# Nonnegative matrix factorization¶

A derivative work by Judson Wilson, 6/2/2014.
Adapted from the CVX example of the same name, by Argyris Zymnis, Joelle Skaf and Stephen Boyd

## Introduction¶

We are given a matrix $A \in \mathbf{\mbox{R}}^{m \times n}$ and are interested in solving the problem: \begin{array}{ll} \mbox{minimize} & | A - YX |_F \ \mbox{subject to} & Y \succeq 0 \ & X \succeq 0, \end{array} where $Y \in \mathbf{\mbox{R}}^{m \times k}$ and $X \in \mathbf{\mbox{R}}^{k \times n}$.

This example generates a random matrix $A$ and obtains an approximate solution to the above problem by first generating a random initial guess for $Y$ and then alternatively minimizing over $X$ and $Y$ for a fixed number of iterations.

## Generate problem data¶

In [1]:
import cvxpy as cvx
import numpy as np

# Ensure repeatably random problem data.
np.random.seed(0)

# Generate random data matrix A.
m = 10
n = 10
k = 5
A = np.random.rand(m, k).dot(np.random.rand(k, n))

# Initialize Y randomly.
Y_init = np.random.rand(m, k)


## Perform alternating minimization¶

In [2]:
# Ensure same initial random Y, rather than generate new one
# when executing this cell.
Y = Y_init

# Perform alternating minimization.
MAX_ITERS = 30
residual = np.zeros(MAX_ITERS)
for iter_num in range(1, 1+MAX_ITERS):
# At the beginning of an iteration, X and Y are NumPy
# array types, NOT CVXPY variables.

# For odd iterations, treat Y constant, optimize over X.
if iter_num % 2 == 1:
X = cvx.Variable(k, n)
constraint = [X >= 0]
# For even iterations, treat X constant, optimize over Y.
else:
Y = cvx.Variable(m, k)
constraint = [Y >= 0]

# Solve the problem.
obj = cvx.Minimize(cvx.norm(A - Y*X, 'fro'))
prob = cvx.Problem(obj, constraint)
prob.solve(solver=cvx.SCS)

if prob.status != cvx.OPTIMAL:
raise Exception("Solver did not converge!")

print 'Iteration {}, residual norm {}'.format(iter_num, prob.value)
residual[iter_num-1] = prob.value

# Convert variable to NumPy array constant for next iteration.
if iter_num % 2 == 1:
X = X.value
else:
Y = Y.value

Iteration 1, residual norm 2.76585686659
Iteration 2, residual norm 0.577758799504
Iteration 3, residual norm 0.46343315761
Iteration 4, residual norm 0.300312085357
Iteration 5, residual norm 0.172468695929
Iteration 6, residual norm 0.117552622713
Iteration 7, residual norm 0.0855259222075
Iteration 8, residual norm 0.0660380454036
Iteration 9, residual norm 0.0530018181734
Iteration 10, residual norm 0.0442728793651
Iteration 11, residual norm 0.0364005958705
Iteration 12, residual norm 0.0308842140499
Iteration 13, residual norm 0.0256059616668
Iteration 14, residual norm 0.0226869576657
Iteration 15, residual norm 0.0191546943234
Iteration 16, residual norm 0.0166449632154
Iteration 17, residual norm 0.0135201384604
Iteration 18, residual norm 0.0119471133563
Iteration 19, residual norm 0.0149438374084
Iteration 20, residual norm 0.0138663023673
Iteration 21, residual norm 0.00922230392493
Iteration 22, residual norm 0.00857605731059
Iteration 23, residual norm 0.0074862441594
Iteration 24, residual norm 0.00739813239648
Iteration 25, residual norm 0.0100134191882
Iteration 26, residual norm 0.00944406772568
Iteration 27, residual norm 0.008678201611
Iteration 28, residual norm 0.00873112072225
Iteration 29, residual norm 0.00798267920957
Iteration 30, residual norm 0.00846182763828


## Output results¶

In [3]:
#
# Plot residuals.
#

import matplotlib.pyplot as plt

# Show plot inline in ipython.
%matplotlib inline

# Set plot properties.
plt.rc('text', usetex=True)
plt.rc('font', family='serif')
font = {'family' : 'normal',
'weight' : 'normal',
'size'   : 16}
plt.rc('font', **font)

# Create the plot.
plt.plot(residual)
plt.xlabel('Iteration Number')
plt.ylabel('Residual Norm')
plt.show()

#
# Print results.
#
print 'Original matrix:'
print A
print 'Left factor Y:'
print Y
print 'Right factor X:'
print X
print 'Residual A - Y * X:'
print A - Y * X
print 'Residual after {} iterations: {}'.format(iter_num, prob.value)

