# Imputing values with non-negative matrix factorization

X is a DataFrame w/ about 90% missing values and around 10% actual values. My goal is to use nmf in a successive imputation loop to predict the actual values I have hidden. The mask, msk, selects a random 80% of the actual values (or 80% of the 10% actual values). I initialize all but these 80% to 0 and begin to impute them. Line 2 looks odd because I couldn't find a way to get a random 80% (train set) of the values who weren't np.nan so if I add an np.nan to a number, the value stays np.nan. Then if I subtract that X.values back off the only values that are effected are the non-null values of the array X_imputed. This allows me to get a random 80% of the non-null values.

import pandas as pd
from pandas import DataFrame
import numpy as np

# toy example data, actual data is ~500 by ~ 250
customers = range(20)
features = range(15)

toy_vals = np.random.random(20*15).reshape((20,15))

X = DataFrame(toy_vals, index=customers, columns=features)
# end toy example data gen.

# imputation w/ nmf loops
X_imputed = X.copy()
msk = (X.values + np.random.randn(*X.shape) - X.values) < 0.8
X_imputed.values[~msk] = 0
W = nmf_model.fit_transform(X_imputed.values)
H = nmf_model.components_

while nmf_model.reconstruction_err_**2 > 10:
nmf_model.fit_transform(X_imputed.values)
W = nmf_model.fit_transform(X_imputed.values)
H = nmf_model.components_
X_imputed.values[~msk] = W.dot(H)[~msk]


I'm pretty sure this can be written in fewer lines but I'm not sure how to do it.

• I get your code to run, but the while starts only when I crank up the number of dimensions of the matrix to 200 and 150. I think for smaller matrices the bound you have for the approximation error is met after a single iterations. – Curt F. Jul 14 '15 at 16:18
• Also, strangely, when I try to access the nmf_model.n_iter_ parameter, it isn't there, which seems to conflict with the documentation for the module you are using. Is this a scikit-learn bug? – Curt F. Jul 14 '15 at 16:19
• Yeah, I should have changed the break condition for the while loop. It runs for at least 60s on my machine with the actual data so either scaling up the toy dimensions or scaling down the while loop condition (10 to 10e-3 or something?). I have acces to nmf.model.n_iter_ parameter on my end. Could be a bug. Try to update or reinstall maybe? – pmdaly Jul 14 '15 at 16:31

1. In the while loop, the first call you make to nmf_model.fit_transform() is superfluous and can be removed. You aren't even using the results of the transformation calculation. The next line, where you have W = nmf_model.fit_transform(X_imputed.values) is doing all the work. Removing this line halves the number of model fits and speeds things up by ~twofold.

2. You don't need to assign H outside/before the while loop.

3. If minimization of code lines is the goal, you can avoid assigning to temporary variables and just put the expression that you would have used to define the variable in the code line that uses it. I did that for H in the while loop. It is more compact but probably harder to understand.

4. You don't seem to need to full pandas module, so that import can be removed.

5. I didn't change anything in my code below, but why are you squaring nmf_model.reconstruction_err_? According to the docs this error is the Frobenius norm of the difference matrix (X - WH), so it will always be positive even without squaring.

This is a tad more compact and significantly faster (because of item 1):

from pandas import DataFrame
import numpy as np

# Example data matrix X
nrows, ncols = 200, 150
toy_vals = np.random.random(nrows*ncols).reshape((nrows, ncols))
toy_vals[toy_vals < 0.9] = np.nan
X = DataFrame(toy_vals, index=range(nrows), columns=range(ncols))

# Hiding values to test imputation
X_imputed = X.copy()
msk = (X.values + np.random.randn(*X.shape) - X.values) < 0.8
X_imputed.values[~msk] = 0

# Initializing model

• If you want, the while loop could be a single (but long and confusing) line: X_imputed.values[~msk] = nmf_model.fit_transform(X_imputed.values).dot(nmf_model.components_)[~msk]. – Curt F. Jul 14 '15 at 17:02
• I actually like the version with explicit assignment to W and H (only inside the while loop) better but since you'd asked about "fewer lines" I just thought I'd mention it. There should be essentially no difference in speed either way. – Curt F. Jul 15 '15 at 15:56