Understanding Asymptotic Notation in Data Structure

This blog will help you understand the different asymptotic notations in data structures in detail. We will also discuss the common growth rates and compare the different notations. So, what are you waiting for? Let us get insight into big O notation, omega notation, and theta notation in asymptotic notation.

Table of Contents

• What is Asymptotic Notation in Data Structure?
• Why Do We Need Asymptotic Notation in Data Structure?
• Big O Notation
• Omega Notation
• Theta Notation
• Common Growth Rates for Asymptotic Notation
• Comparing Different Asymptotic Notations in Data Structure
• Conclusion
• FAQs

Watch the video below to understand the time and space complexity of data structures:

What is Asymptotic Notation in Data Structure?

Asymptotic notation in data structures is a mathematical way to express the efficiency of algorithms in terms of input size. It helps analyze how the algorithm’s performance scales as the input grows.

In other terms, it is a tool that allows one to discuss and compare algorithms in a standardized way, focusing on their efficiency and scalability without getting bogged down by implementation details. There are primarily three types of asymptotic notations:

• Big-O Notation (O-notation)
• Omega Notation (Ω-notation)
• Theta Notation (Θ-notation)

Why Do We Need Asymptotic Notation in Data Structure?

Asymptotic notation in data structures is crucial for varied reasons. Some of the reasons why we need them are listed below.

• Abstraction of Complexity: It simplifies the analysis of algorithm efficiency by focusing on its behavior as the input size becomes very large, abstracting away specific details.
• Algorithmic Comparison: It allows for a clear and concise way to compare different algorithms, helping in the selection of the most efficient algorithm for a specific task.
• Platform Independence: It provides a high-level understanding of algorithm efficiency that is not dependent on hardware, programming languages, or specific implementation details.
• Scales with Input: Asymptotic analysis is scalable, making it applicable to various problem sizes and helping predict how an algorithm will perform as the input grows.
• Optimization Guidance: It guides developers in optimizing code by identifying areas that might impact performance, enabling the creation of more efficient algorithms.

Big O Notation

The Big O notation is a mathematical notation that describes the limiting behavior of a function when the argument approaches infinity. In simpler terms, it is a way to express how the runtime or space requirements of an algorithm grow as the input size increases.

In the Big O notation graph, the x-axis represents the input size, and the y-axis represents the algorithm’s complexity. The Big O notation characterizes the upper bound of the graph, showing the worst-case scenario.

As the input size increases, Big O helps us understand the rate at which the algorithm’s performance increases. The lower the Big O, the more efficient the algorithm.

Mathematical Expression

The Big o notation is expressed as follows:

f(n) = O(g(n))

In this expression, 

• f(n) represents the actual time or space complexity of the algorithm.
• g(n) represents the mathematical function that acts as an upper bound on the growth rate of f(n).

This expression signifies that for sufficiently large values of n, the growth rate of f(n) is at most proportional to the growth rate of g(n). In simpler terms, g(n) describes an upper limit on the complexity of f(n).

The graph associated with this mathematical expression typically shows two functions, f(n) and g(n), on the same coordinate axes.

• The x-axis represents the input size n.
• The y-axis represents the computational steps or units of space used.
• The graph of f(n) represents the actual complexity of the algorithm.
• The graph of g(n) represents the upper bound function, indicating the growth rate that f(n) should not exceed.

Note: As n becomes sufficiently large, the graph of f(n) should not surpass the graph of g(n). If it does, it means the algorithm’s complexity is growing faster than the established upper bound, violating the Big O notation.

Example

def linearsearch(arr, target):
    for num in arr:
        if num == target:
            return True
    return False
arr = [1,2,3,4,5]
target=3
result=linearsearch(arr,target)
print(result)

Output: True

The time complexity of this linear search algorithm is O(n). As the array size grows, the number of operations the algorithm needs to perform grows linearly. If there are n elements in the array, the worst-case scenario would be checking each element once.

In the above example, the output would be True because the target element 3 is present in the array. 

But if you call the function with a target element not in the array, for example,

arr = [1,2,3,4,5]
target= 9
result=linearsearch(arr,target)
print(result)

Output: False

The output will be False since the target element 9 is not in the array.

