Enrol to start learning
Reading is open to everyone. Enrolling is free, and it is what unlocks the audio lessons, practice tests and progress tracking.
20.3.4. Example of BFS Execution
Learn content
Interactive Audio Lesson
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Today, we will discuss how to represent graphs for executing BFS. Remember, a graph is made up of vertices and edges. Can anyone explain what those terms mean?
Vertices are the points in the graph, like the dots in a dot-to-dot picture, and edges are the lines connecting them!
Exactly! Now, we can represent a graph with an adjacency matrix or an adjacency list. Which one do you think would use more space?
I think the adjacency matrix because it shows all possible connections, even if some are just zero?
That's correct! The adjacency matrix can become inefficient for large graphs where most pairs are not connected. Now, let’s look at an adjacency list. What are its advantages?
It only lists the connected vertices, so it saves space!
Well done! You are all getting the hang of this. Let's dive deeper into how BFS actually works next.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Now that we understand graph representation, let's discuss how BFS operates. We start from a source vertex. Can anyone tell me what happens next?
We explore all the vertices connected to that source vertex?
Right! We mark those new vertices as visited. How do we keep track of the order in which we explore these vertices?
By using a queue!
Exactly! Can anyone explain why a queue is suitable for this task?
Because it works in a first-in, first-out manner, so we explore all vertices at the current level before going deeper!
Good point! Remember, BFS explores level by level. Let’s summarize this part.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
As we implement BFS, we need to track visited vertices. We can also track parents to reconstruct paths. Why do you think this is important?
So we can figure out how to get back to the source vertex!
Exactly! Thus, when we mark a vertex as visited, we also record which vertex led to that visit. What value do we set for a vertex's parent if it starts unvisited?
Maybe 'undefined' or a negative value, like -1?
Fantastic! This helps differentiate visited from unvisited vertices. Let’s recall how leveling fits into BFS.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Now, who remembers the time complexity of BFS with the adjacency matrix?
That would be O(n^2) since we need to check every entry in the matrix.
Correct! And how does this change when using an adjacency list?
It drops to O(n + m) because we only check the edges that exist.
Exactly! This makes BFS more efficient for sparse graphs. Can anyone summarize why it's useful to know the complexity?
So we can choose the right representation depending on our graph’s structure!
Well said! This understanding helps us optimize our algorithms. Let's wrap up this session!
Overview
Short Summary
This section discusses the concept of Breadth First Search (BFS) for exploring graphs, explaining how it operates, its implementation, and its efficiency.
Medium Summary
The section details the Breadth First Search algorithm, including how graphs are represented, the BFS execution process with a queue, data structures involved, and the computational complexity related to different graph representations.
Detailed Summary
Detailed Summary of BFS Execution
Breadth First Search (BFS) is an algorithm used to explore a graph systematically. It starts from a given source vertex and examines all its neighboring vertices before moving on to the vertices at the next level. The search continues level by level until all reachable vertices are explored. To represent the graph, two methods are commonly used: an adjacency matrix and an adjacency list. In an adjacency matrix, the presence of edges between vertices is represented in a 2D format, while in an adjacency list, each vertex maintains a list of its neighbors which is more space-efficient for sparse graphs.
When performing BFS, vertices are marked as visited to avoid re-exploration. The algorithm employs a queue to keep track of vertices whose neighbors are yet to be explored. The marking of vertices as visited happens the first time they are encountered. BFS efficiently identifies the shortest path in terms of edges in unweighted graphs and can also keep track of the parent vertices to enable path reconstruction. Its computational complexity varies based on the representation used; in an adjacency matrix, it runs in O(n^2), while in an adjacency list, it runs in O(n + m), where n is the number of vertices and m is the number of edges. This makes BFS particularly suitable for sparse graphs where the number of edges (m) is significantly less than the square of the number of vertices (n^2).
Reference YouTube Videos
Audio Book
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountSo, the idea breadth first search is to explore the graph level by level. So, we start at the source vertex and we now explore all the vertices, which are one step away, connected by a direct edge to the source vertex. Then, we look at all vertices which are newly connected one step away from level 1 vertices. So, these are two steps away from the source vertex, then for three steps and so on.
Detailed Explanation
The Breadth First Search (BFS) algorithm is designed to explore a graph in a systematic manner. It begins at a chosen starting point, referred to as the source vertex. From this point, the algorithm first visits all nodes that are directly connected to the source. This first round of exploration identifies all vertices that are one step away. In subsequent rounds, BFS explores nodes that are two steps away, and continues this pattern, incrementally going further out from the source vertex.
