Expressions

Overview

In Z3, all terms, formulas, and types are represented as Abstract Syntax Trees (ASTs). The expression system is the foundation for building and manipulating logical formulas.

AST Hierarchy

Z3’s AST nodes form a hierarchy:

Core AST Types

Z3_sort - Type/Sort

Represents types in Z3’s type system:

Basic sorts: Bool, Int, Real
Bit-vector sorts: (_ BitVec n)
Array sorts: (Array Domain Range)
Datatypes: user-defined algebraic types
Uninterpreted sorts: abstract types

Z3_func_decl - Function Declaration

Declares functions and constants:

Constants: 0-arity functions
Uninterpreted functions: no specified meaning
Interpreted functions: built-in operations (+, *, etc.)

Z3_app - Function Application

Applications of functions to arguments:

Constants are 0-arity applications
Operations like x + y are applications of + to [x, y]

Creating Sorts

from z3 import *

# Built-in sorts
bool_sort = BoolSort()
int_sort = IntSort()
real_sort = RealSort()

# Bit-vector sort (32 bits)
bv_sort = BitVecSort(32)

# Array sort (Int -> Int)
array_sort = ArraySort(IntSort(), IntSort())

# Uninterpreted sort
U = DeclareSort('U')

# Datatype (recursive list)
List = Datatype('List')
List.declare('nil')
List.declare('cons', ('head', IntSort()), ('tail', List))
ListSort = List.create()

Creating Expressions

Constants and Variables

from z3 import *

# Boolean constant
p = Bool('p')

# Integer constants
x = Int('x')
y = Int('y')

# Real constant
z = Real('z')

# Bit-vector constant (32-bit)
a = BitVec('a', 32)

# Array constant
A = Array('A', IntSort(), IntSort())

Numerals

# Integer literals
zero = IntVal(0)
ten = IntVal(10)

# Real literals (exact rational)
half = RealVal("1/2")  # Exact: 1/2
approx = RealVal(0.5)   # May lose precision
frac = RealVal(1, 2)    # Exact: 1/2

# Bit-vector literals
bv_val = BitVecVal(42, 32)  # 32-bit value 42

Function Applications

from z3 import *

x, y = Ints('x y')

# Arithmetic operations are function applications
expr1 = x + y       # Application of '+'
expr2 = x * 2       # Application of '*'
expr3 = x > y       # Application of '>'

# Uninterpreted function
f = Function('f', IntSort(), IntSort(), IntSort())
expr4 = f(x, y)     # Application of 'f'

# Check structure
print(expr1.decl())          # +
print(expr1.num_args())      # 2
print(expr1.arg(0))          # x
print(expr1.arg(1))          # y

Expression Kinds

From z3_api.h:170, Z3 defines these AST kinds:

typedef enum {
    Z3_NUMERAL_AST,      // Numeric constants
    Z3_APP_AST,          // Function applications
    Z3_VAR_AST,          // Bound variables (in quantifiers)
    Z3_QUANTIFIER_AST,   // Quantified formulas
    Z3_SORT_AST,         // Sorts/types
    Z3_FUNC_DECL_AST,    // Function declarations
    Z3_UNKNOWN_AST       // Internal
} Z3_ast_kind;

from z3 import *

x = Int('x')
expr = x + 10

# Query expression kind
print(expr.decl().kind())  # Z3_OP_ADD
print(is_app(expr))        # True
print(is_const(x))         # True

Boolean Formulas

from z3 import *

p, q, r = Bools('p q r')

# Logical connectives
formula1 = And(p, q)
formula2 = Or(p, q, r)
formula3 = Not(p)
formula4 = Implies(p, q)
formula5 = p == q  # Equivalence (iff)

# De Morgan's laws
conjecture = (Not(And(p, q)) == Or(Not(p), Not(q)))

# Verify it's a tautology
prove(conjecture)

Arithmetic Expressions

from z3 import *

x, y, z = Ints('x y z')

# Linear arithmetic
linear = 2*x + 3*y - z

# Nonlinear arithmetic
nonlinear = x * y + z * z

# Comparisons
constraint = And(x >= 0, x + y <= 10, z > x)

# Division (integer)
quot = x / y   # Integer division
rem = x % y    # Remainder

Bit-Vector Operations

from z3 import *

x = BitVec('x', 32)
y = BitVec('y', 32)

