Quick Reference: Variables and Expressions (PyQBPP)

Data types in PyQBPP

PyQBPP uses Python’s native int type for coefficients, energy values, and constants. Since Python integers have unlimited precision, there is no need to specify coeff_t or energy_t as in the C++ version.

Printing objects

All PyQBPP objects can be printed using print() or converted to strings using str():

print(obj)
s = str(obj)

Variable classes

  • pyqbpp.Var: A class that holds a unique 32-bit integer ID. The variable name can be retrieved via the name property.

NOTE A pyqbpp.Var object represents a variable symbolically. No specific data type is associated with it. It can be used to represent binary, spin, or other types of variables.

Variable creation functions

The following functions are provided to create variables:

  • pyqbpp.var("name"): Creates a pyqbpp.Var object with the given name "name".

  • pyqbpp.var("name", s1): Creates a one-dimensional array (vector) of pyqbpp.Var objects with the base name "name". Each element is represented as name[i]. The resulting type is pyqbpp.Vector.

  • pyqbpp.var("name", s1, s2): Creates a two-dimensional array (matrix) of pyqbpp.Var objects with the base name "name". Each element is represented as name[i][j]. The resulting type is a nested pyqbpp.Vector.

  • pyqbpp.var("name", s1, s2, ...): Creates a higher-dimensional array of pyqbpp.Var objects with the base name "name". Each element is represented as name[i][j].... The resulting type is a nested pyqbpp.Vector.

NOTE If "name" is omitted, numbered names such as "{0}", "{1}", … are automatically assigned in creation order.

Examples 

from pyqbpp import var

x = var("x")          # Single variable named "x"
y = var("y", 3)       # Vector: y[0], y[1], y[2]
z = var("z", 2, 3)    # 2x3 matrix: z[0][0], ..., z[1][2]
a = var()             # Single unnamed variable
b = var(5)            # Vector of 5 unnamed variables

pyqbpp.Var properties and methods

For a pyqbpp.Var instance x, the following are available:

  • x.name (property): Returns the name of x as a string.

  • x.index (property): Returns the unique integer ID of x.

Usually, there is no need to use these properties explicitly in PyQBPP programs.

Integer variable class

  • pyqbpp.VarInt: A class derived from pyqbpp.Expr that represents an integer variable with a specified range.

Integer variable creation functions

The following functions are provided to create integer variables:

  • pyqbpp.var_int("name"): Returns an internally used helper object (pyqbpp.VarIntCore) and does not create a pyqbpp.VarInt by itself. To define a pyqbpp.VarInt, the range must be specified using the between() function, as shown below.

  • pyqbpp.between(pyqbpp.var_int("name"), l, u): Here, l and u must be integers. This expression creates a pyqbpp.VarInt object with the name "name", which internally contains a pyqbpp.Expr object representing all integers in the range [l, u]. Internally, this also creates pyqbpp.Var objects used in the underlying expression.

  • pyqbpp.between(pyqbpp.var_int("name", s1), l, u): Creates a one-dimensional array (vector) of pyqbpp.VarInt objects with the base name "name" and the same range [l, u]. Each element is represented as name[i]. The resulting type is pyqbpp.Vector. Higher-dimensional arrays of pyqbpp.VarInt objects can be created in the same way as pyqbpp.Var objects.

Examples 

from pyqbpp import var_int, between

x = between(var_int("x"), 0, 10)       # Integer variable x in [0, 10]
y = between(var_int("y", 3), -5, 5)    # Vector of 3 integer variables in [-5, 5]
z = between(var_int("z", 2, 3), 1, 8)  # 2x3 matrix of integer variables in [1, 8]

Integer variable properties

For a pyqbpp.VarInt instance x, the following are available:

  • x.min_val (property): Returns the minimum value l of x.

  • x.max_val (property): Returns the maximum value u of x.

  • x.int_vars (property): Returns the list of pyqbpp.Var objects used to represent the integer variable.

  • x.coeffs (property): Returns a list of integer coefficients.

The following expression is equivalent to the expression stored in x:

x.min_val + sum(c * v for c, v in zip(x.coeffs, x.int_vars))

Comparison with C++ QUBO++

C++ QUBO++ PyQBPP
l <= qbpp::var_int("name") <= u between(var_int("name"), l, u)
l <= qbpp::var_int("name", s1) <= u between(var_int("name", s1), l, u)
x.name() x.name (property)
x.str() str(x)
x.min_val() x.min_val (property)
x.max_val() x.max_val (property)
x.vars() x.int_vars (property)
x.coeffs() x.coeffs (property)