Original matrix:
[[ 1.323426    1.11061189  1.69137835  1.20020115  1.13216889  0.5980743
1.64965406  0.340611    1.69871738  0.78278448]
[ 1.73721109  1.40464204  1.90898877  1.60774132  1.53717253  0.62647405
1.76242265  0.41151492  1.8048194   1.20313124]
[ 1.4071438   1.10269406  1.75323063  1.18928983  1.23428169  0.60364688
1.63792853  0.40855006  1.57257432  1.17227344]
[ 1.3905141   1.33367163  1.07723947  1.67735654  1.33039096  0.42003169
1.22641711  0.21470465  1.47350799  0.84931787]
[ 1.42153652  1.13598552  2.00816457  1.11463462  1.17914429  0.69942578
1.90353699  0.45664487  1.81023916  1.09668578]
[ 1.60813803  1.23214532  1.73741086  1.3148874   1.27589039  0.40755835
1.31904948  0.3469129   1.34256526  0.76924618]
[ 0.90607895  0.6632877   1.25412229  0.81696721  0.87218892  0.50032884
1.245879    0.25079329  1.25017792  0.72155621]
[ 1.5691922   1.47359672  1.76518996  1.66268312  1.43746574  0.72486628
1.97409333  0.39239642  2.09234807  1.16325748]
[ 1.18723548  1.00282008  1.41532595  1.03836298  0.90382914  0.38460446
1.213473    0.23641422  1.32784402  0.27179726]
[ 0.75789915  0.75119989  0.99502166  0.65444815  0.56073096  0.341146
1.02555143  0.24273668  1.01035919  0.49427978]]
Left factor Y:
[[  7.38991833e-01   3.15957978e-01   8.46211348e-01   7.90522539e-01
8.82326030e-01]
[  6.37868033e-01   8.22907024e-01   5.32198000e-01   5.70689637e-01
6.21191813e+00]
[  5.59748656e-01   6.34112010e-01   7.99615283e-01   1.72054035e-01
6.92576630e+00]
[  2.61288516e-01   9.41947419e-01   4.03583183e-02   1.09118729e+00
9.07778543e-07]
[  7.89189550e-01   3.41453292e-01   1.17654458e+00   3.93009044e-01
5.50024762e+00]
[  7.39615442e-01   4.74493175e-01  -2.23332571e-04   6.74749299e-01
8.42579458e+00]
[  4.73914127e-01   3.70454244e-01   8.08948369e-01   1.36848129e-01
3.44366220e-06]
[  5.88504809e-01   7.27646377e-01   1.00390505e+00   1.03542480e+00
3.71366168e-01]
[  8.14822860e-01   8.87015769e-04   2.91164377e-01   1.17787451e+00
1.24901335e-06]
[  4.22680617e-01   7.77641517e-02   5.87259008e-01   6.51086033e-01
2.66173216e+00]]
Right factor X:
[[  1.13055890e+00   4.05899679e-01   1.59181960e+00   6.82867774e-01
9.75411818e-01   3.23464160e-01   8.83710480e-01   1.64529269e-01
9.23391090e-01   1.03847861e-01]
[  9.03465524e-01   6.86715676e-01   6.62169881e-01   1.12490745e+00
1.03933855e+00   3.06370001e-01   7.29180054e-01   1.18625225e-01
8.75435486e-01   8.00971786e-01]
[  6.63783207e-03   1.79085385e-01   3.11550072e-01   2.62447584e-02
1.60660298e-02   2.77495461e-01   6.46185026e-01   1.51538848e-01
5.43725876e-01   4.58269799e-01]
[  2.23736169e-01   5.25847565e-01   2.32705796e-02   4.01864284e-01
8.80884850e-02   3.23502266e-02   2.59210460e-01   4.94700824e-02
3.53704441e-01   4.54771630e-02]
[  2.28693369e-02   2.99295139e-02   2.74091017e-02   5.18043712e-04
1.64409925e-04   2.93841883e-04   1.72556068e-02   1.61648660e-02
6.42191908e-04   3.35760370e-02]]
Residual A - Y * X:
[[ -1.68636983e-04   3.60207904e-05  -3.98077462e-04  -2.04516172e-04
-4.16031924e-04   1.58422455e-03  -7.36906450e-04  -5.91624286e-05
-5.46879309e-04  -4.00508769e-04]
[ -6.83414765e-04  -6.95866828e-04  -6.36012200e-04  -5.76770312e-05
-1.33274006e-04   6.27824831e-05  -3.32923224e-04  -3.46132086e-04
2.01209611e-04  -6.50509595e-04]
[ -7.73470728e-04  -9.20826799e-04  -6.31154497e-04   2.22522673e-05
9.78991659e-05  -1.17532606e-03   8.50142570e-05  -4.05047780e-04
5.07774407e-04  -5.64939614e-04]
[ -3.10948903e-04  -2.61145596e-04  -3.80145546e-04  -2.41030729e-04
-2.44643018e-04   4.30377374e-04  -2.61672784e-04  -1.20679505e-04
-2.79346735e-04  -4.08818401e-04]
[ -7.07080893e-04  -8.12121976e-04  -6.37850976e-04  -4.46396107e-05
4.76451453e-05  -1.27571378e-03   9.33258194e-05  -3.50044450e-04
3.29244878e-04  -4.88627114e-04]
[ -3.85692817e-04  -8.61465250e-04  -6.94274594e-04   5.50232944e-04
4.82360881e-04  -1.29338044e-03  -6.97353986e-04  -6.10252918e-04
2.71414348e-04  -1.10507632e-03]
[ -3.89135610e-04  -3.02813339e-04  -7.79701822e-04   3.94758648e-04
-1.51231975e-04   4.63196558e-03  -1.25457666e-03  -4.81723061e-04
1.10896155e-05  -1.32234306e-03]
[ -3.69191741e-04  -3.38504643e-04  -4.69486964e-04  -3.62561200e-04
-2.38561015e-04  -6.07402720e-04  -6.92616183e-05  -1.03315580e-04
-4.01477226e-04  -2.98251672e-04]
[ -2.37003094e-04  -5.11457693e-05  -4.34540394e-04  -3.83365150e-05
-3.15650926e-04   1.86541629e-03  -7.04768572e-04  -1.45497987e-04
-2.65728261e-04  -5.29103341e-04]
[ -6.65085219e-04  -9.73758356e-04  -3.70136974e-04  -1.03883251e-04
3.94287162e-04  -4.20725740e-03   1.14398039e-03  -2.59815037e-04
6.72891426e-04  -4.62543141e-06]]
Residual after 30 iterations: 0.00846182763828

In [ ]: