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# 6 Migration options

### Migration.Do

This option switches migration between subpopulations on or off. Migration is executed by migrate.

• Default value: 1
• Type: integer in {0, 1}
• Two variants are available:
• 0: no migration
• 1: do migration between subpopulations
• Example - switch migration off:
`GeaOpt = geaoptset(GeaOpt, 'Migration.Do', 0);`
• When only one subpopulation is defined (see NumberSubpopulation) no migration takes place.
• Option number in previous versions: 31 (2.x), not available/internal option (1.x)

### Migration.Interval

This option defines the number of generations between successive migration. Thus, the frequency of migration is specified (how often a migration takes place). This option is often called isolation time.

• Default value: 20
• Type: positive integer in [1, Inf]
• Example - set migration interval to every 8 generations:
`GeaOpt = geaoptset(GeaOpt, 'Migration.Interval', 8);`
• Small values of Migration.Interval decrease the isolation of the individuals.
• A migration takes place (that means the function migrate is called) when the reminder of Run.Generation and Migration.Interval is zero.
• Option number in previous versions: 33 (2.x), not available/internal option (1.x)

### Migration.Rate

This option defines the fraction of every population to migrate during a migration.

• Default value: 0.1
• Type: scalar in [0, 1]
• Example - set migration rate to 15%:
`GeaOpt = geaoptset(GeaOpt, 'Migration.Rate', 0.15);`
• A value of 0.15 means that 15% of the individuals of every subpopulation are copied (exported) to the migration pole. From this pole the same number of individuals is selected and migrates (is imported) to the subpopulation. It is assured that no individuals from the own subpopulation are reimported. The selection of the individuals is controlled by the option Migration.Selection
• Option number in previous versions: 32 (2.x), 26 (1.x)

### Migration.Topology

This option defines the used topology of the subpopulations for migration.

• Default value: 0
• Type: integer in {0, 1, 2}
• The following variants are available:
• 0: complete net structure (unconstrained migration)
• 1: 1-D neighborhood structure
• 2: 1-D ring structure
• Example - use 1-D ring structure between subpopulations:
`GeaOpt = geaoptset(GeaOpt, 'Migration.Topology', 2);`
• Option number in previous versions: 34 (2.x), 27 (1.x)

### Migration.Selection

This option controls the selection of individuals for migration.

• Default value: 1
• Type: integer in {0, 1}
• Two variants are available:
• 0: the exported individuals are selected uniform at random
• 1: the best individuals are exported
• Example - select the individuals for migration uniform at random:
`GeaOpt = geaoptset(GeaOpt, 'Migration.Selection', 0);`
• Option number in previous versions: not available/internal option (2.x and 1.x)

 GEATbx: Main page  Tutorial  Algorithms  M-functions  Parameter/Options  Example functions  www.geatbx.com

This document is part of version 3.8 of the GEATbx: Genetic and Evolutionary Algorithm Toolbox for use with Matlab - www.geatbx.com.
The Genetic and Evolutionary Algorithm Toolbox is not public domain.