Input format

The raw (plain text) input data for VW should have one example per line. Each example should be formatted as follows.

[Label] [Importance [Tag]]|Namespace Features |Namespace Features ... |Namespace Features

where

Namespace=String[:Value]

and

Features=(String[:Value] )*

Label is the real number that we are trying to predict for this example. If the label is omitted, then no training will be performed with the corresponding example, although VW will still compute a prediction.
Importance (importance weight) is a non-negative real number indicating the relative importance of this example over the others. Omitting this gives a default importance of 1 to the example.
Tag is a string that serves as an identifier for the example. It is reported back when predictions are made. It doesn't have to be unique. The default value if it is not provided is the empty string. If you provide a tag, you must also provide an importance. If you don't provide a tag, put a space before the vertical bar.
Namespace is an identifier of a source of information for the example optionally followed by a float (e.g., MetricFeatures:3.28), which acts as a global scaling of all the values of the features in this namespace. If value is omitted, the default is 1.
Features is a sequence of whitespace separated strings, each of which is optionally followed by a float (e.g., NumberOfLegs:4.0 HasStripes). Each string is a feature and the value is the feature value for that example. Omitting a feature means that its value is zero. Including a feature but omitting its value means that its value is 1.

Currently, the only characters that can't be used in feature names are vertical bar, colon, space, and newline.

Example

1 1.0 |MetricFeatures:3.28 height:1.5 length:2.0 |Says black with white stripes |OtherFeatures NumberOfLegs:4.0 HasStripes

1 1.0 zebra|MetricFeatures:3.28 height:1.5 length:2.0 |Says black with white stripes |OtherFeatures NumberOfLegs:4.0 HasStripes

When using logistic or hinge loss, the labels need to be from the set {+1,-1} (documented in the V6.1 tutorial slide deck, but not elsewhere)

The spacing around the | characters is important and significant:

Around the 1st | if there's no space preceding it, the string that touches the | is considered a tag (id of example)
After any | ** If there's a space, the next non-space token is considered a regular feature name ** If there's no space, the next non-space token is considered a name-space ** name-spaces are considered as feature name prefixes, they are prepended to all feature names in the name-space