# Memoization

Dive into the world of computer science, delving specifically into the concept of memoization, a powerful technique used in optimising functions with overlapping inputs. This comprehensive guide brings you from the basic principles, the origins and algorithms of memoization, to real-world examples, demonstrating its practical efficiency in problem-solving situations. Further explore how the technique integrates into Python programming through practical coding samples. The exploration is rounded off by looking into the broader impact and applications of memoization in varied domains, while also critically recognising its advantages and potential drawbacks. Let this be your essential resource to understanding and mastering memoization in computer science.

#### Create learning materials about Memoization with our free learning app!

• Flashcards, notes, mock-exams and more
• Everything you need to ace your exams

## Understanding Memoization in Computer Science

When plowing the fields of computer programming and algorithm design, you may come across an interesting technique called memoization. But what exactly is it and why is it important? Dive in to unravel the depths of memoization.

### The Basics of Memoization

Say you're climbing a mountain, and as you go higher, you're marking the track you follow. If you need to climb the mountain again, your previous markings provide a guide so you don't have to figure out the route anew. This is, in a way, what memoization does in the realm of computer science.

Memoization is an optimization technique used primarily to speed up computer programs by storing the results of expensive function calls and reusing them when the same inputs occur again.

Memoization finds its value in situations where the same expensive computations are performed multiple times. Rather than carrying out the same calculations repeatedly, it's far more efficient to store and re-utilize previous results. Essentially, memoization trades memory space for improved runtime efficiency.

  function memoize(func) {
let cache = {};
return (...args) => {
let n = args[0];
if (n in cache) {
console.log('Fetching from cache');
return cache[n];
}
else {
console.log('Calculating result');
let result = func(n);
cache[n] = result;
return result;
}
}
}


As a concept, it employs two key principles:

• Recurring subproblems: These appear in recursive algorithms, where the same problem is broken down into smaller ones.
• Overlapping computations: These occur when the same computation is performed multiple times within different subproblems.

#### What is Memoization: A Simple Explanation

Let's consider an analogy to elucidate the concept of memoization further. Imagine you're reading a book. Suddenly, you find a word you don't know. Naturally, you look it up in a dictionary, note the meaning, and move on. After a while, you encounter the same word again. Would you look it up again? Or would you recall the meaning from your memory? Precisely. You'd recall it. That is memoization in a nutshell.

Consider the problem of calculating the nth Fibonacci number: a very common example of using memoization. Normally, it has a time complexity of O(2^n), but with memoization, it can be calculated in O(n) time.

It's important to mention that memoization is a specific form of caching, but while all memoization is caching, not all caching is memoization. Caching could be related to any arbitrary expensive computation, but memoization strictly deals with the re-computation of function outputs.

#### Tracing the Origins of Memoization within Computer Science

In the story of computer science, the term "memoization" was introduced by Donald Michie in the year 1968. It's a portmanteau of 'memorandum', meaning 'to remember something', and 'optimization'.

Although the term was coined in 1968, the technique has been implicitly used for a long time before that. For instance, Richard Bellman's dynamic programming theory—which predates memoization—is fundamentally a memoization concept.

Today, memoization is used in a multitude of languages and frameworks. It has proven to be an asset in reducing the time complexity of programs and is a frequent weapon of choice for many programmers dealing with recursive function calls. From the 'memoize' function in JavaScript, 'lru_cache' in Python, to various libraries in Java and C++, its footprint is truly well-established.

## The Structure and Function of Memoization Techniques

Memoization techniques are a cornerstone in optimising recursive algorithms, cutting down on computational expenses. To understand how these techniques function, it's essential to grasp their underlying structure.

### Delving Deeper into the Memoization Technique

The heart of the memoization technique lies in its ability to store results of complex function calls, ensuring these are not needlessly recalculated. This gives memoization its edge—an impressive increase in efficiency for recursive algorithms.

A recursive algorithm continually breaks a problem down into smaller subproblems, solving each until reaching a simple case that can be solved directly. Recursive algorithms are frequently used in numerous branches of computer science.

