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Fix array index offset bug in simple-graph traversal examples 

+ Add comments to explain when to offset and when to use key values
This commit is contained in:
Shaun Reed 2021-06-28 06:56:58 -04:00
parent 1fc34d2dd4
commit 21ed349c39
2 changed files with 17 additions and 14 deletions
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2
cpp/algorithms/graphs/simple/graph.cpp
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@ -78,6 +78,8 @@ int main (const int argc, const char * argv[])
{9, {}},
}
);
// The graph traversed in this example is seen in MIT Intro to Algorithms
// + Chapter 22, Figure 22.7 on Topological Sort
// + Each node was replaced with a value from left-to-right, top-to-bottom

27
cpp/algorithms/graphs/simple/lib-graph.cpp
View File

@ -14,14 +14,13 @@
void Graph::BFS(int startNode)
{
// Track the nodes we have discovered
std::vector<bool> discovered(nodes_.size());
for (bool node : discovered) node = false; // Initialize all nodes to false
std::vector<bool> discovered(nodes_.size(), false);
// Create a queue to visit discovered nodes in FIFO order
std::queue<int> visitQueue;
// Visit the startNode
discovered[startNode] = true;
discovered[startNode - 1] = true;
visitQueue.push(startNode);
// Continue to visit nodes until there are none left in the graph
@ -33,15 +32,17 @@ void Graph::BFS(int startNode)
visitQueue.pop();
// Check if we have already discovered all the adjacentNodes to thisNode
// + Do not offset this by 1, since we are using the key value for a map
for (const auto &adjacent : nodes_[thisNode]) {
if (!discovered[adjacent]) {
if (!discovered[adjacent - 1]) {
std::cout << "Found undiscovered adjacentNode: " << adjacent << "\n";
// Mark the adjacent node as discovered
// + If this were done out of the for loop we could discover nodes twice
// + This would result in visiting the node twice, since it appears
// In the visitQueue twice
discovered[adjacent] = true;
discovered[adjacent - 1] = true;
// Add the discovered node the the visitQueue
// + Since this value will later be used as a map key, dont offset by 1
visitQueue.push(adjacent);
}
}
@ -53,17 +54,16 @@ void Graph::BFS(int startNode)
void Graph::DFS()
{
// Track the nodes we have discovered
std::vector<bool> discovered(nodes_.size());
for (auto node : discovered) node = false; // Initialize nodes to false
std::vector<bool> discovered(nodes_.size(), false);
// Visit each node in the graph
for (const auto &node : nodes_) {
std::cout << "Visiting node " << node.first << std::endl;
// If the node is undiscovered, visit it
if (!discovered[node.first]) {
if (!discovered[node.first - 1]) {
std::cout << "Found undiscovered node: " << node.first << std::endl;
// Mark the node as visited so we don't visit it twice
discovered[node.first] = true;
discovered[node.first - 1] = true;
// Visiting the undiscovered node will check it's adjacent nodes
DFSVisit(node.first, discovered);
}
@ -74,12 +74,13 @@ void Graph::DFS()
void Graph::DFSVisit(int startNode, std::vector<bool> &discovered)
{
// Check the adjacent nodes of the startNode
// + Do not offset startNode by 1, since we use it as a key to a map
for (auto &adjacent : nodes_[startNode]) {
// If the adjacentNode is undiscovered, visit it
if (!discovered[adjacent]) {
if (!discovered[adjacent - 1]) {
std::cout << "Found undiscovered adjacentNode: " << adjacent << std::endl;
// Mark the node as visited so we don't visit it twice
discovered[adjacent] = true;
discovered[adjacent - 1] = true;
// Visiting the undiscovered node will check it's adjacent nodes
DFSVisit(adjacent, discovered);
@ -93,8 +94,7 @@ std::vector<int> Graph::TopologicalSort()
std::vector<int> topologicalOrder;
// Track the nodes we have discovered
std::vector<bool> discovered(nodes_.size());
for (auto node : discovered) node = false; // Initialize nodes to false
std::vector<bool> discovered(nodes_.size(), false);
// Visit each node in the graph
for (const auto &node : nodes_) {
@ -122,6 +122,7 @@ void Graph::TopologicalVisit(
discovered[startNode - 1] = true;
// Check the adjacent nodes of the startNode
// + Do not offset by 1, since startNode is used as a key to the map
for (auto &adjacent : nodes_[startNode]) {
// If the adjacentNode is undiscovered, visit it
if (!discovered[adjacent - 1]) {