copy_colvals

Should I use copy_colvals or get_colvals?

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[1] = 5
>>> y[1] = 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.