Let's examine the key aspects of the memoization technique:

• Storage: The heart of memoization lies in its ability to remember previously calculated results. The technique employs a storage structure, frequently a hash table or an array. Each time a function call with a new input takes place, the result is stored with its input as a key.
• Lookup: When a function is called, the memoization technique first looks up the result in the storage structure. If the result is found, it's instantly returned, bypassing the need for computation.
• Computation: If the result is not found in the cache, the function carries out the calculation and stores the result in the cache for future use.
function memoize(func) {
let cache = {};
return (...args) => {
if (args in cache)
return cache[args];
else {
const result = func(...args);
cache[args] = result;
return result;
}
}
}


#### Principles and Concepts Behind Memoization Algorithms

Memoization algorithms operate on two primary principles: overlapping subproblems and optimal substructure.

Overlapping subproblems: This principle states that in certain situations, the same subproblems are solved multiple times while computing the overall solution. Memoization mitigates this redundancy by saving the result once and recalling it on subsequent calls.

Optimal substructure: An optimal solution to a problem incorporates optimal solutions to its subproblems. For a problem exhibiting optimal substructure, a global optimum can be arrived at from the local optima.

In certain challenges, such as the longest common subsequence (LCS) problem and the knapsack problem, the recursive solutions involve solving the same subproblems multiple times. Here, memoization techniques come into play in increasing the efficiency of the solutions by avoiding repetition of the same computations.

#### How Does Memoization Work?: A Step-by-step Process

Let's explore how memoization works using a detailed step-by-step process:

1. Initialization: An empty table or array is created, serving as a cache to store computed results.
2. Function Call: When a function is called with an input, the algorithm first checks if the result for this input already exists in the cache.
3. Result Lookup:
• If the result is present in the cache, it is directly returned, eliminating the need for recomputation.
• If it is not in the cache, the function proceeds to compute the result.
4. Computation: The function executes the necessary computation and stores the result in the cache, associating it with the relevant input.
5. Return: The function then returns the computed result.
function memoize(func) {
let cache = {};
return (...args) => {
if (args in cache)
return cache[args];
else {
const result = func(...args);
cache[args] = result;
return result;
}
}
}


In this way, by applying the memoization technique, you can significantly increase the efficacy of your code, making it both cleaner and faster.

## Exploring Real-World Memoization Examples

If you've ever wondered how memoization applies in real-world scenarios, you're in the right place. Below, we'll delve into some practical examples and case studies, helping to demystify this integral computer science concept.

### Practical Memoization Examples in Various Situations

Memoization can turn turtle-paced algorithms into greyhounds of execution efficiency. It's employed in myriad ways, across an array of algorithmic challenges. Here, we'll explore its practical implementation, focusing primarily on two popular problems: Fibonacci sequence computation and the Longest Common Subsequence problem.

A Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding ones, usually starting with 0 and 1.

The Longest Common Subsequence (LCS) problem is a classic computer science problem, the analysis of which forms the basis of data comparison programs such as version comparison in version control systems.

Let's start with the Fibonacci sequence, which lends itself well to deterioration in performance when computed recursively. Calculating the nth term demands not just the computation of the (n-1)th and (n-2)th terms, but also an avalanche of repeat computations. Memoization preempts this inefficiency by caching previously computed terms.

function fibonacciMemo(n, memo = {}) {
if (n <= 1)
return 1;
if (!memo[n])
memo[n] = fibonacciMemo(n - 1, memo) + fibonacciMemo(n - 2, memo);
return memo[n];
}


The LCS problem offers another instance of overlapping subproblems that need not be computed more than once. In this case, memoization ensures that the length of the LCS for each pair of prefixes is stored and recalled, rather than recalculated.

function LCS(X, Y, m, n, dp)
{
if (m == 0 || n == 0)
return 0;

if (dp[m - 1][n - 1] != -1)
return dp[m - 1][n - 1];

if (X[m - 1] == Y[n - 1]) {
dp[m - 1][n - 1] = 1 + LCS(X, Y, m - 1, n - 1, dp);
return dp[m - 1][n - 1];
}
else {
dp[m - 1][n - 1] = Math.max(LCS(X, Y, m, n - 1, dp),
LCS(X, Y, m - 1, n, dp));
return dp[m - 1][n - 1];
}
}


These instances offer a glimpse into how memoization can be deployed to optimise recursive algorithms and enhance the time efficiency of your programs.

