get_indep

Context: data

Synopsis

Return the independent axes of a data set.

Syntax

get_indep(id=None, filter=False, bkg_id=None)

id - int or str, optional
filter - bool, optional
bkg_id - int or str, optional

Description

This function returns the coordinates of each point, or pixel, in the data set. The `get_axes` function may be be preferred in some situations.

Examples

Example 1

For a one-dimensional data set, the X values are returned:

>>> load_arrays(1, [10, 15, 19], [4, 5, 9], Data1D)
>>> get_indep()
(array([10, 15, 19]),)

Example 2

For a 2D data set the X0 and X1 values are returned:

>>> x0 = [10, 15, 12, 19]
>>> x1 = [12, 14, 10, 17]
>>> y = [4, 5, 9, -2]
>>> load_arrays(2, x0, x1, y, Data2D)
>>> get_indep(2)
(array([10, 15, 12, 19]), array([12, 14, 10, 17]))

Example 3

For PHA data sets the return value is in channel units:

>>> load_pha('spec', 'src.pi')
>>> set_analysis('spec', 'energy')
>>> (chans,) = get_indep('spec')
>>> chans[0:6]
array([ 1.,  2.,  3.,  4.,  5.,  6.])

Example 4

If the filter flag is set then the return will be limited to the data that is used in the fit:

>>> notice_id('spec', 0.5, 7)
>>> (nchans,) = get_indep('spec', filter=True)
>>> nchans[0:5]
array([ 35.,  36.,  37.,  38.,  39.])

Example 5

For images the pixel coordinates of each pixel are returned, as 1D arrays, one value for each pixel:

>>> load_image('img', 'image.fits')
>>> (xvals, yvals) = get_indep('img')
>>> xvals.shape
(65536,)
>>> yvals.shape
(65536,)
>>> xvals[0:5]
array([ 1.,  2.,  3.,  4.,  5.])
>>> yvals[0:5]
array([ 1.,  1.,  1.,  1.,  1.])

Example 6

The coordinate system for image axes is determinated by the `set_coord` setting for the data set:

>>> set_coord('img', 'physical')
>>> (avals, bvals) = get_indep('img')
>>> avals[0:5]
array([  16.5,   48.5,   80.5,  112.5,  144.5])

PARAMETERS

The parameters for this function are:

Parameter	Definition
id	The identifier for the data set to use. If not given then the default identifier is used, as returned by `get_default_id` .
filter	Should the filter attached to the data set be applied to the return value or not. The default is False .
bkg_id	Set if the values returned should be from the given background component, instead of the source data set.

Return value

The return value from this function is:

axis -- The independent axis values. These are the values at which the model is evaluated during fitting. The values returned depend on the coordinate system in use for the data set (as set by `set_coord` ). For PHA data sets the value returned is always in channels, whatever the `set_analysis` setting is, and does not follow any grouping setting for the data set.

Notes

For a two-dimensional image, with size n by m pixels, the `get_dep` function will return two arrays, each of size n * m, which contain the coordinate of the center of each pixel. The `get_axes` function will instead return the coordinates of each axis separately, i.e. arrays of size n and m.

Bugs

See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.