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Understanding Prim's Algorithm

Understanding Prim's Algorithm

This blog will help you understand Prim’s algorithm in detail, along with its workings and implementation. By the end of this blog, you’ll appreciate Prim’s algorithm’s brilliance and see how it quietly shapes the digital world around us. Are you ready to be amazed? Let’s dive in!

Table of Contents

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What is a Spanning Tree?

A spanning tree is a fundamental concept in graph theory, specifically in the domain of connected graphs. To understand it, let’s define a few key terms:

  • Graph: A graph is a collection of vertices (or nodes) and edges that connect pairs of vertices.
  • Connected Graph: A graph is considered connected if there is a path between every pair of vertices. In simpler terms, you can reach any vertex from any other vertex by following the edges.
  • Tree: A tree is a type of graph without cycles, meaning there are no closed loops or circuits.

Now, when we bring these concepts together, we will understand the following:

  • In a connected graph, a spanning tree is a subgraph that has all of the graph’s nodes.
  • It must be a tree, meaning it is connected and has no cycles.
  • The spanning tree retains the connectivity of the original graph but removes any unnecessary edges that might create loops.
  • In simpler terms, a spanning tree is like a simplified version of the original connected graph. It maintains the essential connections between all vertices without any redundant paths that would form cycles.

Why is this concept important? 

Well, in various optimization algorithms like Prim’s Algorithm, working with a spanning tree allows for streamlined and efficient solutions. It’s a way of extracting the essential structure of a connected graph without unnecessary complexity.

What is Prim’s Algorithm?

Prim’s algorithm is a method used in graph theory to find the minimum spanning tree for a connected, weighted graph. A minimum spanning tree is a subset of the graph’s edges that connects all the vertices with the minimum possible total edge weight. The algorithm starts with an arbitrary vertex and adds the shortest edge at each step, ensuring that the tree remains connected without forming cycles. 

Now that you know what Prim’s algorithm is, let us discuss the time complexity and space complexity of Prim’s algorithm.

Prim’s Algorithm Time Complexity

The time complexity of Prim’s algorithm depends on the data structures used for implementation. Typically, it is implemented using a priority queue or a min-heap to efficiently select the edge with the smallest weight at each step.

With Priority Queue/Min-Heap: O(E log V)

  • E is the number of edges.
  • V is the number of vertices.

The log V factor arises from the insertion and extraction operations on the priority queue or min-heap. Prim’s algorithm efficiently explores the graph by always selecting the minimum-weight edge, making it a relatively fast algorithm.

Prim’s Algorithm Space Complexity

The space complexity of Prim’s algorithm is mainly influenced by the data structures used for bookkeeping during the algorithm’s execution.

Using Priority Queue/Min-Heap: O(V + E)

  • V is the number of vertices.
  • E is the number of edges.

In addition to storing the graph itself, the algorithm uses additional space for maintaining the priority queue or min-heap. The space complexity is considered reasonable, making it practical for application in various scenarios.

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How Does Prim’s Algorithm Work?

Prim’s algorithm is a systematic, step-by-step process that grows a minimum-spanning tree by always choosing the edge with the lowest weight. This ensures efficiency in connecting all vertices while minimizing the overall weight of the edges—a key principle in graph theory and optimization. 

Let us now discuss the working of Prim’s algorithm in detail.

Greedy Algorithm Approach

Prim’s algorithm falls into the category of greedy algorithms. Greedy algorithms make locally optimal choices at each step with the hope of finding a global optimum. In the case of Prim’s algorithm, it starts from an arbitrary vertex and systematically adds edges with the lowest weights to build a minimum spanning tree.

Initializing the Minimum Spanning Tree

The process begins by initializing the minimum spanning tree with a single vertex, which can be chosen at random. This initial vertex serves as the starting point for the algorithm to grow the tree.

Finding and Adding Minimum Weight Edges

The algorithm then identifies all the edges that connect the existing tree to new vertices. From these candidate edges, it selects the one with the minimum weight and adds it to the tree. This step ensures that the tree keeps expanding while minimizing its overall weight.

Iterative Process

Steps 2 and 3 are repeated iteratively. In each iteration, the algorithm identifies new edges and consistently adds the one with the smallest weight to the minimum spanning tree. This process continues until all vertices are part of the tree.

Termination

Prim’s algorithm concludes when all vertices are included in the minimum spanning tree. At this point, you have a tree that efficiently connects all vertices with the minimum total edge weight.

Once termination is carried out, Prim’s algorithm can be widely used in various applications, such as network design, where the goal is to connect points with the least possible cost. 

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Step-by-Step Implementation of Prim’s Algorithm

Step 1: Understand the Problem

Prim’s algorithm is used to find the minimum spanning tree in a connected, undirected graph. A spanning tree is a subgraph that includes all the vertices of the original graph and forms a tree without any cycles.

Step 2: Choose a Starting Point

Pick any vertex from the graph to start the algorithm. This will be the initial point for building the spanning tree.

