A dataset is made up of a table (or matrix) of samples comprised of features which are usually scalar variables. Each row (or sample) is a sequential array with exactly the same number of elements as the rest. The columns of the data table contain the values for the particular feature represented by that column. The dimensionality of a sample is equal to the number of features it has. For example, the samples below are said to be 3-dimensional because they contain 3 feature columns. You'll notice that samples can be made up of a heterogeneous mix of data types which we'll describe in detail in the next sections.

Example

``````\$samples = [
[0.1, 21.5, 'furry'],
[2.0, -5, 'rough'],
[0.001, -10, 'rough'],
];
``````

## High-level Data Types#

In addition to PHP's internal type system, the library adds a layer on top which distinguishes types that are continuous (numerical), categorical (discrete), or some other type. Continuous features represent some quantitative property of the sample such as `age` or `velocity`, whereas, categorical features form a qualitative property such as `rough` or `furry`. We make this distinction because different learners are compatible with different data types. For example, the Naive Bayes classifier is compatible with only categorical features but Gaussian Naive Bayes is compatible with continuous - and Random Forest is compatible with both.

Rubix ML Data Type PHP Internal Type
Continuous Integer or Float
Categorical String
Image GD Resource

## Quantities#

A quantity is a property that describes either the magnitude or multitude of something. For example, `temperature`, `income`, and `age` are all quantitative features. In Rubix ML, quantities are represented as one of the continuous data types such as integers or floating point numbers and their distances are assumed to be equally-spaced. For example, the distance between 10 years old and 11 is exactly 1 year. Quantities can further be broken down into ratios, intervals, or counts depending on the feature they are describing.

## Categories#

Categories are discrete values that describe some qualitative property of a sample such as `species`, `gender`, or `nationality`. They are represented as strings and have no numerical relationship between the values. Unlike ratios and intervals, which can take on an infinite number of values, categorical variables can only take on 1 of a finite set of values.

## Booleans#

A boolean (or binary) variable is a special case of a categorical variable in which the number of possible categories is strictly two. For example, to denote if a subject is tall or not you can use the `tall` and `not tall` categories respectively.

## Ordinals#

Even though PHP treats numeric strings such as `'1'` and `'2'` as if they were numeric, they are still considered categorical variables within the library. This conveniently allows you to represent ordinal variables as ordered categories. For example, instead of the integers `1`, `2`, `3`, `...`, which imply a precise interval, you could use the strings `'1'`, `'2'`, `'3'`, `...` to signal ordinal values in which the distances between values could be arbitrary.

## Date/Time#

There are a number of ways datetime features can be represented in a dataset. One way is to discretize the value into days, months, and/or years using categories like `1`, `2`, `3`, `...`, `june`, `july`, `august`, and `2019`, `2020` etc. Datetimes can also be represented as a continuous feature by converting them to integer timestamps.

## Images#

Images are represented as the GD resource type. A resource is a special variable that holds a reference to some external data such as an image file. For this reason, resources must eventually be converted into a scalar type for compatibility with a learner. In the case of images, they will most often be converted to raw color channel data by reading the RGB values of each pixel.

## Text#

Text data are a product of a process called language communication and can be viewed as an encoding of many individual features. Initially, text blobs are imported as categorical, however, they have little meaning as a category because the features are still encoded in the language. Thus, import text blobs as simple strings and use a preprocessing step to extract features such as word counts, weighted term frequencies, or word embeddings.

Null values are often used to indicate the absence of a value, however since it does not give any information as to the type of the variable that is missing, it cannot be used in a dataset. Instead, represent missing values as either `NaN` for continuous variables or use a separate category (such as `?`) to denote missing categorical values.