# Arithmetic
expr1 = x + y
expr2 = x * y
expr3 = -x

# Bitwise operations
expr4 = x & y      # AND
expr5 = x | y      # OR
expr6 = x ^ y      # XOR
expr7 = ~x         # NOT

# Shifts
expr8 = x << 2     # Left shift
expr9 = x >> 3     # Logical right shift

# Comparison (signed vs unsigned)
expr10 = x < y      # Signed comparison
expr11 = ULT(x, y)  # Unsigned comparison

# Extract and concatenate
low_bits = Extract(15, 0, x)   # Extract bits 15-0
high_bits = Extract(31, 16, x) # Extract bits 31-16
combined = Concat(x, y)         # 64-bit concatenation

Array Expressions

from z3 import *

A = Array('A', IntSort(), IntSort())
i, j, v = Ints('i j v')

# Array operations
read = A[i]                    # Select: read at index i  
write = Store(A, i, v)         # Store: update at index i
read_after = write[j]          # Read from updated array

# Array axioms are built-in
s = Solver()
s.add(i == j)
s.add(Store(A, i, v)[j] != v)
print(s.check())  # unsat - array axiom contradiction

Expression Traversal

From examples/c++/example.cpp:807:

from z3 import *

def visit(e, seen=None):
    """Recursively visit all subexpressions"""
    if seen is None:
        seen = set()
    
    if e.get_id() in seen:
        return
    seen.add(e.get_id())
    
    if is_app(e):
        print(f"Application of {e.decl().name()}: {e}")
        for child in e.children():
            visit(child, seen)
    elif is_quantifier(e):
        print(f"Quantifier: {e}")
        visit(e.body(), seen)
    else:
        print(f"Leaf: {e}")

# Example usage
x, y = Ints('x y')
f = x*x + x*x - y*y >= 0
visit(f)

Expression Simplification

from z3 import *

x, y = Ints('x y')

# Create complex expression
expr = And(x > 0, x > 0, Or(y < 0, False))

# Simplify
simplified = simplify(expr)
print(simplified)  # And(x > 0, y < 0)

# Algebraic simplification
algebraic = simplify(x + x + x)
print(algebraic)  # 3*x

If-Then-Else Terms

from z3 import *

x, y = Ints('x y')
b = Bool('b')

# Conditional expression
result = If(b, x, y)

# Type must match both branches
result2 = If(x > 0, x + 1, x - 1)  # Both branches are Int

Substitution

From examples/c++/example.cpp:1147:

from z3 import *

x = Int('x')
f = Or(x == 2, x == 1)

print(f)  # Or(x == 2, x == 1)

# Substitute 2 with 3
f_subst = substitute(f, (IntVal(2), IntVal(3)))
print(f_subst)  # Or(x == 3, x == 1)

Memory Management

Z3 uses reference counting for memory management. In C/C++:

Always increment refs with Z3_inc_ref
Decrement with Z3_dec_ref when done
The C++ API handles this automatically
Python API is fully garbage-collected

// C API - manual reference counting
Z3_context ctx = Z3_mk_context(cfg);
Z3_ast x = Z3_mk_int_const(ctx, Z3_mk_string_symbol(ctx, "x"));
Z3_inc_ref(ctx, x);  // Increment ref
// ... use x ...
Z3_dec_ref(ctx, x);  // Decrement when done

SMT Solving

Learn about satisfiability checking

Solvers

Using expressions with solvers

Quantifiers

Quantified expressions

References

Z3 API: src/api/z3_api.h:7-179
AST implementation: src/api/api_ast.cpp
Examples: examples/c++/example.cpp

Getting Started

Core Concepts

Building Z3

Guides

Overview

AST Hierarchy

Core AST Types

Creating Sorts

Creating Expressions

Constants and Variables

Numerals

Function Applications

Expression Kinds

Boolean Formulas

Arithmetic Expressions

Bit-Vector Operations

Array Expressions

Expression Traversal

Expression Simplification

If-Then-Else Terms

Substitution

Memory Management

SMT Solving

Solvers

Quantifiers

References

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Building Z3

Guides

Documentation Index

​Overview

​AST Hierarchy

​Core AST Types

​Creating Sorts

​Creating Expressions

​Constants and Variables

​Numerals

​Function Applications

​Expression Kinds

​Boolean Formulas

​Arithmetic Expressions

​Bit-Vector Operations

​Array Expressions

​Expression Traversal

​Expression Simplification

​If-Then-Else Terms

​Substitution

​Memory Management

​Related Topics

SMT Solving

Solvers

Quantifiers

​References

Build docs developers (and LLMs) love

Overview

AST Hierarchy

Core AST Types

Creating Sorts

Creating Expressions

Constants and Variables

Numerals

Function Applications

Expression Kinds

Boolean Formulas

Arithmetic Expressions

Bit-Vector Operations

Array Expressions

Expression Traversal

Expression Simplification

If-Then-Else Terms

Substitution

Memory Management

Related Topics

References