Variable¶

class Variable¶

The Variable object stores the information regarding a variable in the algebraic model. It can contain multiple variable instances, or can be scalar. Generally, numerical values are available for each specific instance, which are accessible through the function Variable.get. For scalar variables, direct accessors to the values are available (see for Example function Variable.value below). For ease of use, values for the entire algebraic entity can be accessed directly and assigned to MATLAB vectors using the DataFrame object. To assign values in this way, see AMPL.setData. To get the values use Variable.getValues.

get(key)¶

classmethod Variable.get(key)¶

Syntax

v = get(key)

Description

v = get(key) returns the VariableInstance corresponding to the specified key. Note that the same effect can be achieved using MATLAB’s indexing operator (round brackets).

Input Arguments

The key of the variable instance to look up.

Output Arguments

v: The specified variable instance

Example

Create an indexed variable, get an instance for each, and fix them to some value

ampl.eval('var x{1..5}; var y{1..2, 1..2};');
x = ampl.getVariable('x');
y = ampl.getVariable('y');
x2 = x.get(2);
y2_2 = y.get({2,2});
x2.fix(3);
y2_2.fix(4.4);
x.display
y.display

gives:

getValues¶

classmethod Variable.getValues()¶

Syntax

df = v.getValues()

df = v.getValues(suffixes)

Description

These functions return a dataframe with all the specified values of the variable, decoupled from the underlying AMPL. To gain access to the data, see DataFrame.

v.getValues() gets the AMPL value (equivalent to the suffix val)

v.getValues(suffixes) allows the user to specify which suffixes to get

Input Arguments

suffixes: Strings specifying which suffixes of the variable to get (see AMPL built-in suffixes).

Output Arguments

df: A dataframe containing all the specified values of the variable and its indices

Example

Create an algebraic variable in AMPL, then get the values and the lower and upper bounds for all instances in a dataframe.

ampl.eval('var x{i in 1..2} <= i, >= -i;');
x = ampl.getVariable('x');
df = x.getValues('val', 'lb', 'ub')

gives:

df =
i in 1 .. 2  |  value  lb    ub
1.0          |  0.0    -1.0  1.0
2.0          |  0.0    -2.0  2.0

value()¶

classmethod Variable.value()¶

Syntax

v = value()

Description

v = value() Get the current value of the variable. Valid only for scalar entities.

Output Arguments

v: The current value of the variable

Example

Create a scalar variable x and an indexed variable y (using AMPL.eval) Try to access their values and see that x returns its current value (0) and y throws an error because it is not a scalar variable.

>> ampl.eval('var x; var y{1..2};');
>> x = ampl.getVariable('x');
>> y = ampl.getVariable('y');
>> v = x.value
 v = 0
>> y.value
   Java exception occurred:
   com.ampl.AMPLException: The entity y is not a scalar entity
   at com.ampl.Entity.checkNoIndexing(Entity.java:429)
   at com.ampl.Variable.value(Variable.java:214)

setValue(v)¶

classmethod Variable.setValue()¶

Syntax

setValue(v)

Description

setValue(v) Assigns the specified value to the variable. Valid only for scalar variables. Equivalent to the AMPL code:

var x;
let x := v;

Input Arguments

v: The value to be assigned

Example

Create a variable x and sets its initial value. Then displays it:

ampl.eval('var x;');
x = ampl.getVariable('x');
x.setValue(4);
x.value

gives:

ans =
  4

dual¶

classmethod Variable.dual()¶

Syntax

v = dual

Description

dual Access the current dual value of the variable (if scalar). Valid only for scalar entities.

Output Arguments

v: The dual value of the variable

Example

Create a scalar variable x and an indexed variable y (using AMPL.eval). Set the system to fetch all suffixes from the variables and get the values.

ampl.eval('var x; var y{1..2};');
x = ampl.getVariable('x');
x.setGetAllProperties(1);
v = x.dual

gives:

v = 0

lb¶

classmethod Variable.lb()¶

Syntax

v = lb

Description

lb Get the current lower bound of the variable. Valid only for scalar entities. See Note on variables suffixes.

Output Arguments

v: The lower bound of the variable

Example

Create a scalar variable x and an indexed variable y (using AMPL.eval). Set the system to fetch all suffixes from the variables and get the value.

ampl.eval('var x; var y{1..2};');
x = ampl.getVariable('x');
x.setGetAllProperties(1);
v = x.lb

gives:

v = 0

ub¶

classmethod Variable.ub()¶

Syntax

v = ub

Description

ub Get the current upper bound of the variable. Valid only for scalar entities. See Note on variables suffixes.

Output Arguments

v: The upper bound of the variable

Example

Create a scalar variable x and an indexed variable y (using AMPL.eval). Set the system to fetch all suffixes from the variables and get the value.

ampl.eval('var x; var y{1..2};');
x = ampl.getVariable('x');
x.setGetAllProperties(1);
v = x.ub

gives:

v = 0

getIntegrality¶

classmethod Variable.getIntegrality()¶

Syntax

v = getIntegrality

Description

v = getIntegrality returns the integrality type of this Variable

Output Arguments

v: Integrality type

Example

Create a continous, an integer and a binary variable and check their integrality

ampl.eval('var x; var y integer; var z binary;');
x = ampl.getVariable('x');
y = ampl.getVariable('y');
z = ampl.getVariable('z');
v = x.getIntegrality
v = y.getIntegrality
v = z.getIntegrality

gives:

 v =
   CONTINUOUS

v =
   INTEGER

v =
   BINARY

fix¶

classmethod Variable.fix()¶

Syntax

fix

Description

fix Fix all instances in the variable to their current values

Example

Create a scalar variable x and an indexed variable y (using AMPL.eval) and fix their values.

  ampl.eval('var x<=3; var y{1..3}<=4;');
  ampl.eval('maximize z: x + sum{i in 1..3} y[i];');
  x = ampl.getVariable('x');
  y = ampl.getVariable('y');
  x.fix;      % fix variables
  y.fix;
  ampl.solve; % solve
  x.display   % display solution
  y.display
  x.unfix;    % unfix variables
  y.unfix;
  ampl.solve; % solve and display
  x.display   % display solution
  y.display

gives::

  ans =
     x = 0

  ans =
     y [*] :=
     1  0
     2  0
     3  0
     ;

  ans =
     x = 3

  ans =
     y [*] :=
     1  4
     2  4
     3  4
     ;

unfix¶

classmethod Variable.unfix()¶

Syntax

unfix

Description

unfix Fix all instances in the variable to their current values

Example

Create a scalar variable x and an indexed variable y (using AMPL.eval), fix their values, solve and unfix them

  ampl.eval('var x<=3; var y{1..3}<=4;');
  ampl.eval('maximize z: x + sum{i in 1..3} y[i];');
  x = ampl.getVariable('x');
  y = ampl.getVariable('y');
  x.fix;      % fix variables
  y.fix;
  ampl.solve; % solve
  x.display   % display solution
  y.display
  x.unfix;    % unfix variables
  y.unfix;
  ampl.solve; % solve and display
  x.display   % display solution
  y.display

gives::

  ans =
     x = 0

  ans =
     y [*] :=
     1  0
     2  0
     3  0
     ;

  ans =
     x = 3

  ans =
     y [*] :=
     1  4
     2  4
     3  4
     ;

Tuple

VariableInstance