#### Case Studies: Applying Memoization in Problem-solving Scenarios

Memoization assumes a crucial role in problem-solving scenarios, most notably in dynamic programming, a problem-solving paradigm that solves complex problems by breaking them down into simpler, overlapping subproblems.

One classic case is in the computation of Factorials. Factorials are known for involving an extensive recursive operation that includes multiplication of a sequence of descending natural numbers. This is an ideal condition for memoization because of the inherent overlapping subproblems. A factorial program without memoization would have a time complexity of $$O(n!)$$, but with memoization it can be reduced to $$O(n)$$.

function factorial(n, memo = {}) {
if (n <= 1)
return 1;
if (!memo[n])
memo[n] = n * factorial(n-1, memo);
return memo[n];
}


Memoization also shines in graph traversal, commonly used in video games or routing algorithms. For instance, let's consider a knight in a game of chess. In some instances, you may need to calculate the shortest path for the knight to move from one position to another. Using a standard breadth-first search (BFS) might explore many unnecessary paths, leading to a considerable waste of computational resources. Here, memoization can optimize the search. By caching visited positions, you prevent the BFS algorithm from revisiting those positions in subsequent searches, effectively reducing redundant computations.

#### Evaluating the Efficiency of the Memoization Technique Through Examples

Let's take Fibonacci sequences: they're a prime example of a problem that initially seems easy to calculate. But there's a trap: the naive recursive algorithm for calculating Fibonacci numbers results in an exponential time complexity of $$O(2^n)$$. For any significant value of n, this quickly becomes untenable.

Now, let's say you deploy memoization. By storing each calculated Fibonacci number and checking the cache before calculation, the time complexity reduces to linear, $$O(n)$$. This significant performance boost exhibits the power of memoization as an optimization technique.

Evaluating time complexities through a comparison table:

 Fibonacci without Memoization Fibonacci with Memoization Time Complexity: $$O(2^n)$$ Time Complexity: $$O(n)$$

Thus, from the examples and evaluations provided, you can visualise the transformative leap in efficiency memoization grants to a program burdened down by gratuitous redundant computation, making it a powerful tool in the programmer's toolkit.

## Unravelling Memoization in Python

Memoization boasts a hugely significant role in programming, especially in languages like Python. Its application significantly enhances the efficiency and speed of your code by dramatically reducing the computation time for costly function calls.

### The Role of Memoization within Python Programming

Deep Dive: Python is an incredibly versatile programming language, making it an ideal platform to delve into the world of memoization. Python even provides built-in support for memoization through a technique known as decorator functions, showcasing the importance of this optimization strategy.

Python is renowned for its ease of use and flexibility, but even Python programs can become computationally expensive—especially when dealing with recurrent function calls in recursive programming or iterative algorithms. Memoization assumes a crucial role here by enabling the storage of the return values of expensive function calls and reusing the results when the same inputs occur. This ability to remember the result of a function with a particular argument reduces unnecessary computation time and optimises code.

Common use-cases of memoization in Python programming include:

• Solving Recursive Problems: Recursive algorithms, used for problems like calculating the Fibonacci series or Factorials, often involve heavy repetition. Memoization allows efficient computation by remembering previously calculated results.
• Improving Dynamic Programming: Dynamic programming decomposes large problems into smaller subproblems—with optimal solutions requiring optimum solutions for each subproblem. By caching the results of these subproblems, memoization enables dynamic programming to run faster and more efficiently.
• Optimising Function Calls: Functions in Python can become expensive—especially if they contain complicated operations or feature in intensive loops. Memoisation allows these expensive function calls to be stored and directly retrieved, eliminating superfluous computation.

#### Python Coding: Integrating Memoization in Your Codes

Example: A simple demonstration of memoization can be shown in Python with the calculation of the Fibonacci series.

Python's versatility facilitates the application of memoization in various ways—ranging from explicit to implicit methods. For explicit implementations, a data structure (like a list or a dictionary) is used to store the results of function calls. Conversely, an implicit implementation utilises Python's built-in functionality—primarily through the @functools.lru_cache decorator, which automatically performs caching and invalidation.

