doc_what_is_astnode Class Reference

Detailed Description

An AST node in libSBML is a recursive structure containing a pointer to the node's value (which might be, for example, a number or a symbol) and a list of children nodes. Each ASTNode node may have none, one, two, or more children depending on its type. The following diagram illustrates an example of how the mathematical expression "1 + 2" is represented as an AST with one plus node having two integer children nodes for the numbers 1 and 2. The figure also shows the corresponding MathML representation:

Example AST representation of a mathematical expression.

Infix	AST	MathML
1 + 2		<math xmlns="http://www.w3.org/1998/Math/MathML">   <apply>     <plus/>     <cn type="integer"> 1 </cn>     <cn type="integer"> 2 </cn>   </apply> </math>

The following are other noteworthy points about the AST representation in libSBML:

• A numerical value represented in MathML as a real number with an exponent is preserved as such in the AST node representation, even if the number could be stored in a double data type. This is done so that when an SBML model is read in and then written out again, the amount of change introduced by libSBML to the SBML during the round-trip activity is minimized.

• Rational numbers are represented in an AST node using separate numerator and denominator values. These can be retrieved using the methods ASTNode::getNumerator() and ASTNode::getDenominator().

• The children of an ASTNode are other ASTNode objects. The list of children is empty for nodes that are leaf elements, such as numbers. For nodes that are actually roots of expression subtrees, the list of children points to the parsed objects that make up the rest of the expression.

For many applications, the details of ASTs are irrelevant because the applications can use the text-string based translation functions such as SBML_formulaToString(), SBML_parseL3Formula() and SBML_parseFormula(). If you find the complexity of using the AST representation of expressions too high for your purposes, perhaps the string-based functions will be more suitable.

Finally, it is worth noting that the AST and MathML handling code in libSBML remains written in C, not C++. (All of libSBML was originally written in C.) Readers may occasionally wonder why some aspects are more C-like and less object oriented, and that's one of the reasons.