ConditionalExpression¶
Status: Stable
documented, exercised by the test suite and/or worked examples, with no known limitations recorded.
Description¶
ConditionalExpression[expr, cond]
is a symbolic construct that represents expr when cond is True.
ConditionalExpression[expr, True] evaluates to expr.
ConditionalExpression[expr, False] evaluates to Undefined.
Nested forms collapse: ConditionalExpression[ConditionalExpression[e, c1], c2] evaluates to ConditionalExpression[e, c1 && c2].
ConditionalExpression has attribute Protected.
Examples¶
All examples below are verified against the current Mathilda build.
In[1]:= ConditionalExpression[a, True]
Out[1]= a
In[2]:= ConditionalExpression[a, False]
Out[2]= Undefined
In[3]:= ConditionalExpression[x^2, x > 0]
Out[3]= ConditionalExpression[x^2, x > 0]
In[4]:= ConditionalExpression[ConditionalExpression[e, c1], c2]
Out[4]= ConditionalExpression[e, c1 && c2]
Implementation notes¶
Algorithm. builtin_conditional_expression takes ConditionalExpression[expr, cond] (two args; ATTR_PROTECTED, no Hold, so both are pre-evaluated). When cond is the interned True it yields expr (stealing the slot via args[0] = NULL so the evaluator's free of res doesn't double-free); when cond is False it yields the symbol Undefined. Nested forms ConditionalExpression[ConditionalExpression[e, c1], c2] are flattened to ConditionalExpression[e, And[c1, c2]], with the combined And run through evaluate so contradictory or redundant conditions collapse. Any other condition leaves the call unevaluated (NULL).
Attributes: Protected.
Implementation status¶
Stable — documented, exercised by the test suite and/or worked examples, with no known limitations recorded.
References¶
- Source:
src/boolean.c - Specification:
docs/spec/builtins/control-flow.md
Notes & additional examples¶
Worked examples¶
In[1]:= ConditionalExpression[ConditionalExpression[e, a > 0], b > 0]
Out[1]= ConditionalExpression[e, a > 0 && b > 0]
Notes¶
ConditionalExpression[expr, cond] carries a value together with the assumption under which it is valid. It evaluates to expr when the condition is True and to Undefined when it is False; nested forms merge their conditions with And.