Add Expression::remove_variables() method

dimod/include/dimod/expression.h

@@ -16,6 +16,7 @@
 
 #include <limits>
 #include <unordered_map>
+#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -267,6 +268,10 @@ class Expression : public abc::QuadraticModelBase<Bias, Index> {
 
     virtual void remove_variable(index_type v);
 
+    /// Remove several variables from the expression.
+    template<class Iter>
+    void remove_variables(Iter first, Iter last);
+
     /// Set the linear bias of variable `v`.
     void set_linear(index_type v, bias_type bias);
 
@@ -624,6 +629,38 @@ void Expression<bias_type, index_type>::remove_variable(index_type v) {
     }
 }
 
+template <class bias_type, class index_type>
+template <class Iter>
+void Expression<bias_type, index_type>::remove_variables(Iter first, Iter last) {
+    std::unordered_set<index_type> to_remove;
+    for (auto it = first; it != last; ++it) {
+        if (indices_.find(*it) != indices_.end()) {
+            to_remove.emplace(*it);
+        }
+    }
+
+    if (!to_remove.size()) {
+        return;  // nothing to remove
+    }
+
+    // now remove any variables found in to_remove
+    size_type i = 0;
+    while (i < this->num_variables()) {
+        if (to_remove.count(variables_[i])) {
+            base_type::remove_variable(i);
+            variables_.erase(variables_.begin() + i);
+        } else {
+            ++i;
+        }
+    }
+
+    // finally fix the indices by rebuilding from scratch
+    indices_.clear();
+    for (size_type i = 0, end = variables_.size(); i < end; ++i) {
+        indices_[variables_[i]] = i;
+    }
+}
+
 template <class bias_type, class index_type>
 void Expression<bias_type, index_type>::set_linear(index_type v, bias_type bias) {
     base_type::set_linear(enforce_variable(v), bias);

releasenotes/notes/expression-remove-variables-4da6fef4b08ee743.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Add ``dimod::Expression::remove_variables()`` C++ method.
+    This method is useful for removing variables in bulk from the objective
+    or constraints of a constrained quadratic model.

testscpp/tests/test_constrained_quadratic_model.cpp

@@ -901,4 +901,58 @@ TEST_CASE("Test Expression::add_quadratic()") {
     }
 }
 
+TEST_CASE("Test Expression::remove_variables()") {
+    GIVEN("A CQM with five variables, an objective, and one constraint") {
+        auto cqm = ConstrainedQuadraticModel<double>();
+
+        cqm.add_variables(Vartype::BINARY, 10);
+        cqm.objective.set_linear(2, 1);
+        cqm.objective.set_linear(3, 2);
+        cqm.objective.set_linear(5, 3);
+        cqm.objective.set_linear(8, 4);
+        cqm.objective.set_quadratic(5, 8, 5);
+
+        auto c = cqm.add_linear_constraint({2, 3, 5, 8}, {1, 2, 3, 4}, Sense::EQ, 1);
+        cqm.constraint_ref(c).set_quadratic(5, 8, 5);
+
+        std::vector<int> fixed = {3, 8};
+
+        WHEN("We remove several variables from the objective") {
+            cqm.objective.remove_variables(fixed.cbegin(), fixed.cend());  // works with cbegin/cend
+
+            THEN("they are removed as expected") {
+                CHECK(cqm.num_variables() == 10);  // not changed
+
+                CHECK(cqm.objective.num_variables() == 2);
+                CHECK(cqm.objective.num_interactions() == 0);
+
+                CHECK(cqm.objective.linear(2) == 1);
+                CHECK(cqm.objective.linear(3) == 0);  // removed
+                CHECK(cqm.objective.linear(5) == 3);
+                CHECK(cqm.objective.linear(8) == 0);  // removed
+
+                CHECK(cqm.objective.variables() == std::vector<int>{2, 5});
+            }
+        }
+
+        WHEN("We remove several variables from the constraint") {
+            cqm.constraint_ref(c).remove_variables(fixed.begin(), fixed.end());
+
+            THEN("they are removed as expected") {
+                CHECK(cqm.num_variables() == 10);  // not changed
+
+                CHECK(cqm.constraint_ref(c).num_variables() == 2);
+                CHECK(cqm.constraint_ref(c).num_interactions() == 0);
+
+                CHECK(cqm.constraint_ref(c).linear(2) == 1);
+                CHECK(cqm.constraint_ref(c).linear(3) == 0);  // removed
+                CHECK(cqm.constraint_ref(c).linear(5) == 3);
+                CHECK(cqm.constraint_ref(c).linear(8) == 0);  // removed
+
+                CHECK(cqm.constraint_ref(c).variables() == std::vector<int>{2, 5});
+            }
+        }
+    }
+}
+
 }  // namespace dimod