Cache vs Memoization

Caching and memoization are related concepts, but they have some key differences:

Caching is a broader term that refers to storing frequently accessed data in a faster storage location to improve performance. Caches can be implemented at various levels, such as CPU caches, disk caches, or application-level caches. Caches are typically used to store the results of expensive operations or frequently accessed data.

Memoization, on the other hand, is a specific form of caching that is applied to function calls. It involves storing the results of a function for a given set of arguments, so that if the function is called again with the same arguments, the cached result can be returned instead of recomputing it.

Here's an example to illustrate the difference:

1#include <iostream>
2#include <unordered_map>
3
4// Memoization
5std::unordered_map<int, int> fibCache;
6
7int fibonacci(int n) {
8  if (fibCache.count(n) > 0) {
9    return fibCache[n];  // Return cached result
10  }
11
12  if (n <= 1) {
13    return n;
14  }
15
16  int result = fibonacci(n - 1) + fibonacci(n - 2);
17  fibCache[n] = result;  // Store result in cache
18  return result;
19}
20
21// Caching
22std::unordered_map<std::string, int> userAgeCache;
23
24int getUserAge(const std::string& userId) {
25  if (userAgeCache.count(userId) > 0) {
26    // Return cached result
27    return userAgeCache[userId];
28  }
29
30  // Simulating an expensive operation to
31  // retrieve user age from a database
32  int age{getUserAgeFromDatabase(userId)};
33
34  // Store result in cache
35  userAgeCache[userId] = age;
36  return age;
37}

In the first example, the fibonacci function uses memoization to cache the results of previous function calls. If the result for a given input n is already in the cache, it is returned directly, avoiding redundant calculations.

In the second example, the getUserAge function uses caching to store the ages of users. If the age for a given userId is already in the cache, it is returned directly, avoiding the need to retrieve it from the database again.

In summary, memoization is a specific technique for caching the results of function calls, while caching is a more general concept that can be applied to various scenarios beyond just function calls.