Step 3: Create a Set for Selected Vertices

Create an empty set to keep track of the vertices that will be included in the spanning tree. Initially, this set will only contain the starting vertex.

Step 4: Create a Priority Queue for Edges

Build a priority queue to store all the edges connected to the selected vertices. The priority should be based on the weight of the edges, with the smallest weight having the highest priority.

Step 5: Explore Edges

Repeat the following steps until all vertices are included in the spanning tree:

  • Pick the edge with the smallest weight from the priority queue.
  • Check if adding this edge will form a cycle in the selected vertices. If it doesn’t, add the edge to the spanning tree.
  • Add the vertex at the other end of the chosen edge to the set of selected vertices.
  • Update the priority queue with all the edges connected to the newly added vertex.

Step 6: Termination

Continue this process until all vertices are included in the spanning tree. The algorithm terminates when the set of selected vertices contains all the vertices from the original graph.

Step 7: Final Result

The final result is a minimum spanning tree that connects all vertices with the minimum possible total edge weight.

Example of Prim’s Algorithm

Prim’s algorithm is a greedy algorithm used to find the minimum spanning tree (MST) of a connected, undirected graph. The minimum spanning tree is a subset of the edges that connects all the vertices in the graph with the minimum possible total edge weight without forming any cycles.

Let us now understand Prim’s algorithm with an example. 

Imagine you have a map with cities connected by roads, and each road has a certain distance. You want to build the most efficient network of roads to connect all the cities. Prim’s algorithm will help you do this. Let’s break down how it works:

Consider the map below for the cities connected via the roads.

Step 1: Choose a starting city. Let’s call it Q.

Step 2: Look at all the roads connected to Q, and choose the shortest one. In our case, the roads are QC (11 units) and QR (7 units). Pick the shorter one, QR, and add it to your network.

Step 3: Now, look at all the roads connected to the cities in your network (which are now just Q and S). Choose the shortest road again. In this case, it’s ST (3 units). Add ST to your network and explore the adjacent cities, which are R and P.

Step 4: Repeat the process. Look at all the roads connected to your network (QS, ST, and RS now) and pick the shortest one. RS is the shortest here, so add it to your network.

Step 5: Keep going until all cities are connected. In this case, choose the shortest road from your network (QS, ST, RS, and RP now). RP is the shortest remaining road, so add it to your network.

At this point, you’ve connected all the cities in the most efficient way possible. The roads you’ve chosen form a minimum spanning tree (MST), and its total cost is the sum of the weights of all the roads you’ve selected. In this example, it’s 7 + 3 + 2 + 5 = 17 units.

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Optimizing Prim’s Algorithm for Efficiency

Optimizing Prim’s algorithm for efficiency involves making strategic choices to reduce unnecessary computations and improve the overall performance of the algorithm. Let us find out how you can optimize Prim’s algorithm for efficiency.

  • Use a Priority Queue: Instead of scanning the entire list of vertices to find the minimum edge each time, use a priority queue. This data structure keeps track of the vertices with their corresponding weights and allows you to extract the minimum efficiently.
  • Lazy Prim’s Algorithm: In the standard Prim’s algorithm, you eagerly add edges to the MST right away. In the lazy version, you delay the decision to add an edge until it’s necessary. This reduces redundant computations and makes the algorithm more efficient.
  • Avoid Duplicate Edges: While adding edges to the MST, ensure that you don’t add duplicates. Keep track of the vertices already included in the MST to avoid unnecessary computations and to maintain the correctness of the algorithm.
  • Optimize Priority Queue Operations: If using a priority queue, choose an efficient implementation. Binary heaps, or Fibonacci heaps, are commonly used for Prim’s algorithm. These data structures offer faster insertion and extraction of minimum elements.
  • Update Key in Priority Queue: When a vertex’s key (distance) changes, update it in the priority queue. This ensures that you always have the correct and minimum key for each vertex, without the need to insert the same vertex multiple times.

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Advantages and Disadvantages of Prim’s Algorithm

There are a lot of advantages and disadvantages to Prim’s algorithm. Let us discuss them in detail ahead. Let us first discuss the advantages of Prim’s algorithm.

Advantages

  • Optimal Solution: Prim’s algorithm guarantees the generation of a minimum spanning tree, ensuring that the total weight or cost of the tree is minimized. This is advantageous when efficiency and cost-effectiveness are crucial considerations.
  • Efficiency: The algorithm is efficient and runs in O(V^2) time complexity with an adjacency matrix representation, where V is the number of vertices. However, using advanced data structures, like a Fibonacci heap, can reduce the time complexity to O(E + V log V), making it highly practical for large graphs.
  • Incremental Construction: Prim’s algorithm builds the minimum spanning tree incrementally, adding one vertex at a time. This allows for easy implementation and understanding, making it suitable for practical applications.
  • Distributed Nature: The algorithm is well-suited for distributed systems and can be adapted for parallel processing, making it applicable in scenarios where resources are distributed across multiple nodes.
  • Versatility: Prim’s algorithm applies to both weighted and unweighted graphs, and it can handle scenarios where the weights represent distances, costs, or any other relevant metric.