Here's an example of explicit memoization in a recursive Fibonacci series calculation:

def fib(n, memo = {}):
if n in memo:
return memo[n]
elif n <= 2:
return 1
else:
memo[n] = fib(n-1, memo) + fib(n-2, memo)
return memo[n]


With implicit memoization using @functools.lru_cache, the definition simplifies dramatically:

from functools import lru_cache

@lru_cache(maxsize=None)
def fib(n):
if n < 2:
return n
else:
return fib(n-1) + fib(n-2)


#### Sample Python Programs Using Memoization

Example: Another instance where memoization proves beneficial is in the calculation of Factorials, typically a recursively heavy task.

The recursive factorial function undergoes multiple repeated calculations for larger arguments. By employing memoization—either explicitly with a dictionary or by utilising Python's built-in decorator—an efficient factorial function emerges:

Explicit Memoization:

def fact(n, memo = {}):
if n in memo:
return memo[n]
elif n <= 1:
return 1
else:
memo[n] = n * fact(n-1, memo)
return memo[n]


Implicit Memoization with @functools.lru_cache:

from functools import lru_cache

@lru_cache(maxsize=None)
def fact(n):
if n < 2:
return 1
else:
return n * fact(n-1)


These examples elucidate the transformative effect that integrating memoization can have on your Python code's performance—boosting efficiency, improving the readability of code, and ensuring smoother, faster execution.

## The Wide Array of Memoization Use Cases

Memoization, the technique of storing the results of expensive functions to avoid unnecessary recomputation, has a myriad of use cases. Besides computer programming, you'll find examples of its application in various disciplines and domains, from mathematics and physics to artificial intelligence and big data analysis. Understanding these different manifestations of memoization can help you tap into its full potential to optimise and supercharge your projects.

### Memoization in Different Disciplines and its Impact

Definition: Memoization is an optimisation technique used primarily in computer science to speed up programs by storing the results of expensive function calls and reusing them when the same inputs occur.

Memoization's inevitable mark on Computer Science has spilled over into numerous other disciplines. Its ability to remember previously computed results has wide-ranging implications and benefits in various fields.

Example: For instance, in Mathematics, computational problems such as finding the greatest common divisor (GCD) often involve repeated sub-problems. Here, memoization can step in, remembering previously calculated results, to speed up the calculation process.

def gcd(m,n, memo={}):
if (m,n) in memo:
return memo[(m,n)]
elif m % n == 0:
return n
else:
memo[(m,n)] = gcd(n, m % n)
return memo[(m,n)]


Another illustration is in Physics, where large-scale numerical simulations are commonplace. Often, these simulations carry out the same computational tasks repeatedly, leading to inefficient use of resources. Memoization can contribute significantly here, augmenting the speed and efficiency of these simulations.

The field of Artificial Intelligence (AI) and Machine Learning (ML) are another arena where memoization sees broad applications. Synthetic algorithms often rely on recursive methods. Techniques like Dynamic Programming that utilise memoization hence offer an efficient means of solving AI or ML problems, such as reinforcement learning tasks, optimisation problems, and more.

Lastly, big data analysis and cloud applications, where speed and efficiency are paramount, also benefit from memoization. By storing and reusing computation results, unnecessary repetition of processes is avoided, leading to faster data retrieval and overall performance improvement.

#### The Prolific Use Cases of Memoization in Various Domains

Given memoization's widespread implications, multiple domains exercise this technique for optimisation. Here are few:

• Animation Industry: Creating animations, especially 3D, is an intensive process that includes repetitive rendering tasks. Memoization techniques aid in drastically reducing rendering time.
• Video Game Development: Games often require the same calculations or data rendering. Employing memoization in computations improves the gameplay experience by enhancing speed and fluidity.
• Data Mining: Large data sets usually involve repetitive calculations. Memoization optimises these computations, reducing algorithmic time complexity and boosting data processing speed.
• Computer Graphics: Calculations such as pixel rendering and light reflectance become faster and more efficient with memoization, enhancing graphics performance.
• Combinatorial Optimization Problems: Combinatorial problems like the Knapsack problem can be computed more efficiently using memoization, helping overcome a massive problem—redundant computations.

#### Recognising the Benefits and Drawbacks: How can Memoization Influence Your Coding Experience?

Like everything else, memoization too has its set of benefits and drawbacks. Recognising these aspects will offer a nuanced overview of its impact.

