Gurobi——GRBLinExpr

GRBLinExpr

  Gurobi Linear expression object . A linear expression consists of a constant term and a list of coefficient variable pairs that capture the linear term . Linear expressions are used to construct constraints . They are usually short-lived temporary objects .

  GRBLinExpr Class is an abstract base class GRBExpr Subclasses of .

   You usually use overloaded operators to build linear expressions . for example , If x yes GRBVar object , be x + 1 yes GRBLinExpr object . Expressions can consist of constants （ for example expr = 0）、 Variable （ for example expr = 1 * x + 2 * y） Or other expressions （ for example expr2 = 2 * expr1 + x or expr3 = expr1 + 2 * expr2). You can also modify existing expressions （ for example expr += x or expr2 -= expr1）.

   Another option for building expressions is to use addTerms Method , It adds a new set of terms at a time . You can also use remove Remove the term... From the expression .

   Please note that , The cost of building expressions depends largely on the method you use . Although you can usually ignore this problem when building small expressions , But there are some efficiency issues when building large expressions ：

• You should avoid using... In loops expr = expr + x. This will result in a runtime with a quadratic number of items in the expression .
• Use expr += x（ or expr -= x） Than expr = expr + x More effective . Through the loop += Statement to build large expressions is quite effective , But it's not the most effective way .
• The most effective way to build large expressions is to addTerms Make a call .

   To add linear constraints to your model , You usually build one or two linear expression objects （expr1 and expr2）, Then use overloaded comparison operators to build GRBModel::addConstr Parameters of . Take a few examples ：

   After adding constraints to the model , Subsequent changes to the expression object used to build the constraint will not change the constraint （ You will use... For this GRBModel::chgCoeff）.

   have access to getVar、getCoeff and getConstant Method to query each item in a linear expression . You can use size Method to query the number of items in the expression .

   Please note that , A linear expression may contain multiple terms involving the same variable . These duplicates are merged when creating constraints from expressions , But they may be visible when examining individual items in an expression （ for example , Use getVar when ）.

GRBLinExpr()

Linear expression constructor . Create a constant expression or an expression with a term .

GRBLinExpr 	GRBLinExpr (double 	constant=0.0 )

Create a constant linear expression .

Parameters	Detailed explanation
constant (optional)	The constant value of the expression .
Return value	A constant expression object .

GRBLinExpr 	GRBLinExpr(GRBVar 	var, double 	coeff=1.0 )

Create an expression with one word .

Parameters	Detailed explanation
var	Variables of expression terms .
coeff（ Optional ）	The coefficient of the expression term .
Return value	An expression object that contains a linear term .

GRBLinExpr::addTerms()

Add a new item to the linear expression .

void addTerms (const double* coeffs,
			   const GRBVar* vars,
			   int count）

Parameters	Detailed explanation
coeffs	The coefficient of the new term .
vars	Variables of the new term .
count	Number of items to add to the expression .

GRBLinExpr::clear()

Set linear expression to 0.

You should use overloaded expr = 0. clear Method is mainly to keep consistent with our interface to non overloaded language .

void clear()

GRBLinExpr::getConstant()

Retrieve constant terms from linear expressions .

double getConstant ( )

Return value ： Constants from expressions .

GRBLinExpr::getCoeff()

Retrieve coefficients from a single item of an expression .

double getCoeff ( int i )

Parameters	Detailed explanation
i	Coefficient of interest index .
Return value	Index in expression i The coefficient of the term at .

GRBLinExpr::getValue()

Calculate the value of the linear expression of the current solution .

double getValue()

Return value ： The value of the expression for the current solution .

GRBLinExpr::getVar()

Retrieves a variable object from a single item in an expression .

GRBVar 	getVar ( 	int 	i )

Parameters	Detailed explanation
i	Index of items of interest .
Return value	Index in expression i Variables in terms .

GRBLinExpr::operator=

Set one expression equal to another .

GRBLinExpr	operator=(const GRBLinExpr&	rhs)

Parameters	Detailed explanation
rhs	Source expression .
Return value	New expression object .

GRBLinExpr::operator+

Add one expression to another , Generate the result expression .

GRBLinExpr 	operator+(const GRBLinExpr&	rhs)

Parameters	Detailed explanation
rhs	The expression to add .
Return value	Expression object , It is equal to the sum of the call expression and the parameter expression .

GRBLinExpr::operator-

Subtract an expression from another expression , Produce a result expression .

GRBLinExpr 	operator-(const GRBLinExpr&	rhs)

Parameters	Detailed explanation
rhs	The expression to subtract .
Return value	Expression object , It is equal to the call expression minus the parameter expression .

GRBLinExpr::operator+=

Add an expression to the call expression .

void  operator+=(const GRBLinExpr&  expr)

Parameters	Detailed explanation
expr	The expression to add .
Return value	nothing .

GRBLinExpr::operator-=

Subtract an expression from an expression call .

void 	operator-= ( 	const GRBLinExpr& 	expr )

Parameters	Detailed explanation
expr	The expression to subtract .
Return value	nothing .

GRBLinExpr::operator*=

Multiply the call expression by a constant .

void 	operator*=(double 	multiplier)

Parameters	Detailed explanation
multiplier	Constant multiplier .
Return value	nothing .

GRBLinExpr::remove()

Delete an item from a linear expression .

void remove(int i)

Delete stored in the expression index i Item at .

Parameters	Detailed explanation
i	Index of the entry to be deleted .

boolean 	remove(GRBVar var)

Remove variables from the expression var All associated items .

Parameters	Detailed explanation
var	The variable whose item should be deleted .
Return value	If the variable appears in a linear expression （ And has been deleted ）, Then return to true.

GRBLinExpr::size()

Retrieves the number of items in a linear expression （ Excluding constants ）.

unsigned int size()

Return value ： The number of items in the expression .