krahets/hello-algo · error

error

Error message

error

What it means

Panic thrown by addEdge(vet1, vet2) on the undirected adjacency-list graph (Traditional Chinese build) when the edge is illegal: an endpoint vertex is not in the adjacency map, or vet1 == vet2 (no self-loops). The graph only connects two distinct, already-registered vertices, so it aborts with 'error' otherwise.

Source

Thrown at zh-hant/codes/go/chapter_graph/graph_adjacency_list.go:45

	for _, edge := range edges {
		g.addVertex(edge[0])
		g.addVertex(edge[1])
		g.addEdge(edge[0], edge[1])
	}
	return g
}

/* 獲取頂點數量 */
func (g *graphAdjList) size() int {
	return len(g.adjList)
}

/* 新增邊 */
func (g *graphAdjList) addEdge(vet1 Vertex, vet2 Vertex) {
	_, ok1 := g.adjList[vet1]
	_, ok2 := g.adjList[vet2]
	if !ok1 || !ok2 || vet1 == vet2 {
		panic("error")
	}
	// 新增邊 vet1 - vet2, 新增匿名 struct{},
	g.adjList[vet1] = append(g.adjList[vet1], vet2)
	g.adjList[vet2] = append(g.adjList[vet2], vet1)
}

/* 刪除邊 */
func (g *graphAdjList) removeEdge(vet1 Vertex, vet2 Vertex) {
	_, ok1 := g.adjList[vet1]
	_, ok2 := g.adjList[vet2]
	if !ok1 || !ok2 || vet1 == vet2 {
		panic("error")
	}
	// 刪除邊 vet1 - vet2
	g.adjList[vet1] = DeleteSliceElms(g.adjList[vet1], vet2)
	g.adjList[vet2] = DeleteSliceElms(g.adjList[vet2], vet1)
}

View on GitHub (pinned to 69932aed18)

Solutions

1. Register both endpoints via addVertex() before addEdge().
2. Guard: ensure vet1 != vet2 and both vertices are present in the map.
3. Two-pass graph construction: collect distinct vertices, add them, then add edges.
4. Drop or log edges referencing unknown/duplicate vertices instead of panicking.

Example fix

// before: panics on missing vertex or self-loop
g.addEdge(a, b)

// after
if a != b {
    g.addVertex(a); g.addVertex(b)
    g.addEdge(a, b)
}

if vet1 != vet2 && g.hasVertex(vet1) && g.hasVertex(vet2) {
    g.addEdge(vet1, vet2)
}

defer func() {
    if r := recover(); r != nil {
        // addEdge rejected
    }
}()
g.addEdge(vet1, vet2)

When it happens

Trigger: addEdge() before both addVertex() calls; vet1 == vet2; referencing a vertex removed by removeVertex().

Common situations: Loading an edge list without first extracting/registering the vertex set; assuming the graph constructor created vertices it actually skipped; pipeline that deletes a vertex then re-adds an edge touching it.

Related errors

• error
• Illegal Argument Exception
• Illegal Argument Exception
• Illegal Argument Exception
• Illegal Argument Exception

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/28803473b53d40bb. Report an issue: GitHub.