TheAlgorithms/Python · error · ValueError

Incorrect input: {vertex} already exists in this graph.

Error message

Incorrect input: {vertex} already exists in this graph.

What it means

Raised by GraphAdjacencyMatrix.add_vertex when contains_vertex(vertex) is True, i.e. the vertex already has an index in vertex_to_index. Vertices act as unique keys; re-adding one would corrupt the index mapping and matrix dimensions, so the request is rejected. It commonly fires from the constructor's loop over the `vertices` argument when that list contains duplicates.

Source

Thrown at graphs/graph_adjacency_matrix.py:125

                f"{source_vertex} and {destination_vertex}"
            )
            raise ValueError(msg)

        # Get the indices of the corresponding vertices and set their edge value to 0.
        u: int = self.vertex_to_index[source_vertex]
        v: int = self.vertex_to_index[destination_vertex]
        self.adj_matrix[u][v] = 0
        if not self.directed:
            self.adj_matrix[v][u] = 0

    def add_vertex(self, vertex: T) -> None:
        """
        Adds a vertex to the graph. If the given vertex already exists,
        a ValueError will be thrown.
        """
        if self.contains_vertex(vertex):
            msg = f"Incorrect input: {vertex} already exists in this graph."
            raise ValueError(msg)

        # build column for vertex
        for row in self.adj_matrix:
            row.append(0)

        # build row for vertex and update other data structures
        self.adj_matrix.append([0] * (len(self.adj_matrix) + 1))
        self.vertex_to_index[vertex] = len(self.adj_matrix) - 1

    def remove_vertex(self, vertex: T) -> None:
        """
        Removes the given vertex from the graph and deletes all incoming and
        outgoing edges from the given vertex as well. If the given vertex
        does not exist, a ValueError will be thrown.
        """
        if not self.contains_vertex(vertex):
            msg = f"Incorrect input: {vertex} does not exist in this graph."
            raise ValueError(msg)

View on GitHub (pinned to f5988cc097)

Solutions

1. Guard: `if not graph.contains_vertex(v): graph.add_vertex(v)`.
2. De-duplicate the vertices list before passing it to the constructor: `vertices = list(dict.fromkeys(vertices))`.
3. Use a set to track added vertices during ingestion.

Example fix

# before
GraphAdjacencyMatrix(vertices=[1, 2, 2, 3])  # duplicate 2

# after
GraphAdjacencyMatrix(vertices=list(dict.fromkeys([1, 2, 2, 3])))

vertices = list(dict.fromkeys(vertices))  # order-preserving de-dup
g = GraphAdjacencyMatrix(vertices=vertices)

def no_duplicates(vertices) -> bool:
    return len(vertices) == len(set(vertices))

When it happens

Trigger: Calling add_vertex on an existing vertex; passing a `vertices` list with duplicate entries to __init__; adding a vertex twice in a data-ingestion loop without de-duplication.

Common situations: Ingesting node lists from data with repeated ids; combining vertex sets from multiple sources without set operations; re-running initialization code over an existing graph object.

Related errors

• Invalid input: {edge} must have length 2.
• Incorrect input: Either {source_vertex} or {destination_vert
• Incorrect input: The edge already exists between {source_ver
• Incorrect input: The edge does NOT exist between {source_ver
• Incorrect input: {vertex} does not exist in this graph.

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/042c440e7c8d035d. Report an issue: GitHub.