krahets/hello-algo · error

error

Error message

error

What it means

Panic thrown by addEdge(vet1, vet2) on the undirected adjacency-list graph when the operation is illegal. The guard aborts if either endpoint vertex is not present in the adjacency map, or if both arguments are the same vertex (self-loops are disallowed). This keeps the graph representation consistent: an edge can only connect two distinct, pre-registered vertices.

Source

Thrown at ru/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
}

/* Получить число вершин */
func (g *graphAdjList) size() int {
	return len(g.adjList)
}

/* Добавление ребра */
func (g *graphAdjList) addEdge(vet1 Vertex, vet2 Vertex) {
	_, ok1 := g.adjList[vet1]
	_, ok2 := g.adjList[vet2]
	if !ok1 || !ok2 || vet1 == vet2 {
		panic("error")
	}
	// Добавить ребро vet1 - vet2, добавив анонимную struct{}
	g.adjList[vet1] = append(g.adjList[vet1], vet2)
	g.adjList[vet2] = append(g.adjList[vet2], vet1)
}

/* Удаление ребра */
func (g *graphAdjList) removeEdge(vet1 Vertex, vet2 Vertex) {
	_, ok1 := g.adjList[vet1]
	_, ok2 := g.adjList[vet2]
	if !ok1 || !ok2 || vet1 == vet2 {
		panic("error")
	}
	// Удалить ребро 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. Call addVertex() for both endpoints before addEdge().
2. Guard the call: confirm both vertices exist and are distinct (vet1 != vet2).
3. When loading edges, deduplicate/register all distinct vertices in a first pass, then add edges.
4. After any removeVertex(), stop issuing edge calls that reference the removed vertex.

Example fix

// before: panics if a vertex is missing or vet1 == vet2
g.addEdge(a, b)

// after: ensure vertices exist and differ
if a != b {
    g.addVertex(a)
    g.addVertex(b)
    g.addEdge(a, b)
}

// Membership helper (adjList is unexported; expose a method on the type).
func (g *graphAdjList) hasVertex(v Vertex) bool {
    _, ok := g.adjList[v]
    return ok
}

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

defer func() {
    if r := recover(); r != nil {
        // edge rejected: missing vertex or self-loop
    }
}()
g.addEdge(vet1, vet2)

When it happens

Trigger: Calling addEdge() with a vertex that was never added via addVertex(); passing vet1 == vet2; adding an edge whose endpoint was previously removeVertex()'d.

Common situations: Building a graph from raw edge data and forgetting to register vertices first; reusing a graph instance after removing a vertex but still referencing it in edge calls; assuming the constructor registered a vertex that was actually skipped.

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/7c677ca86e7dc9c2. Report an issue: GitHub.