Extracting an undefined number of items from a list

[vsoler]

Hi everyone,

say that 'db' is a list of values
say 'i' is a list of indexes
I'd like to get a list where each item is i-th element of db.

For example:

db=[10,20,30,40,50,60,70,80,90] #undefined length
i=[3,5,7] #undefined length
then result=[30,50,70] # resulting list

how can I do this?

 

[Patrick Maupin]

Hi everyone,

say that 'db' is a list of values
say 'i' is a list of indexes
I'd like to get a list where each item is i-th element of db.

For example:

db=[10,20,30,40,50,60,70,80,90]      #undefined length
i=[3,5,7]                                         #undefined length
then result=[30,50,70]                     # resulting list

how can I do this?

With a list comprehension:

db=[10,20,30,40,50,60,70,80,90]
i=[3,5,7]
[db[x-1] for x in i]

[30, 50, 70]

Regards,
Pat

 

[vsoler]

Hi everyone,

say that 'db' is a list of values
say 'i' is a list of indexes
I'd like to get a list where each item is i-th element of db.

For example:

db=[10,20,30,40,50,60,70,80,90]      #undefined length
i=[3,5,7]                                         #undefined length
then result=[30,50,70]                     # resulting list

how can I do this?

With a list comprehension:

db=[10,20,30,40,50,60,70,80,90]
i=[3,5,7]
[db[x-1] for x in i]

[30, 50, 70]

Regards,
Pat

Thank you Patrick,

Your quick answer only tells me how much I still have to learn.

 

[Chris Rebert]

Hi everyone,

say that 'db' is a list of values
say 'i' is a list of indexes
I'd like to get a list where each item is i-th element of db.

For example:

db=[10,20,30,40,50,60,70,80,90]      #undefined length
i=[3,5,7]                                         #undefined length
then result=[30,50,70]                     # resulting list

how can I do this?

result = [db[k-1] for k in i] # your indices seem to be 1-based

See "List comprehensions":
http://docs.python.org/tutorial/datastructures.html#list-comprehensions

Cheers,
Chris

 

[Steven D'Aprano]

Hi everyone,

say that 'db' is a list of values
say 'i' is a list of indexes
I'd like to get a list where each item is i-th element of db.

For example:

db=[10,20,30,40,50,60,70,80,90] #undefined length
i=[3,5,7] #undefined length then
result=[30,50,70] # resulting list

how can I do this?

Others have given a simple solution using list comprehensions. Here's
another few ways.


# The beginner's way
result = []
for index in i: # "i" is a bad name for a list of indexes
x = db[index - 1] # shift from one-based counts to zero-based
result.append(x)


# The advanced functional way
from operator import itemgetter
indexes = map(lambda n: n-1, i) # adjust for one-based counts
result = itemgetter(*indexes)(db)



# The obfuscated, fragile way
map( itemgetter(0), sorted(
zip(db, range(1, len(db)+1)), key=lambda t: t[1] if t[1] in i else -1
)[-len(i):] )

 

[Patrick Maupin]

# The obfuscated, fragile way
map( itemgetter(0), sorted(
     zip(db, range(1, len(db)+1)), key=lambda t: t[1] if t[1] in i else -1
     )[-len(i):] )

I have to hand it to you that this might, in fact, be "the"
obfuscated, fragile way, but I gotta tell you that I'm certain there
exist other ways that are considerably less obfuscated and
considerably less fragile that I, personally would still consider to
be obfuscated and fragile...

The only thing you get points off for is the lack of a multiply or
divide by 10. The original data set is just crying out for such a
thing, and, while it might not do all that much for the obfuscation,
it would certainly help with the fragility. ;-)

Regards,
Pat