TheAlgorithms/Java · error · Error

Please call solve() before fetching the solution.

Error message

Please call solve() before fetching the solution.

What it means

Thrown by `TwoSat.getSolutions` when `solve()` has not been called. The assignment array is only computed during `solve()`, so reading it earlier would return a default all-false array. Like the sibling check, this is an `Error` indicating API misuse.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/graphs/TwoSat.java:214

     * @throws Error if called before {@link #solve()}
     */
    boolean isSolutionExists() {
        if (!isSolved) {
            throw new Error("Please call solve() before checking for a solution.");
        }
        return hasSolution;
    }

    /**
     * Returns one valid assignment of variables that satisfies the boolean formula.
     *
     * @return a boolean array where {@code result[i]} represents the truth value of
     *         variable {@code xᵢ}
     * @throws Error if called before {@link #solve()} or if no solution exists
     */
    boolean[] getSolutions() {
        if (!isSolved) {
            throw new Error("Please call solve() before fetching the solution.");
        }
        if (!hasSolution) {
            throw new Error("No satisfying assignment exists for the given expression.");
        }
        return variableAssignments.clone();
    }

    /** Performs DFS to compute topological order. */
    private void dfsForTopologicalOrder(int u, boolean[] visited, Stack<Integer> topologicalOrder) {
        visited[u] = true;
        for (int v : graph[u]) {
            if (!visited[v]) {
                dfsForTopologicalOrder(v, visited, topologicalOrder);
            }
        }
        topologicalOrder.push(u);
    }

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Call `solve()` before `getSolutions()`
2. Gate `getSolutions()` behind an `isSolutionExists()` check that itself requires solve()
3. Encapsulate the full sequence in a single facade method

Example fix

// before
boolean[] s = ts.getSolutions(); // throws Error
// after
ts.solve();
boolean[] s = ts.getSolutions();

ts.solve(); // must run before getSolutions()

try {
    ts.getSolutions();
} catch (Error e) {
    // solve() not called first
}

When it happens

Trigger: Calling `getSolutions()` without a prior `solve()`, or when `solve()` sits behind a branch that did not execute.

Common situations: Forgetting to call solve(); refactoring that drops the call; assuming construction implies solving.

Related errors

• Please call solve() before checking for a solution.
• Number of variables cannot be negative.
• Variable number must be between 1 and {numberOfVariables}
• No satisfying assignment exists for the given expression.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/dc6f9e2a456572b6. Report an issue: GitHub.