Property in Golang struct not getting modified

Question

I'm coming from Java and trying to figure out how Go's type system works. I want to create a simple graph data structure and implement breadth first search. This is what I have so far.

package graph

import "fmt"

type Node struct {
    neighbors []Edge
    visited   bool
    name      string
}

type Edge struct {
    neighbor Node
}

type Graph struct {
    nodes []Node
}

func (g *Graph) addNode(node Node) {
    g.nodes = append(g.nodes, node)
}

func (n *Node) addEdge(neighbor Node) {
    edge := Edge{
        neighbor: neighbor,
    }
    n.neighbors = append(n.neighbors, edge)
}

func (g Graph) String() {
    for _, node := range g.nodes {
        //fmt.Printf("nodename: %v", node.name)
        fmt.Println(len(node.neighbors))
        if len(node.neighbors) > 0 {
            fmt.Print("node: %v, edges: ", node.name)
            for _, e := range node.neighbors {
                fmt.Print(e.neighbor.name)
            }
        }
    }
}

when I try to run it with test code:

func TestGraph(t *testing.T) {
    graph := Graph{}
    n1 := Node { name: "abc", }
    n2 := Node { name: "def", }
    graph.addNode(n1) 
    graph.addNode(n2) 
    n1.addEdge(n2)

    graph.String()
}

in my String() method, the len(node.neighbors) is always 0. What am I doing wrong? I thought that since I take in a reference type in addEdge, it modifies the node reference but I'm obviously I'm missing something about Go's type system.

FWIW String() is usually a method String() string that fulfils the fmt.Stringer (golang.org/pkg/fmt/#Stringer) interface. Most Go programmers will be surprised seeing a String() that doesn't return string — John Weldon
– John Weldon, Commented Oct 3, 2017 at 3:39

John Weldon · Accepted Answer · 2017-10-03 03:42:37Z

This isn't a type system issue, but an issue with how data is passed in Go.

I think the root misunderstanding is about "passing by reference". In Go, everything is passed by value, there is no pass by reference (https://golang.org/doc/faq#pass_by_value)

So when you pass a Node struct in to the addEdge method, it's actually making a copy of that struct.

If you want to refer to the same underlying struct instead of copying it you should pass the pointer to it.

Try the following slightly modified code, that uses pointers to pass the structs: (You can tweak and run the code here: https://play.golang.org/p/Qsbi4LBXS4)

package main

import "fmt"

type Node struct {
    neighbors []*Edge
    visited   bool
    name      string
}

type Edge struct {
    neighbor *Node
}

type Graph struct {
    nodes []*Node
}

func (g *Graph) addNode(node *Node) {
    g.nodes = append(g.nodes, node)
}

func (n *Node) addEdge(neighbor *Node) {
    edge := &Edge{
        neighbor: neighbor,
    }
    n.neighbors = append(n.neighbors, edge)
}

func (g Graph) String() {
    for _, node := range g.nodes {
        //fmt.Printf("nodename: %v", node.name)
        fmt.Printf("number of neighbors: %d\n", len(node.neighbors))
        if len(node.neighbors) > 0 {
            fmt.Printf("node: %v, edges: ", node.name)
            for _, e := range node.neighbors {
                fmt.Printf("%q", e.neighbor.name)
            }
            fmt.Println()
        }
    }
}

func main() {
    graph := &Graph{}
    n1 := &Node{name: "abc"}
    n2 := &Node{name: "def"}
    graph.addNode(n1)
    graph.addNode(n2)
    n1.addEdge(n2)

    graph.String()
}

Bruno Grieder · Accepted Answer · 2017-10-03 03:36:45Z

1

You need to pass your variables by pointer or a new value is created

 func (g *Graph) addNode(node *Node) {
    g.nodes = append(g.nodes, node)
 }

 ...
 n1 := Node { name: "abc", }
 graph.addNode(&n1)
 ...
 n1.addEdge(&n2)

answered Oct 3, 2017 at 3:36

Bruno Grieder

30k10 gold badges73 silver badges104 bronze badges

Collectives™ on Stack Overflow

Property in Golang struct not getting modified

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related