - S(n + 7) = 226 - 8 = 218
   \]
   This does not match the required difference of 2018. Hence, we need to reconsider the construction of \( n \).

   Instead, consider \( n = 10^k - 1 \) for large \( k \). Then:
   \[
   S(n) = 9k
   \]
   \[
   n + 7 = 10^k - 1 + 7 = 10^k + 6
   \]
   \[
   S(n + 7) = 1 + 6 = 7
   \]
   Therefore:
   \[
   S(n) - S(n + 7) = 9k - 7
   \]
   Setting \( 9k - 7 = 2018 \):
   \[
   9k = 2025 \quad \Rightarrow \quad k = 225
   \]
   Thus, for \( k = 225 \), we have:
   \[
   S(n) - S(n + 7) = 2018
   \]
   This confirms that \( a = 7 \) satisfies the condition.

The final answer is \( \boxed{7} \).