Floating point exception during Breadth First Search

Question

Everything else seems to work just fine, but it returns a Floating point exception during execution of BFS in this graph. Earlier, the program worked fine during debugging, BFS worked successfully but crashed during runtime. And now, that too isn't happening anymore.

Can't figure out what's wrong.

The code:

#include<stdio.h>
#include<stdlib.h>

//structure to define a node
struct node{
    int vertex;
    struct node *next;
};

//structure to define a graph
struct Graph{
    int numVertices;
    struct node** array;
};

//structure to define a queue
struct Q{
    int front, rear, size;
    int *array;
};

//create a node
struct node* createNode(int dest){
    struct node *new=(struct node*)malloc(sizeof(struct node));
    new->vertex=dest;
    new->next=NULL;
    return new;
}

//create a graph
struct Graph* createGraph(int num){
    struct Graph *graph=(struct Graph*)malloc(sizeof(struct Graph));
    graph->numVertices=num;
    graph->array=(struct node**)malloc(num*(sizeof(struct node*)));
    
    int i;
    for(i=0;i<num;i++)
        graph->array[i]=NULL;
    return graph;
}
//create a queue
struct Q* createQ(int size){
    struct Q *new=(struct Q*)malloc(sizeof(struct Q));
    new->front=new->rear=0;
    new->array=(int*)malloc(size*sizeof(int));
    return new;
}

//enqueue
void enqueue(struct Q *q,int data){
    if((q->rear+1)%q->size==q->front)
        printf("Queue is full.\n");
    else{
        q->rear=(q->rear+1)%q->size;
        q->array[q->rear]=data;
    }
}

//dequeue
int dequeue(struct Q *q){
    if(q->front==q->rear)
        printf("Queue is empty.\n");
    else{ 
        int x;
        q->front=(q->front+1)%q->size;
        x=q->array[q->front];
        return x;
    }
}

//is empty queue
int isEmpty(struct Q *q){
    if(q->front==q->rear)
        return 0;
    else return 1;
}

//BFS
void BFS(struct Graph *graph, int start){
    
    struct Q *q=createQ(graph->numVertices);
    struct node *temp;
    int curr;
    int *visited=(int*)malloc(graph->numVertices*sizeof(int));
    
    visited[start]=1;
    enqueue(q,start);
    
    while(!isEmpty(q)){
        curr=dequeue(q);
        printf("%d ",curr);
        
        temp=graph->array[curr];
        while(temp!=NULL){
            if(visited[temp->vertex]!=1){
                visited[temp->vertex]=1;
                enqueue(q,temp->vertex);
            }
            temp=temp->next;
        }
    }                
}

//add an edge to graph
void addEdge(struct Graph *graph,int src, int dest){
    
    //edge from src to dest
    struct node *new=createNode(dest);
    new->next=graph->array[src];
    graph->array[src]=new;
    
    //edge from dest to src
    new=createNode(src);
    new->next=graph->array[dest];
    graph->array[dest]=new;
    
}

//print a graph
void printGraph(struct Graph *graph){
    struct node *temp;
    int i;
    for(i=0;i<graph->numVertices;i++){
        temp=graph->array[i];
        printf("Adj List of Vertex %d",i);
        while(temp!=NULL){
            printf("-> %d",temp->vertex);
            temp=temp->next;
        }
        printf("\n");
    }
}

int main(){
    
    struct Graph *graph=createGraph(5);
    addEdge(graph, 0, 3); 
    addEdge(graph, 0, 4); 
    addEdge(graph, 1, 2); 
    addEdge(graph, 1, 3); 
    addEdge(graph, 1, 4); 
    addEdge(graph, 2, 3); 
    addEdge(graph, 3, 4); 
    
    printGraph(graph);
    
    BFS(graph,0);
}

You have missed to initialize size (queue size) in createQ(). Try changing that and see — Sai Sreenivas, Jul 13 '20 at 16:07

score 3 · Accepted Answer · answered Jul 13 '20 at 16:08

3

You are getting the SIGFPE in enqueue:

if ((q->rear + 1) % q->size == q->front)

This is because q->size is zero and you're getting a divide-by-zero exception.

q->size is never set correctly. So, in createQ, add:

new->size = size;

answered Jul 13 '20 at 16:08

Craig Estey

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Floating point exception during Breadth First Search

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