Disadvantages

  • Limited to Connected Graphs: Prim’s algorithm can only be applied to connected graphs. If the input graph consists of multiple disconnected components, the algorithm needs to be applied separately to each component.
  • Dependency on Data Structures: The efficiency of Prim’s algorithm is highly dependent on the choice of data structures, such as priority queues or heaps. Using less efficient data structures can impact the algorithm’s overall performance.
  • Sensitivity to Edge Weights: If the edge weights are subject to frequent changes, applying Prim’s algorithm each time can be computationally expensive. In such cases, dynamic programming approaches or alternative algorithms may be more suitable.

Prim’s Algorithm Application

Prim’s algorithm finds applications in various fields. Some of them have been listed below.

  • Network Design: Prim’s algorithm is used to design efficient and cost-effective communication networks by determining the optimal connections between nodes.
  • Circuit Design: In electronic circuit design, the algorithm helps in minimizing the total wire length while ensuring connectivity, which is crucial for optimizing circuit performance.
  • Transportation Networks: It is employed in designing transportation networks like road systems, where the goal is to connect cities or locations with the least overall cost.
  • Resource Management: Prim’s algorithm is used in resource management scenarios, such as minimizing the cost of laying pipelines or cables to connect different locations.
  • Computer Networks: The algorithm is applied in computer networks to establish the most efficient connections between routers or servers, optimizing data transmission paths.

Prim’s Vs. Kruskal’s Algorithm

Below is a tabular comparison of Prim’s and Kruskal’s algorithms in terms of key features. These algorithms are both effective ways to find the minimum spanning tree of a graph, but the choice between them may depend on the characteristics of the specific graph being considered.

FeaturePrim’s AlgorithmKruskal’s Algorithm
ObjectiveFinds the minimum spanning tree (MST) of a connected, undirected graphFinds the minimum spanning tree (MST) of a connected, undirected graph
Selection of EdgesSelects the edge with the minimum weight that connects a vertex in the MST to a vertex outside the MSTSorts all the edges in the graph by weight and adds them to the MST in ascending order, avoiding cycles
Data StructuresTypically implemented using a priority queue or a min-heap to efficiently select the minimum-weight edgeUses a disjoint set data structure (Union-Find) to keep track of connected components and detect cycles
Algorithm TypeGreedy algorithm, as it makes locally optimal choices at each stepGreedy algorithm, as it makes locally optimal choices by selecting the smallest edge at each step
EfficiencyEfficient for dense graphs (graphs with many edges)Efficient for sparse graphs (graphs with few edges)
Time ComplexityO((V + E) log V), where V is the number of vertices and E is the number of edgesO(E log V), where E is the number of edges and V is the number of vertices
Memory RequirementTypically requires less memory than Kruskal’s algorithmRequires more memory due to the need to store all edges and their sorting
ParallelizationLess suitable for parallelizationMore suitable for parallelization due to its independence in edge selection
Use CasesSuitable for graphs with dense edge connectionsSuitable for graphs with sparse edge connections
Cycle DetectionNot explicitly required, as the algorithm naturally avoids cyclesRequires cycle detection to avoid forming cycles in the MST

Conclusion

Prim’s algorithm is a greedy algorithm for finding a minimum spanning tree in a connected, undirected graph. It is a simple, efficient, and versatile algorithm that is widely used in practice. It is used for finding a minimum spanning tree in a connected, undirected graph. It starts with any vertex in the graph and keeps adding the nearest vertex to the tree until all vertices are in the tree.

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FAQs

What is a minimum spanning tree (MST)?

A minimum spanning tree is a subset of the edges of a connected, undirected graph that connects all the vertices with the minimum possible total edge weight without forming any cycles.

When should I use Prim's algorithm over Kruskal's algorithm?

Prim’s algorithm is more suitable for dense graphs where there are many edge connections between vertices. Its efficiency is notable in such scenarios, making it a preferable choice.

In which situations is Kruskal's algorithm more advantageous?

Kruskal’s algorithm is more efficient for sparse graphs, where there are fewer edge connections. It excels at handling graphs with a limited number of edges.

Does Prim's algorithm guarantee the same minimum spanning tree for any given graph?

Yes, Prim’s algorithm guarantees to find the same minimum spanning tree for a connected, undirected graph. The order in which edges are selected may vary, but the resulting MST will have the same total weight.

Is cycle detection necessary in Prim's algorithm?

No, cycle detection is not explicitly required in Prim’s algorithm. The nature of the algorithm naturally avoids the formation of cycles as it selects edges that connect the current minimum spanning tree to vertices outside it.

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About the Author

Senior Consultant Analytics & Data Science

Presenting Sahil Mattoo, a Senior Consultant Analytics & Data Science at Eli Lilly and Company is an accomplished professional with 14 years of experience across data science, analytics, and technical leadership domains, demonstrates a remarkable ability to drive business insights. Sahil holds a Post Graduate Program in Business Analytics and Business Intelligence from Great Lakes Institute of Management.