 Benefits Drawbacks 1. Optimises time complexity of algorithms 1. Increased space complexity due to storage of results 2. Avoids redundant computations 2. Not beneficial for problems with unique subproblems 3. Enhances software performance 3. Impractical in multi-threaded environments due to synchronisation 4. Ideal for Dynamic Programming problems 4. Requires careful management to prevent excessive memory consumption

Knowing how and when to use memoization effectively is key to leveraging its advantages while mitigating its drawbacks. As a rule of thumb, memoization proves to be an excellent strategy when you confront a problem with overlapping subproblems, which is a characteristic trait of most Dynamic Programming issues. Conversely, in instances with unique subproblems or if memory is a constraint, it might be prudent to consider other optimisation techniques.

In conclusion, memoization is a potent technique that has left an indelible mark on not just Computer Science, but several disciplines. Understanding its use cases, advantages, and limitations can greatly benefit your programming skills and computational problem-solving abilities.

## Memoization - Key takeaways

• Memoization: It is a technique used in computer science to speed up computer programs by storing the results of expensive function calls and reusing them when the same inputs occur.
• Memoization Storage: The technique uses a hash table or an array to store the results of previous function calls with their inputs as a key, reducing the need for recomputation for subsequent calls with the same inputs.
• Memoization Principles: The technique operates on two primary principles: overlapping subproblems, where the same subproblems are solved multiple times in a problem, and optimal substructure, where optimal solutions to subproblems contribute to the optimal solution of the entire problem.
• Memoization Implementation: When a function is called with an input, the algorithm first checks if the result is present in the storage structure. If it is, the result is returned directly. If not, the function carries out the calculation, stores the result for future use, and then returns the calculated result.
• Memoization in Python: Python provides built-in support for memoization through decorator functions. It is especially useful for optimising recursive problems, improving dynamic programming, and optimising expensive function calls.

#### Flashcards in Memoization 15

###### Learn with 15 Memoization flashcards in the free StudySmarter app

We have 14,000 flashcards about Dynamic Landscapes.

What is the significance of Memoization in Computer Science?
Memoization in computer science is significant because it enhances efficiency by reducing redundant computation. It stores the results of expensive function calls which, when called upon in the future, can be retrieved quickly from the cache.
How is Memoization utilised in programming and algorithms?
Memoization is utilised in programming and algorithms by storing the results of expensive function calls and reusing them when the same inputs occur again, reducing computation time and optimising speed, particularly in repetitive, recursive calculations.
What are the potential challenges and limitations associated with Memoization in Computer Science?
The main challenges and limitations of memoization include higher memory usage because results are stored for re-use, complexity in implementing for certain problems, issues with system performance if the memoization table gets too large, and poor efficiency when dealing with recursive calls or repeated calculations.
What are some common applications and examples of Memoization in Computer Science?
Common applications of memoization in computer science include dynamic programming algorithms such as the Fibonacci sequence and the travelling salesperson problem, algorithms for combinatorial problems, and recursive function optimisation. It's widely used in optimising computation time and complexity.
What is the difference between Memoization and normal caching in the context of Computer Science?
Memoization is a specific form of caching that stores the return values of a function based on its input parameters. Normal caching broadly addresses storing data for quicker future retrievals, not specifically tied to function invocations or parameters.

## Test your knowledge with multiple choice flashcards

What is memoization in computer science?

What are the key principles of memoization in computer science?

Who introduced the term "memoization" and what does it mean?

StudySmarter is a globally recognized educational technology company, offering a holistic learning platform designed for students of all ages and educational levels. Our platform provides learning support for a wide range of subjects, including STEM, Social Sciences, and Languages and also helps students to successfully master various tests and exams worldwide, such as GCSE, A Level, SAT, ACT, Abitur, and more. We offer an extensive library of learning materials, including interactive flashcards, comprehensive textbook solutions, and detailed explanations. The cutting-edge technology and tools we provide help students create their own learning materials. StudySmarter’s content is not only expert-verified but also regularly updated to ensure accuracy and relevance.

##### StudySmarter Editorial Team

Team Computer Science Teachers

• Checked by StudySmarter Editorial Team