0

I have data which I need to center and scale so that is centered around the origin. Then the data needs to be rotated so that the direction of maximum variance is on the x-axis. The mean of the data and the covariance is then calculated. I need the first element of the covariance matrix to be 1. I think this is done by adjusting the scaling factor, but I can't figure out what the scaling factor should be.

To center the data I take away the mean, and to rotate I use SVD, but the scaling is still my problem.

```
signature = numpy.loadtxt(name, comments = '%', usecols = (0,cols-1))
signature = numpy.transpose(signature)
#SVD to get D so that data can be scaled by 1/(highest singular value in D)
U, D, Vt = numpy.linalg.svd( signature , full_matrices=0)
cs = utils.centerscale(signature, scale=False)
signature = cs[0]
#plt.scatter(cs[0][0],cs[0][1],color='r')
#SVD so that data can be rotated so that direction of most variance is on x-axis
U, D, Vt = numpy.linalg.svd( signature , full_matrices=0)
cs = utils.centerscale(signature, center=False, scalefactor=D[0])
U, D, Vt = numpy.linalg.svd( cs[0] , full_matrices=0)
D = numpy.diag(D)
norm = numpy.dot(D,Vt)
```

The following are examples of results of the mean and cov of *norm* (the test cases use res).

```
**********************************************************************
Failed example:
print numpy.mean(res, axis=1)
Expected:
[ 7.52074907e-18 -6.59917722e-18]
Got:
[ -1.22008884e-17 2.41126563e-17]
**********************************************************************
Failed example:
print numpy.cov(res, bias=1)
Expected:
[[ 1.00000000e+00 9.02112676e-18]
[ 9.02112676e-18 1.40592827e-01]]
Got:
[[ 4.16666667e-03 -1.57698124e-19]
[ -1.57698124e-19 5.85803446e-04]]
**********************************************************************
1 items had failures:
2 of 4 in __main__.processfile
***Test Failed*** 2 failures.
```

All values are irrelevant except for the first element of the covariance matrix, that needs to be one.

I have tried looking everywhere and can't find an answer. Any help would be appreciated.

0

I don't know what utils.centerscale is or does, but if you want to scale a matrix by a constant factor so that the upper left term of its covariance matrix is 1, you can simply divide the matrix by the square root of the unscaled covariance term:

```
>>> import numpy
>>> numpy.random.seed(17)
>>> m = numpy.random.rand(5,4)
>>> m
array([[ 0.294665 , 0.53058676, 0.19152079, 0.06790036],
[ 0.78698546, 0.65633352, 0.6375209 , 0.57560289],
[ 0.03906292, 0.3578136 , 0.94568319, 0.06004468],
[ 0.8640421 , 0.87729053, 0.05119367, 0.65241862],
[ 0.55175137, 0.59751325, 0.48352862, 0.28298816]])
>>> c = numpy.cov(m,bias=1)
>>> c
array([[ 0.0288779 , 0.00524455, 0.00155373, 0.02779861, 0.01798404],
[ 0.00524455, 0.00592484, -0.00711072, 0.01006019, 0.00631144],
[ 0.00155373, -0.00711072, 0.13391344, -0.10551922, 0.00945934],
[ 0.02779861, 0.01006019, -0.10551922, 0.11250984, 0.00982862],
[ 0.01798404, 0.00631144, 0.00945934, 0.00982862, 0.01444482]])
>>> numpy.cov(m/c[0][0]**0.5, bias=1)
array([[ 1. , 0.18161135, 0.05380354, 0.96262562, 0.62276138],
[ 0.18161135, 0.20516847, -0.24623392, 0.3483699 , 0.21855613],
[ 0.05380354, -0.24623392, 4.63722877, -3.65397781, 0.32756326],
[ 0.96262562, 0.3483699 , -3.65397781, 3.89605297, 0.34035085],
[ 0.62276138, 0.21855613, 0.32756326, 0.34035085, 0.5002033 ]])
```

But this has the same effect as simply dividing the covariance matrix by the upper left member:

```
>>> (numpy.cov(m,bias=1)/numpy.cov(m,bias=1)[0][0])/(numpy.cov(m/c[0][0]**0.5, bias=1))
array([[ 1., 1., 1., 1., 1.],
[ 1., 1., 1., 1., 1.],
[ 1., 1., 1., 1., 1.],
[ 1., 1., 1., 1., 1.],
[ 1., 1., 1., 1., 1.]])
```

Depending on what you're doing, you might also be interested in `numpy.corrcoef`

, which gives the correlation coefficient matrix instead.

Thanks this has helped a lot - luke417 2012-04-05 16:33