What causes "Edge index out of bounds or equal " in krahets/hello-algo?

Pass vertex INDICES (positions in vertices[]), not values — addEdge(graph, 0, 2) connects the 1st and 3rd vertices.

Edge index out of bounds or equal

Error message

Edge index out of bounds or equal

What it means

Emitted by addEdge() when either endpoint index is negative, >= graph->size, or when i == j. This implementation models a simple undirected graph (no self-loops, no multi-edges), so identical endpoints are forbidden alongside out-of-range ones. The function logs to stderr and returns void without modifying adjMat.

Source

Thrown at en/codes/c/chapter_graph/graph_adjacency_matrix.c:80

    for (int i = index; i < graph->size - 1; i++) {
        for (int j = 0; j < graph->size; j++) {
            graph->adjMat[i][j] = graph->adjMat[i + 1][j];
        }
    }
    // Remove the column at index from the adjacency matrix
    for (int i = 0; i < graph->size; i++) {
        for (int j = index; j < graph->size - 1; j++) {
            graph->adjMat[i][j] = graph->adjMat[i][j + 1];
        }
    }
    graph->size--;
}

/* Add edge */
// Parameters i, j correspond to the vertices element indices
void addEdge(GraphAdjMat *graph, int i, int j) {
    if (i < 0 || j < 0 || i >= graph->size || j >= graph->size || i == j) {
        fprintf(stderr, "Edge index out of bounds or equal\n");
        return;
    }
    graph->adjMat[i][j] = 1;
    graph->adjMat[j][i] = 1;
}

/* Remove edge */
// Parameters i, j correspond to the vertices element indices
void removeEdge(GraphAdjMat *graph, int i, int j) {
    if (i < 0 || j < 0 || i >= graph->size || j >= graph->size || i == j) {
        fprintf(stderr, "Edge index out of bounds or equal\n");
        return;
    }
    graph->adjMat[i][j] = 0;
    graph->adjMat[j][i] = 0;
}

/* Print adjacency matrix */

View on GitHub (pinned to 69932aed18)

Solutions

Pass vertex INDICES (positions in vertices[]), not values — addEdge(graph, 0, 2) connects the 1st and 3rd vertices.
Insert both endpoints with addVertex() first so that i and j are always < graph->size.
Reject self-loops at the call site: this implementation forbids i == j by design.

Example fix

// before
addEdge(graph, 1, 3);   // 1,3 are vertex VALUES -> index 1,3 may be out of bounds

// after
int i = indexOf(graph, 1), j = indexOf(graph, 3);
if (i >= 0 && j >= 0 && i != j) addEdge(graph, i, j);

Defensive patterns

Strategy: validation

Validate before calling

static inline int graphValidEdge(const GraphAdjMat *g, int i, int j) {
    return g != NULL && i >= 0 && j >= 0
        && i < g->size && j < g->size && i != j;
}

/* usage */
if (graphValidEdge(graph, i, j)) {
    addEdge(graph, i, j);
}

Prevention

Name call-site variables to separate indices from values (idxA, idxB vs valA, valB).
Insert all vertices first, then add edges, so endpoints are always in range.
If you need self-loops or multi-edges, this simple-undirected model is the wrong tool — use a different structure.

When it happens

Trigger: addEdge(graph, i, j) where i < 0, j < 0, i >= graph->size, j >= graph->size, or i == j. Concrete cases: addEdge(graph, 0, 0) (self-loop); addEdge(graph, 5, 3) where 5 and 3 are vertex VALUES mistaken for indices; addEdge before the vertices exist.

Common situations: Passing vertex values instead of array indices (the API uses 0-based positions in vertices[]); attempting self-loops in a simple-graph model; creating an edge before both endpoints have been inserted via addVertex; off-by-one from a <= vs < loop bound.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/4b094dcf993afd25. Report an issue: GitHub.