具有复杂类型的 Swift CustomStringConvertible

Question

我希望 CustomStringConvertible 为我提供节点的描述，特别是它包含的边。

作为背景，我一直在研究图论并创建了一个节点:

class Node : CustomStringConvertible {
    // unique identifier required for each node
    var identifier : Int
    var distance : Int = Int.max
    var edges = [Edge]()
    var visited = false

    var description: String {
        return "identifier: " + identifier.description + ", Edges: " + ( "edgesString" )
    }

    init(visited: Bool, identifier: Int, edges: [Edge]) {
        self.visited = visited
        self.identifier = identifier
        self.edges = edges
    }

    static func == (lhs: Node, rhs: Node) -> Bool {
        return lhs.identifier == rhs.identifier
    }
}

有边

class Edge {
    var from: Node // does not actually need to be stored!
    var to: Node
    var weight: Int
    var description : String {
        return "from: " + from.description + ", to: " + to.description + ", weight: " + weight.description
    }
    init(to: Node, from: Node, weight: Int) {
        self.to = to
        self.weight = weight
        self.from = from
    }
}

我可以很容易地打印出每个节点的每条边

testGraph.nodes.forEach { $0.edges.forEach{ print ($0.description)}}

但是我无法通过节点的描述来实现 in。

我尝试为每个语句写出与我的等效的内容

var description: String {
    var edgesString = String()
    edges.forEach{  edgesString.append($0.description)}
    return "identifier: " + identifier.description + ", Edges: " + ( edgesString )
}

但在这种情况下，执行会给出 EXC_BAD_ACCESS，实际上我无法完成任何代码，也无法提供节点的描述和其中包含的边。

如何完成节点的描述字符串，然后继续描述边？

Answer 1

您的description 代码似乎会导致Node.description 和Edge.description 无限循环。

Node 为每条边调用 Edge.description，Edge 为它的 from 和到节点。如果图有循环连接，而不是星形连接，则会导致无限循环。

一个简单的方法是Edge.description只显示Node.identifier而不是详细的描述。

class Edge {
    var from: Node // does not actually need to be stored!
    var to: Node
    var weight: Int
    var description : String {
        return "{ Edge, from: \(from.identifier), to: \(to.identifier), weight: \(weight) }"
    }
    init(to: Node, from: Node, weight: Int) {
        self.to = to
        self.weight = weight
        self.from = from
    }
}
class Node : CustomStringConvertible {
    // unique identifier required for each node
    var identifier : Int
    var distance : Int = Int.max
    var edges = [Edge]()
    var visited = false

    var description: String {
        let edgesString = edges.map { $0.description }.joined(separator: ", ")
        return "{ Node, identifier: \(identifier), Edges: [\(edgesString)] }"
    }

    init(visited: Bool, identifier: Int, edges: [Edge]) {
        self.visited = visited
        self.identifier = identifier
        self.edges = edges
    }

    static func == (lhs: Node, rhs: Node) -> Bool {
        return lhs.identifier == rhs.identifier
    }
}

let rootNode = Node(visited: false, identifier: 10, edges: [])
var edges: [Edge] = []
for i in 0..<3 {
    let node = Node(visited: false, identifier: i, edges: [])
    let edge = Edge(to: node, from: rootNode, weight: i)
    edges.append(edge)
}
rootNode.edges = edges

print(rootNode)
// { Node, identifier: 10, Edges: [{ Edge, from: 10, to: 0, weight: 0 }, { Edge, from: 10, to: 1, weight: 1 }, { Edge, from: 10, to: 2, weight: 2 }] }

如果你想搜索和打印所有节点，你最好再做一个函数来做。

它应该记住你已经访问过哪个节点(或者如果可以的话，使用visited成员)，尽量不要访问那些访问过的节点。