Nodes As Functions
Each of these nodes can also be called a function. However, this type of function (i.e. a node in a parse tree) is not the same as a function in mathematics or computer programming. A function in mathematics takes in zero or more inputs and returns a value.
In parse trees, a function can be a true mathematical function which "uses" its child nodes as inputs, and returns a single output to its parent. More often, however, a node in a parse tree is a static symbol such as a number, letter, or word.
The use of parse tree functions is highly dependent on the area which is using it. In the branch of evolutionary computation called genetic programming, for instance, tree functions are quite often (but not exclusively) used as mathematical-style function. In linguistics and linguistic theory, tree functions are never mathematical functions, except in certain areas of computational linguistics.
Read more about this topic: Terminal Function
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