Examples & Analogies
Imagine you're trying to find all your friends in a large party. You start with your closest friends as your 'source'—those you interact with directly. After meeting everyone around your close friends, you then move outwards to meet their friends, and so forth. By the end of the night, you may have met many people, exploring the party in layers, which is similar to how BFS explores a graph.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountNow, while we are doing this exploration, we have to keep track of two quantities: first, whether a given vertex has been visited; second, for each such vertex, we need to explore its neighbors.
Detailed Explanation
As we explore the graph using BFS, it's crucial to avoid visiting the same vertex multiple times. To manage this, we use a structure to mark each vertex as either 'visited' or 'not visited'. This allows the algorithm to keep track of which vertices have been explored, ensuring that once a vertex is visited, it won't be revisited, thus preventing loops and redundant processing.
Examples & Analogies
Think of playing a game of hide and seek in a park. As you find friends hiding in the bushes, you mark them with a check to indicate that you've found them and won't check the same bush again. This is similar to how BFS keeps track of which vertices have been visited.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountWe need to use some data structures to keep track of these quantities. We can keep an array 'visited' to maintain whether each vertex has been visited. Additionally, a queue can help manage the order of exploration.
Detailed Explanation
To efficiently implement BFS, we utilize two main data structures: an array to keep track of visited vertices and a queue to store vertices that need to be explored next. The array allows quick access to check if a vertex has been visited, while the queue follows the FIFO (First In, First Out) principle, ensuring that the vertices are processed in the order they were discovered.
Examples & Analogies
Consider a printing queue at a cafe. Orders are processed one at a time in the order they were received. Similarly, in BFS, vertices are explored in the order they are added to the queue, which represents the order they were found.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountBefore giving the actual code for breadth first search, let us workout the example that we have and see how this data structures are updated. So, we have these 10 vertices 1 to 10, and we are looking for a path starting at 1...
Detailed Explanation
Let's step through an example of applying BFS to a graph with vertices labeled 1 to 10. We start by initializing the queue with the source vertex, which is the vertex 1 in this case, and marking it as visited. As we explore the neighbors of vertex 1, we add them to the queue and continue the process for each vertex, checking its neighbors and marking them accordingly until all reachable vertices from the source have been visited.
Examples & Analogies
Imagine you're exploring a new city, starting at a landmark (vertex 1). You take a map (data structure) and note down which tourist attractions (neighboring vertices) you can visit from there. You write down your planned itinerary (queue) based on the attractions closest to you and proceed to visit those first before moving further away. By the end of your exploration, you would have noted all the attractions around your starting point and planned how to navigate to them.
--
Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Breadth First Search (BFS):
An algorithm to explore a graph level by level by starting from a source vertex.
- Adjacency Representation:
Graphs can be represented as either an adjacency matrix or an adjacency list.
- Queue:
A data structure suitable for BFS to manage which vertices to explore next.
- Visited Status:
Indicates whether a vertex has already been explored to avoid reprocessing.
- Parent Tracking:
Keeping track of the vertex that leads to another vertex for path reconstruction.
- Complexity Analysis:
BFS has different complexities based on the graph representation used.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
Example 1: When performing BFS on a simple graph with vertices 1 to 5, starting from vertex 1, we visit vertices 2, 3, and 4 in the first level, and then explore their neighbors in subsequent levels.
Example 2: In a graph with 10 vertices, if BFS is started at vertex 1, the algorithm will explore vertices based on their proximity level, queueing newly visited vertices for exploration until all reachable nodes are processed.
Memory aids
Imagine BFS as a curious explorer starting at a base camp, checking all tents nearby (neighbors) before venturing to the next row of tents, ensuring none are left unexplored.
BFS stands for 'Breadth First Search' - think 'Big Fast Steps' when exploring wide areas!
Flash Cards
Glossary
Graph
A collection of vertices (nodes) and edges (connections) representing relationships.
Adjacency Matrix
A 2D array used to represent a graph where entry [i][j] indicates if there is an edge from vertex i to vertex j.
Adjacency List
A collection of lists used to represent a graph; each vertex has a list of adjacent vertices.
Queue
A data structure used to store elements in a first-in, first-out (FIFO) manner.
Visited
A status indicating a vertex has been explored in BFS.
Parent Vertex
The vertex from which a new vertex was reached; useful for path reconstruction.