Chandra X-Ray Observatory
Skip to the navigation links
Last modified: December 2015

AHELP for CIAO 4.9


Context: crates


Copy the column values from a crate.


copy_colvals(crate, colname)
copy_colvals(crate, colnum)


Argument Description
crate The table crate.
colname The column name (case insensitive).
colnum The column number, where the first column is numbered 0.

The copy_colvals command retrieves a numpy array of the values of the specified column within a table crate. The data values are copied into an array and that array is returned.

The get_col_names routine can be used to find the columns in a Crate.

The get_col command returns a CrateData object for the column, which is useful if you are interested in the metadata associated with it.

Example 1

>>> cr = read_file("evt2.fits")
>>> t = copy_colvals(cr, "time")
>>> print(t[0:3])
[ 52379718.80918065 52379718.80918065 52379718.80918065]

Copy the values of the "time" column from the crate "cr", then print out the first three elements of the array.

Example 2

>>> cr = read_file("evt2.fits")
>>> print(get_col_names(cr))
['time' 'ccd_id' 'node_id' 'expno' 'chip(chipx,chipy)'
'det(detx,dety)' 'sky(x,y)' 'pha' 'pha_ro' 'energy' 'pi' 'fltgrade'
'grade' 'status']
>>> vals = copy_colvals(cr, 7)
>>> print(vals.shape)
(603495, 2)
>>> print(vals[0:3])
[[ 3892.52172852 4165.99365234]
[ 3908.08300781 4142.32275391]
[ 3849.05786133 3993.52783203]]

The values from column number 7 - the sky(x,y) column in this file - are copied to the variable "vals".

Example 3

>>> cr = read_file("evt2.fits")
>>> sky = copy_colvals(cr, "sky")
>>> y = copy_colvals(cr, "Y")
>>> print(sky.shape)
(124940, 2)
>>> print(y.shape)
>>> print(get_number_rows(cr))

Use copy_colvals to get the values of vector column SKY(X,Y) from the crate "cr". Then get the Y component values as a separate array.

Should I use copy_colvals or get_colvals?

The general rule of thumb is to use copy_colvals rather than get_colvals, particularly for interactive use from the ChIPS, Sherpa or IPython shells.

The return value from get_colvals reflects the contents of the Crate, so that changes to the return value also change the crate, whereas copy_colvals returns a copy of this data. This can be seen in the example below, which uses as an input file:

unix% cat tbl.dat
# x y
1  2
3  5
9  10

The steps taken below read in the file into a crate, extract the x and y column values, change the middle value of each column, check to see what values are stored in the crate, and then write out the crate to a new file. The x column is accessed using copy_colvals whereas the y column data is retrieved using get_colvals to show the difference in behavior.

>>> cr = read_file("tbl.dat")
>>> x = copy_colvals(cr, "x")
>>> y = get_colvals(cr, "y")
>>> print(x)
[ 1. 3. 9.]
>>> print(y)
[ 2. 5. 10.]
>>> x[1] = 5
>>> y[1] = 9
>>> print(x)
[ 1. 5. 9.]
>>> print(get_colvals(cr, "x"))
[ 1. 3. 9.]
>>> print(y)
[ 2. 9. 10.]
>>> print(get_colvals(cr, "y"))
[ 2. 9. 10.]
>>> write_file(cr, "tbl2.dat[opt kernel=text/simple]")

The output file contains:

unix% cat tbl2.dat
# x y
1.0 2.0
3.0 9.0
9.0 10.0

So changing the values in the x array - which was created using copy_colvals - does not change the values stored in the crate, whereas changes to the values in the y array - which was created with get_colvals - are propagated to the crate, and hence into the output file tbl2.dat.

Note that the input file - here tbl.dat - is not changed by these operations.

It is generally going to be safer to use copy_colvals rather than get_colvals unless:

  • you do want to be able to change values in the crate for either future processing or to write out to disk
  • you have read in a large file and do not want to waste memory by creating another copy of the array

If the column being read in is a virtual one then the data is always going to be copied, so in this particular case there is no difference between copy_colvals and get_colvals.


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

Refer to the CIAO bug pages for an up-to-date listing of known issues.

See Also

add_col, col_exists, delete_col, get_col, get_col_names, get_colvals, get_number_cols, is_virtual, set_colvals

Last modified: December 2015
Smithsonian Institute Smithsonian Institute

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email: Smithsonian Institution, Copyright © 1998-2017. All rights reserved.