Namespaces

vowpal wabbit supports a concept called feature name spaces (aka namespaces).

Two identically named features in two separate name-spaces are effectively different features.

Name spaces are useful for:

• Separating the data-set (by row) into example subsets (think one model including multiple, smaller and separate sub-models)
• Separating features (columns) into subsets
• Ignore/drop a subset of features belonging to a name-space as a group from consideration
• Cross one name-space subset with another, in the same example, on the fly during runtime (dynamic feature-interaction generation).

Example1: row-based name-spaces: suppose you're tracking revenues for N different stores, based on features like: year opened, median income in local zip-code, previous-week gross proceeds, day-of-week. etc. You can use the store name or town as a name-space for all the other features segregating the data-set into N separate sub-models, one for each store. Obviously, when trying to predict, you should be sending the expected name-space with the example you want to predict.

Example2: column-based name-spaces: you have features about a product (price, category, ...) in one name-space P and features about the buyers of the product (age, income, ...) in another name-space B, by using --quadratic PB you can generate the interaction features between the two name-spaces on-the-fly.

Input format:

In essence when you add a name-space to an example, all the features following that name-space get mapped to a new feature name. A name-space can be introduced by dropping the initial space after the | char introducing input features. If a space is present, there are no name-spaces; only regular features (as in the 1st example in the table below). When the space is dropped and a feature-name appears immediately after the | char, that 1st feature name becomes a name-space for all the following feature-names.

Namespace	Original feature-name	Effective feature name	Example syntax
	bar	bar	| bar
foo	bar	foo^bar	|foo bar
foo	baz	foo^baz	|foo baz

The complete Input file format page covers the full syntax.

Command-line options related to name-spaces:

A few of the many vw command-line options, act on name spaces.

Limitation: options that act on name-spaces only use on the 1st char of the name-space. You cannot (for example) drop only one name-space starting with a if you have more than one name-space starting with a.

Option	Meaning
--keep c	Keep a name-space staring with the character c
--ignore c	Ignore a name-space starting with the character c
--redefine a:=b	redefine namespace starting with b as starting with a
--quadratic ab	Cross namespaces starting with a & b on the fly to generate 2-way interacting features
--cubic abc	Cross namespaces starting with a, b, & c on the fly to generate 3-way interacting features

The complete Command line arguments page covers all possible vw options.

Reference and further reading:

• vw command line arguments
• vw input format spec
• (Stackoverflow): Difference between name space and feature
• (Crossvalidated): Finding the best features in interaction models
• (Stackoverflow): Can't use full-name of name space in --keep
• (Stackoverflow): What hash function is used by vw?
• (Stackoverflow): How to use the --keep and --ignore options?
• Feature Hashing for Large Scale Multitask Learning K. Weinberger, A. Dasgupta, J. Attenberg, J. Langford, A. Smola