Omega Notation

Omega notation in asymptotic analysis represents the lower bound on the growth rate of an algorithm; it signifies that the algorithm’s actual running time will not grow slower than a certain rate, even in the best-case scenario.

The curve in the omega notation represents the lower bound of an algorithm’s running time. This curve signifies that the algorithm will not perform better than this. It forms a lower boundary, indicating the least amount of resources (time or space) required for the algorithm.

Mathematical Expression

f(n)= Ω (g(n)) if there exist constants c and n0 such that f(n) ≥ c.g(n) for all n≥ n0.

The graph with two functions, f(n) and g(n), is represented below.

If f(n) =Ω g(n), it implies that g(n) forms a lower bound for f(n) after a certain point. The graph showcases this relationship by demonstrating that, for sufficiently large n, the growth of g(n) is below or equal to the growth of f(n).

Example:

def linear_search(arr, target):
    for element in arr:
        if element == target:
            return True
    return False

Output: The output in this case will be either True (if the target is found) or False (if the target is not found).

Consider the above example with a linear search algorithm, where arr is an array of length n. In the best-case scenario, the target element is found in the first position of the array. The time complexity of this best case is Ω (1). This is because, in the lower bound, the algorithm finds the target in constant time, irrespective of the input size.

Theta Notation

Theta notation, denoted as a Θ, is a mathematical notation used to describe both the upper and lower bounds of the running time of an algorithm. It provides a balanced representation of an algorithm’s performance.

Its graph has two functions f(n) and g(n). If f(n) = Θ(g(n)), it means that g(n) is both an upper and lower bound for f(n) after a certain point. For sufficiently large n, the growth of f(n) is sandwiched between c1*g(n) and c2*g(n), where c1 and c2 are positive constants.

The graph of f(n) = Θ(g(n)) signifies that g(n) provides a tight boundary for the growth of f(n). It neither grows faster nor slower than the multiple of g(n) after a certain point, representing a balanced and predictable performance.

Example

def binarysearch(arr, target):
    low, high=0, len(arr)-1
    while low<=high:
        mid=(low+high)//2
        if arr[mid]==target:
            return True
        elif arr[mid]<target:
            low=mid+1
        else:
            high=mid-1
    return false

Output: The output is either True (if target is found) or False if not found.

This binary search algorithm has arr (which is a sorted array of length n). The time complexity of the binary search in both the worst- and best-case scenarios is Θ(log n), indicating a logarithmic growth in performance and providing a representation of its efficiency.

Common Growth Rates for Asymptotic Notation

There are some growth rates for asymptotic notation, which we generally use to represent the performance of an algorithm. We have mentioned a few of them below:

Constant Time (O(1))

In constant time complexity, the algorithm’s performance remains consistent, regardless of the input size. No matter how large the dataset gets, the execution time stays the same—a fixed, constant speed.

Logarithmic Time (O(log n))

Logarithmic time complexity means the algorithm’s efficiency grows logarithmically with the input size. As the dataset increases, the algorithm’s execution time increases, but not at a linear rate. This is common in binary search algorithms.

Linear Time (O(n))

In linear time complexity, the algorithm’s performance scales proportionally with the input size. If the input doubles, the execution time also doubles. Linear algorithms iterate through each element once, making their efficiency directly tied to the dataset’s size.

Linearithmic Time (O(n log n))

Linearithmic time combines linear and logarithmic growth. Common in efficient sorting algorithms like merge sort and heap sort, it strikes a balance between speed and scalability as the dataset grows.

Quadratic Time (O(n^2)) and Beyond

Quadratic time complexity indicates that the execution time grows quadratically with the input size. For every increase in the dataset, the execution time squares. Algorithms with nested loops often exhibit quadratic time, and it’s a signal to be cautious with larger datasets. Beyond quadratic time, like cubic or higher, efficiency declines rapidly, generally making such algorithms impractical for large inputs.

Comparing Different Asymptotic Notations in Data Structure

Now that we know different types of asymptotic notation in data structures, let us compare Big O notation vs. Omega notation vs. Theta notation.

Conclusion

Understanding asymptotic notations in data structures is crucial; it enables measurement and comparison of algorithmic efficiency. Big O, Omega, and Theta notations will help you gain the ability to predict, optimize, and choose algorithms wisely. These asymptotic notations will help you make informed decisions and build scalable solutions.

FAQs