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pythonnumpyscipysparse-matrixcorrelation
user2855672
user2855672

Reputation: 193

Correlation coefficients for sparse matrix in python?

Does anyone know how to compute a correlation matrix from a very large sparse matrix in python? Basically, I am looking for something like numpy.corrcoef that will work on a scipy sparse matrix.

Upvotes: 17

Views: 14429

Answers (4)

Jake Drew
Jake Drew

Reputation: 2340

I present an answer for a scipy sparse matrix which runs in parallel. Rather than returning a giant correlation matrix, this returns a feature mask of fields to keep after checking all fields for both positive and negative Pearson correlations.

I also try to minimize calculations using the following strategy:

  • Process each column
  • Start at the current column + 1 and calculate correlations moving to the right.
  • For any abs(correlation) >= threshold, mark the current column for removal and calculate no further correlations.
  • Perform these steps for each column in the dataset except the last.

This might be sped up further by keeping a global list of columns marked for removal and skipping further correlation calculations for such columns, since columns will execute out of order. However, I do not know enough about race conditions in python to implement this tonight.

Returning a column mask will obviously allow the code to handle much larger datasets than returning the entire correlation matrix.

Check each column using this function:

def get_corr_row(idx_num, sp_mat, thresh):
    # slice the column at idx_num
    cols = sp_mat.shape[1]
    x = sp_mat[:,idx_num].toarray().ravel()
    start = idx_num + 1
    
    # Now slice each column to the right of idx_num   
    for i in range(start, cols):
        y = sp_mat[:,i].toarray().ravel()
        # Check the pearson correlation
        corr, pVal = pearsonr(x,y)
        # Pearson ranges from -1 to 1.
        # We check both positive and negative correlations >= thresh using abs(corr)
        if abs(corr) >= thresh:
            # stop checking after finding the 1st correlation > thresh   
            return False
            # Mark column at idx_num for removal in the mask  
    return True

Run the column level correlation checks in parallel:

from joblib import Parallel, delayed  
import multiprocessing


def Get_Corr_Mask(sp_mat, thresh, n_jobs=-1):
    
    # we must make sure the matrix is in csc format 
    # before we start doing all these column slices!  
    sp_mat = sp_mat.tocsc()
    cols = sp_mat.shape[1]
    
    if n_jobs == -1:
        # Process the work on all available CPU cores
        num_cores = multiprocessing.cpu_count()
    else:
        # Process the work on the specified number of CPU cores
        num_cores = n_jobs

    # Return a mask of all columns to keep by calling get_corr_row() 
    # once for each column in the matrix     
    return Parallel(n_jobs=num_cores, verbose=5)(delayed(get_corr_row)(i, sp_mat, thresh)for i in range(cols))

General Usage:

#Get the mask using your sparse matrix and threshold.
corr_mask = Get_Corr_Mask(X_t_fpr, 0.95) 

# Remove features that are >= 95% correlated
X_t_fpr_corr = X_t_fpr[:,corr_mask]

Upvotes: 1

Alt
Alt

Reputation: 2755

You do not need to introduce a large dense matrix. Just keep it sparse using Numpy:

import numpy as np    
def sparse_corr(A):
    N = A.shape[0]
    C=((A.T*A -(sum(A).T*sum(A)/N))/(N-1)).todense()
    V=np.sqrt(np.mat(np.diag(C)).T*np.mat(np.diag(C)))
    COR = np.divide(C,V+1e-119)
    return COR

Testing the performance:

A = sparse.rand(1000000, 100, density=0.1, format='csr')
sparse_corr(A)

Upvotes: 12

Amin
Amin

Reputation: 1034

Unfortunately, Alt's answer didn't work out for me. The values given to the np.sqrt function where mostly negative, so the resulting covariance values were nan.

I wasn't able to use ali_m's answer as well, because my matrix was too large that I couldn't fit the centering = rowsum.dot(rowsum.T.conjugate()) / n matrix in my memory (My matrix's dimensions are: 3.5*10^6 x 33)

Instead, I used scikit-learn's StandardScaler to compute the standard sparse matrix and then used a multiplication to obtain the correlation matrix.

from sklearn.preprocessing import StandardScaler

def compute_sparse_correlation_matrix(A):
    scaler = StandardScaler(with_mean=False)
    scaled_A = scaler.fit_transform(A)  # Assuming A is a CSR or CSC matrix
    corr_matrix = (1/scaled_A.shape[0]) * (scaled_A.T @ scaled_A)
    return corr_matrix

I believe that this approach is faster and more robust than the other mentioned approaches. Moreover, it also preserves the sparsity pattern of the input matrix.

Upvotes: 0

ali_m
ali_m

Reputation: 74232

You can compute the correlation coefficients fairly straightforwardly from the covariance matrix like this:

import numpy as np
from scipy import sparse

def sparse_corrcoef(A, B=None):

    if B is not None:
        A = sparse.vstack((A, B), format='csr')

    A = A.astype(np.float64)
    n = A.shape[1]

    # Compute the covariance matrix
    rowsum = A.sum(1)
    centering = rowsum.dot(rowsum.T.conjugate()) / n
    C = (A.dot(A.T.conjugate()) - centering) / (n - 1)

    # The correlation coefficients are given by
    # C_{i,j} / sqrt(C_{i} * C_{j})
    d = np.diag(C)
    coeffs = C / np.sqrt(np.outer(d, d))

    return coeffs

Check that it works OK:

# some smallish sparse random matrices
a = sparse.rand(100, 100000, density=0.1, format='csr')
b = sparse.rand(100, 100000, density=0.1, format='csr')

coeffs1 = sparse_corrcoef(a, b)
coeffs2 = np.corrcoef(a.todense(), b.todense())

print(np.allclose(coeffs1, coeffs2))
# True

Be warned:

The amount of memory required for computing the covariance matrix C will be heavily dependent on the sparsity structure of A (and B, if given). For example, if A is an (m, n) matrix containing just a single column of non-zero values then C will be an (n, n) matrix containing all non-zero values. If n is large then this could be very bad news in terms of memory consumption.

Upvotes: 14

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