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Don't add extra slacks (artificial variables) for ranged rows #98

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Don't add extra slacks (artificial variables) for ranged rows #98

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6f88f67
Don't add extra slacks (artifical variables) for ranged rows
chris-maes Jun 11, 2025
94307c9
Merge branch 'branch-25.05' into less_artificals
rgsl888prabhu Jul 17, 2025
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24 changes: 17 additions & 7 deletions cpp/src/dual_simplex/presolve.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -459,23 +459,33 @@ i_t find_dependent_rows(lp_problem_t<i_t, f_t>& problem,

template <typename i_t, typename f_t>
i_t add_artifical_variables(lp_problem_t<i_t, f_t>& problem,
std::vector<i_t>& equality_rows,
const std::vector<i_t>& range_rows,
const std::vector<i_t>& equality_rows,
std::vector<i_t>& new_slacks)
{
const i_t n = problem.num_cols;
const i_t m = problem.num_rows;
const i_t num_cols = n + equality_rows.size();
const i_t nnz = problem.A.col_start[n] + equality_rows.size();
const i_t num_artificial_vars = equality_rows.size() - range_rows.size();
const i_t num_cols = n + num_artificial_vars;
i_t nnz = problem.A.col_start[n] + num_artificial_vars;
problem.A.col_start.resize(num_cols + 1);
problem.A.i.resize(nnz);
problem.A.x.resize(nnz);
problem.lower.resize(num_cols);
problem.upper.resize(num_cols);
problem.objective.resize(num_cols);

std::vector<bool> is_range_row(problem.num_rows, false);
for (i_t i : range_rows) {
is_range_row[i] = true;
}

i_t p = problem.A.col_start[n];
i_t j = n;
for (i_t i : equality_rows) {
if (is_range_row[i]) {
continue;
}
// Add an artifical variable z to the equation a_i^T x == b
// This now becomes a_i^T x + z == b, 0 <= z =< 0
problem.A.col_start[j] = p;
Expand All @@ -491,8 +501,8 @@ i_t add_artifical_variables(lp_problem_t<i_t, f_t>& problem,
problem.A.col_start[num_cols] = p;
assert(j == num_cols);
assert(p == nnz);
constexpr bool verbose = false;
if (verbose) { printf("Added %d artificial variables\n", num_cols - n); }
constexpr bool verbose = true;
if (verbose) { printf("Added %d artificial variables\n", num_artificial_vars); }
problem.A.n = num_cols;
problem.num_cols = num_cols;
return 0;
Expand Down Expand Up @@ -550,7 +560,7 @@ void convert_user_problem(const user_problem_t<i_t, f_t>& user_problem,
if (verbose) { printf("Constraints < %d = %d > %d\n", less_rows, equal_rows, greater_rows); }

if (user_problem.num_range_rows > 0) {
if (verbose) { printf("Problem has %d range rows\n", user_problem.num_range_rows); }
if (1 || verbose) { printf("Problem has %d range rows\n", user_problem.num_range_rows); }
convert_range_rows(
user_problem, row_sense, problem, less_rows, equal_rows, greater_rows, new_slacks);
}
Expand Down Expand Up @@ -579,7 +589,7 @@ void convert_user_problem(const user_problem_t<i_t, f_t>& user_problem,
}

// Add artifical variables
add_artifical_variables(problem, equality_rows, new_slacks);
add_artifical_variables(problem, user_problem.range_rows, equality_rows, new_slacks);
}

template <typename i_t, typename f_t>
Expand Down