How to efficiently proceed addition and subtraction in python list?

[Daniel Mark]

Hello all:

I have a list AAA = [1, 2, 3] and would like to subtract one from list
AAA
so AAA' = [0, 1, 2]

What should I do?


Thank you
-Daniel

[Tim Chase]

Re: How to efficiently proceed addition and subtraction in pythonlist?

I have a list AAA = [1, 2, 3] and would like to subtract one from list

AAA
so AAA' = [0, 1, 2]
>
What should I do?

Sounds like a list comprehension to me:

>>a = [1,2,3]
>>a_prime = [x-1 for x in a]
>>a_prime

[0, 1, 2]

-tkc

[Chiucs]

Re: How to efficiently proceed addition and subtraction in python list?

try this
a=[1, 2, 3]
b=a[:]

"Daniel Mark" <danielmarkhot@ gmail.com???
news:1158619411 .713865.287810@ m73g2000cwd.goo glegroups.com ???...

Hello all:
>
I have a list AAA = [1, 2, 3] and would like to subtract one from list
AAA
so AAA' = [0, 1, 2]
>
What should I do?
>
>
Thank you
-Daniel
>

[Paul Rubin]

Re: How to efficiently proceed addition and subtraction in python list?

"Daniel Mark" <danielmarkhot@ gmail.comwrites :

I have a list AAA = [1, 2, 3] and would like to subtract one from list
AAA
so AAA' = [0, 1, 2]
>
What should I do?

BBB = [x-1 for x in AAA]

[Ant]

Re: How to efficiently proceed addition and subtraction in python list?


Tim Chase wrote:

I have a list AAA = [1, 2, 3] and would like to subtract one from list
AAA
so AAA' = [0, 1, 2]

What should I do?

>
>
Sounds like a list comprehension to me:

Also the built in function 'map' would work:

>>a = [1,2,3]
>>b = map(lambda x: x-1, a)
>>b

[0, 1, 2]

List comprehensions are more pythonic, but map would probably be faster
if performance was a (real) issue.

[Steve Holden]

Re: How to efficiently proceed addition and subtraction in pythonlist?

Ant wrote:

Tim Chase wrote:
>

>>>I have a list AAA = [1, 2, 3] and would like to subtract one from list
>>>AAA
>>>so AAA' = [0, 1, 2]
>>>
>>>What should I do?

>>
>>
>>Sounds like a list comprehension to me:

>
>
Also the built in function 'map' would work:
>
>

>>>>a = [1,2,3]
>>>>b = map(lambda x: x-1, a)
>>>>b

>
[0, 1, 2]
>
List comprehensions are more pythonic, but map would probably be faster
if performance was a (real) issue.
>

And statements like that are probably going to annoy me ;-)

>>t = timeit.Timer("b = map(lambda x: x-1, a)", setup="a=[1,2,3]")
>>t.timeit()

2.4686168214116 599

>>t = timeit.Timer("b = [x-1 for x in a]", setup="a=[1,2,3]")
>>t.timeit()

0.9930245324475 635

>>>

Any timing prediction involving the word "probably" isn't worth the
paper it's written on (or even less if it's posted in a newsgroup ;-).
If it's "probably" faster, and if performance is *really* important, you
need to benchmark both options to remove the "probably". As the above
test makes clear, your assertions are certainly untrue for 2.4.2 on Windows.

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

[Fredrik Lundh]

Re: How to efficiently proceed addition and subtraction in pythonlist?

Steve Holden wrote:

And statements like that are probably going to annoy me ;-)
>

>>t = timeit.Timer("b = map(lambda x: x-1, a)", setup="a=[1,2,3]")
>>t.timeit()

2.4686168214116 599

>>t = timeit.Timer("b = [x-1 for x in a]", setup="a=[1,2,3]")
>>t.timeit()

0.9930245324475 635

>>>

And now someone's probably reading this and thinking that list comprehensions
are *always* faster than "map"...

</F>

[Steve Holden]

Re: How to efficiently proceed addition and subtraction in pythonlist?

Fredrik Lundh wrote:

Steve Holden wrote:
>
>

>>And statements like that are probably going to annoy me ;-)
>>

>>t = timeit.Timer("b = map(lambda x: x-1, a)", setup="a=[1,2,3]")
>>t.timeit()

>>2.46861682141 16599

>>t = timeit.Timer("b = [x-1 for x in a]", setup="a=[1,2,3]")
>>t.timeit()

>>0.99302453244 75635

>>>

>
>
And now someone's probably reading this and thinking that list comprehensions
are *always* faster than "map"...
>

You are so right. So, let me repeat the important part that Fredrik
didn't quote:

"""if performance is *really* important, you
need to benchmark both options"""

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

[Ant]

Re: How to efficiently proceed addition and subtraction in pythonlist?


Steve Holden wrote:

Fredrik Lundh wrote:

Steve Holden wrote:

....

"""if performance is *really* important, you
need to benchmark both options"""

Fair point.

[Simon Brunning]

Re: How to efficiently proceed addition and subtraction in pythonlist?

On 18 Sep 2006 15:43:31 -0700, Daniel Mark <danielmarkhot@ gmail.comwrote:

Hello all:
>
I have a list AAA = [1, 2, 3] and would like to subtract one from list
AAA
so AAA' = [0, 1, 2]

You've had some excellent suggestions as to how to go about this
assuming that by "efficient" you mean in terms of CPU. If, instead,
you are concerned with memory usage, you might instead do something
like:

for index, value in enumerate(AAA):
AAA[index] = value-1

--
Cheers,
Simon B,
simon@brunningo nline.net