alogos.systems.ge.mapping

Forward and reverse mapping functions for GE.

Functions

forward(grammar, genotype, parameters=None, raise_errors=True, return_derivation_tree=False, verbose=False)

Map a GE genotype to a string phenotype.

reverse(grammar, phenotype_or_derivation_tree, parameters=None, return_derivation_tree=False)

Map a string phenotype (or derivation tree) to a GE genotype.

Detailed object descriptions

alogos.systems.ge.mapping.forward(grammar, genotype, parameters=None, raise_errors=True, return_derivation_tree=False, verbose=False)[source]

Map a GE genotype to a string phenotype.

It converts a genotype (=originally a list of integers) to a derivation tree (=linked nodes that each contain a nonterminal or terminal symbol), which can be further converted to a phenotype (=a string of the grammar’s language).

Parameters:

  • grammar (Grammar)

  • genotype (Genotype or data that can be converted to it)

  • parameters (dict or ParameterCollection, optional) –

    • max_expansions (int): Maximum number of nonterminal expansions allowed in the derivation created by the mapping process.

    • max_wraps (int): Maximum number of times the genotype is allowed to be wrapped in order to have more codons available for making decisions in the mapping process.

  • raise_errors (bool, optional) – Possible values:

    • True: A mapping error will be raised if a derivation is not finished within a limit provided in the parameters.

    • False: A partial derivation is allowed. In this case, the returned string will contain unexpanded nonterminal symbols. Therefore it is not a valid phenotype, i.e. not a string of the grammar’s language but a so-called sentential form.

  • return_derivation_tree (bool, optional) – If True, not only the phenotype is returned but additionally also the derivation tree.

  • verbose (bool, optional) – If True, output about steps of the mapping process is printed.

Returns:

  • phenotype (str) – If return_derivation_tree is False, which is the default.

  • (phenotype, derivation_tree) (tuple with two elements of type str and DerivationTree) – If return_derivation_tree is True.

Raises:

MappingError – If raise_errors is True and the mapping process can not generate a full derivation before reaching a limit provided in the parameters.

alogos.systems.ge.mapping.reverse(grammar, phenotype_or_derivation_tree, parameters=None, return_derivation_tree=False)[source]

Map a string phenotype (or derivation tree) to a GE genotype.

This is a reversal of the mapping procedure of Grammatical Evolution (GE). Note that many different GE genotypes can encode the same derivation tree and phenotype. It is possible to return a deterministic GE genotype that uses the lowest possible integer value for each choice of expansion, or a random GE genotype that uses a random integer value within the codon size limit. The latter is describes in literature as using an “unmod” operation to find a random codon value which is valid, in the sense that after applying the “mod” operation to it the result is the lowest possible integer.

Parameters:

  • grammar (Grammar)

  • phenotype_or_derivation_tree (str or DerivationTree)

  • parameters (dict or ParameterCollection, optional) – Following keyword-value pairs are considered by this function:

    • codon_size (int): Codon size in bits. The number of different integer values a codon can assume is therefore determined by 2**codon_size.

    • codon_randomization (bool): If True, the reverse mapping will use the so-called “unmod” operation and therefore generate random integers that encode the required rule choice. If False, the lowest possible integer will be used and the reverse mapping becomes deterministic.

  • return_derivation_tree (bool, optional) – If True, not only the genotype is returned but additionally also the derivation tree.

Returns:

Raises:

MappingError – If the reverse mapping fails because the string does not belong to the grammar’s language or the derivation tree does not represent a valid derivation.

References