Implementing a graph class

Question

I'm having problems with the BFS method.
When I call it shows me the error "cannot read 'length' of undefined" if I try to use the dequeue method on other queues it works, and it returns the node correctly.
I also tried to use a number instead of u.arr.length in the for loop(where are the problems) and it shows me the same error with u.arr[I].finalnode that says "cannot read final node of undefined".
I think the problem is in the assignment of variable u, but I don't why.

class Node {
  //declaration of graph's nodes
  constructor(nome) {
    this.nome = nome;
    this.ar = [];
  }
}

class Item {
  //declaration of list's elements
  constructor(data, next) {
    this.data = data;
    this.next = null;
  }
}

class Queue {
  //declaration of list class
  constructor(head) {
    this.head = null;
    this.tail = null;
    this.size = 0;
  }
  enqueue(s) {
    var i = new Item(s);
    if (this.size == 0) {
      this.head = i;
      this.tail = i;
      this.size++;
    } else {
      if (this.size == 1) {
        this.tail = i;
        this.head.next = this.tail;
        this.size++;
      }
      if (this.size > 1) {
        this.tail.next = i;
        this.tail = this.tail.next;
        this.size++;
      }
    }
  }

  dequeue() {
    var s = this.head;
    this.head = this.head.next;
    this.size--;
    return s.data;
  }
}

class Graph {
  //declaration of graph class
  constructor() {
    this.arr = [];
  }

  insertnode(n) {
    this.arr.push(n);
  }

  insertedge(n1, n2, p) {
    n1.ar.push({ finalnode: n2, peso: p });
  }

  BFS(s) {
    var i;
    var u;
    var lista = new Queue();
    for (i = 0; i < this.arr.length; i++) {
      this.arr[i].color = "white";
      this.arr[i].d = Infinity;
      this.arr[i].parent = null;
    }
    s.color = "grey";
    s.d = 0;
    lista.enqueue(s);
    console.log(lista);
    while (lista.head != null) {
      u = lista.dequeue();

      for (i = 0; i < u.ar.length; i++) {
        //here is the problem
        u.ar[i].finalnode.color = "grey";
        u.ar[i].finalnode.parent = u;
        u.ar[i].finalnode.d = u.d + 1;
        lista.enqueue(u.ar[i]);
      }
      u.color = "black";
    }
  }

  DFS() {}
}

Can you give us some sample code that uses these classes that fails, so we can look at how the program operates? — Nisala, Apr 29 '21 at 15:16
var G = new Graph() var a = new Node('a') var b = new Node('b') G.insertnode(a) G.insertnode(b) G.insertedge(a, b, 5) G.BFS(a) this is the code i use to test my classes — Antonio Ugolini, Apr 29 '21 at 15:21

score 1 · Answer 1 · answered Apr 30 '21 at 10:34

You're doing lista.enqueue(u.ar[i]); in the BFS, but u.ar[i] is not a Node instance but an edge object { finalnode: n2, peso: p } as created in insertEdge.

You'll want to use

for (i = 0; i < u.ar.length; i++) {
  var finalnode = u.ar[i].finalnode;
  finalnode.color = "grey";
  finalnode.parent = u;
  finalnode.d = u.d + 1;
  lista.enqueue(finalnode);
//              ^^^^^^^^^
}

Implementing a graph class

1 Answers1