|AHELP for CIAO 4.2||
Get the column values from a crate.
- crate - input crate
- colid - column name or number, where the first column is numbered 0
The get_colvals command retrieves an array of the values of the specified column within the crate. Allowed crate types are 'Table', 'ARF', 'PHA', and 'RMF'. The print_col_names command ("ahelp print_col_names") lists all the columns in the file with the corresponding numbers.
The get_col command ("ahelp get_col") returns a CrateData object for the column, which is useful if you are interested in the metadata associated with it.
>>> cr = read_file("evt2.fits") >>> vals = get_colvals(cr, "time") >>> print(vals[0:3]) [ 52379718.80918065 52379718.80918065 52379718.80918065]
Retrieve the values of the "time" column from the crate "cr", then print out the first three elements of the array.
>>> cr = read_file("evt2.fits") print get_col_names(cr) ['time' 'ccd_id' 'node_id' 'expno' 'chip(chipx,chipy)' 'tdet(tdetx,tdety)' 'det(detx,dety)' 'sky(x,y)' 'pha' 'pha_ro' 'energy' 'pi' 'fltgrade' 'grade' 'status'] >>> vals = get_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 stored in the variable "vals".
The copy_colvals routine was added to Crates in CIAO 4.1. It appears similar to get_colvals, so why was it added? 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 = 5 >>> y = 9 >>> print x [ 1. 5. 9.] >>> print get_colvals (cr, "x") [ 1. 3. 9.] >>> print x [ 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 #TEXT/SIMPLE # 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 propogated 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.
- add_col, col_exists, copy_colvals, cratedata, delete_col, get_axis_transform, get_col, get_col_names, get_crate_item_type, get_crate_type, get_key, get_key_names, get_keyval, get_number_cols, get_number_rows, get_piximg, get_piximg_shape, get_piximgvals, get_transform, get_transform_matrix, is_virtual, print_col_names, set_colvals