Best-first feature selection

Best-first feature selection | BigML.com.au

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Created	Wed, 8 Jun 2016 19:25:52 +0000
Published	Thu, 9 Jun 2016 08:09:50 +0000

Description:

Find the best features for modeling using a greedy algorithm

Tags:

Optimization

URL:

https://bigml.com.au/user/whizzml/gallery/script/57587140b85e7535b2000069

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Source code

;; A simple function to get the max value in a list
(define (get-max xs) (reduce (lambda (x y) (if (> x y) x y)) (head xs) xs))

;; Get feature names given ids
(define (feature-names dataset-id ids)
  (let (fields (get (fetch dataset-id) "fields"))
    (map (lambda (id) (get-in fields [id "name"])) ids)))

;; Create a dataset sample
(define (sample-dataset ds-id rate oob)
  (create-and-wait-dataset {"sample_rate" rate
                            "origin_dataset"  ds-id
                            "out_of_bag" oob
                            "seed" "whizzml-example"}))

;; Split a dataset into training and test sets
(define (split-dataset ds-id rate)
  (list (sample-dataset ds-id rate false)
        (sample-dataset ds-id rate true)))

;; Get the default set of input fields for this dataset (all preferred
;; fields minus the objective field).
(define (default-inputs dataset-id obj-id)
  (let (fields-structure (get (fetch dataset-id) "fields")
        fids (keys fields-structure)
        field-val (lambda (fid k) (get-in fields-structure [fid k])))
    (filter (lambda (k) (and (field-val k "preferred") (not (= obj-id k))))
            fids)))

;; Make a list of models using the given dataset, objective field and
;; selected features.  For each model, add one of the potential
;; features in potentials to the input feature list.
(define (make-models dataset-id obj-field selected potentials)
  (let (model-req {"dataset" dataset-id "objective_field" obj-field}
        make-req (lambda (fid)
                   (assoc model-req "input_fields" (cons fid selected)))
        all-reqs (map make-req potentials))
    (create-and-wait* "model" all-reqs)))

;; Given a test dataset, a list of potential features, and a list of
;; model ids corresponding to those features, select the best
;; potential feature by performing an evaluation on each model and
;; returning the feature with the best performance.
(define (select-feature test-dataset-id potentials model-ids)
  (let (eval-req {"dataset" test-dataset-id}
        make-req (lambda (mid) (assoc eval-req "model" mid))
        all-reqs (map make-req model-ids)
        evs (map fetch (create-and-wait* "evaluation" all-reqs))
        vs (map (lambda (ev) (get-in ev ["result" "model" "average_phi"])) evs)
        value-map (make-map potentials vs)
        max-val (get-max vs)
        choose-best (lambda (id) (if (= max-val (get value-map id)) id false)))
    (some choose-best potentials)))

;; Retrieves and checks whether the objective field of the given
;; dataset is categorical, raising an error otherwise.
(define (get-objective ds-id obj-id)
  (let (obj-id (if (empty? obj-id)
                   (dataset-get-objective-id dataset-id)
                   obj-id)
        otype (get-in (fetch ds-id) ["fields" obj-id "optype"]))
    (when (not (= "categorical" otype))
      (raise (str "The dataset's objective field must be categorical, "
                  "but is " otype)))
    obj-id))

;; Do best-first feature selection.  Given a dataset and a target
;; number of features iteratively construct models for each feature,
;; evaluate them, and add the feature corresponding to the best
;; evaluation to the running set of features.  Stop when you reach the
;; target number, or you run out of features.
(define (select-features dataset-id nfeatures objective-id)
  (let (obj-id (get-objective dataset-id objective-id)
        input-ids (default-inputs dataset-id obj-id)
        splits (split-dataset dataset-id 0.5)
        train-id (nth splits 0)
        test-id (nth splits 1))
    (loop (selected []
           potentials input-ids)
      (if (or (>= (count selected) nfeatures) (empty? potentials))
        (feature-names dataset-id selected)
        (let (_ (log-info "Making models...")
              model-ids (make-models dataset-id obj-id selected potentials)
              _ (log-info "Selecting feature...")
              next-feat (select-feature test-id potentials model-ids)
              _ (log-info "Selected feature is " next-feat))
          (recur (cons next-feat selected)
                 (filter (lambda (id) (not (= id next-feat))) potentials)))))))

(define output-features (select-features dataset-id n objective-id))

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