scikit-learn / scikit-learn/scikit-learn

kernel_approximation.Nystroem with precomputed kernel

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Bug
Dominant language
Python
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Description

Describe the bug

I am trying to get a Nystroem approximation of a pre computed kernel but it throws an error if I use n_components anything less than the number of datapoints. Unless my understanding is wrong, does this not defeat the point of the approximation? Please advise, code below:

I have come from this resolved issue https://github.com/scikit-learn/scikit-learn/issues/14641

Steps/Code to Reproduce
from sklearn.svm import SVC
from sklearn.kernel_approximation import Nystroem
 
# data shape (3000,50)
# kernel matrix shape (3000,3000)
clf = SVC()
feature_map_nystroem = Nystroem(
    kernel = 'precomputed',
    random_state=1,
    n_components=300
)
kernel_transformed = feature_map_nystroem.fit_transform(kernel)
clf.fit(kernel_transformed, y)
Expected Results

I expect this to work.

Actual Results

Instead it gives error:

in check_pairwise_arrays(X, Y, precomputed, dtype, accept_sparse, force_all_finite, copy)
    153 if precomputed:
    154     if X.shape[1] != Y.shape[0]:
--> 155 raise ValueError("Precomputed metric requires shape "

    156                 "(n_queries, n_indexed). Got (%d, %d) "
    157                          "for %d indexed." %
    158                          (X.shape[0], X.shape[1], Y.shape[0]))
    159 elif X.shape[1] != Y.shape[1]:
    160     raise ValueError("Incompatible dimension for X and Y matrices: "
    161                      "X.shape[1] == %d while Y.shape[1] == %d" % (
    162                          X.shape[1], Y.shape[1]))
 
ValueError: Precomputed metric requires shape (n_queries, n_indexed). Got (300, 3000) for 300 indexed.
Versions
System:
    python: 3.8.19 (default, Mar 20 2024, 19:58:24)  [GCC 11.2.0]
executable: /opt/conda/miniconda3/envs/python3.8/bin/python
   machine: Linux-6.1.0-21-cloud-amd64-x86_64-with-glibc2.17
 
Python dependencies:
          pip: 24.0
   setuptools: 63.1.0
      sklearn: 0.24.2
        numpy: 1.21.6
        scipy: 1.10.1
       Cython: 3.0.10
       pandas: 1.2.5
   matplotlib: 3.4.3
       joblib: 1.4.2
threadpoolctl: 3.5.0

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with sklearn.kernel_approximation.Nystroem and trace its precomputed-kernel path into check_pairwise_arrays. Reproduce the provided 3000-by-3000 kernel example with n_components=300, then verify that the precomputed case supports the expected approximation shape without the shown validation error.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
machine-learning
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Mostly clear
Newbie friendliness
48/100

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