# [Tutor] finding a maximum between the absolute difference of several columns

Elaina Ann Hyde elainahyde at gmail.com
Fri Feb 17 02:04:12 CET 2012

```Hello all,
I am still scripting away and have reached my next quandry, this one is
much simpler than the last, basically I read in a file with several
columns.
Vmatch3_1=dat[col1]
Vmatch3_2=dat[col2]
Vmatch3_3=dat[col3]
Vdav=5.0
Vhel_fdiff3=max(abs(Vmatch3_1 - Vmatch3_2),abs(Vmatch3_1 -
Vmatch3_3),abs(Vmatch3_3 - Vmatch3_2))
----------------
What I would like this to return is the maximum difference in each case, so
I end up with one column which contains only the largest differences.
now I use this to write the condition:

with_v1_3=(Vhel_fdiff3 >= Vdav)

I know the condition works and plots if

Vhel_fdiff3=(Vmatch3_1 - Vmatch3_2)

for example, and I know this syntax would work if it was numbers instead of
columns.
>>max(abs(1-2),abs(3-7),abs(2-4))
>>4

The error is:
-----------------------
Traceback (most recent call last):
File "double_plot.py", line 109, in <module>
Vhel_fdiff3=max(abs(Vmatch3_1 - Vmatch3_2),abs(Vmatch3_1 -
Vmatch3_3),abs(Vmatch3_3 - Vmatch3_2))
ValueError: The truth value of an array with more than one element is
ambiguous. Use a.any() or a.all()
-----------------
So it can't handle the fact that it's columns of numbers and not single
numbers, so my question is, can I solve this without doing a loop around
it... use numpy, or some other function instead?
I've been searching around and haven't found any good ways forward so I
thought one of you might know.  Thanks
~Elaina

--
PhD Candidate
Department of Physics and Astronomy
Faculty of Science
Macquarie University
North Ryde, NSW 2109, Australia
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