lambda-funcs problem

[dmitrey.kroshko]

hi all,
I need to create a Python list of lambda-funcs that are dependent on
the number of the ones, for example

F = []
for i in xrange(N):
F.append(lambda x: x + i)

however, the example don't work - since i in end is N-1 it yields x+
(N-1) for any func.

So what's the best way to make it valid?
Evaluation speed is also very important to me.

Thank you in advance, D.

[Marc 'BlackJack' Rintsch]

On Wed, 19 Sep 2007 04:39:44 -0700, dmitrey.kroshko wrote:

I need to create a Python list of lambda-funcs that are dependent on
the number of the ones, for example

F = []
for i in xrange(N):
F.append(lambda x: x + i)

however, the example don't work - since i in end is N-1 it yields x+
(N-1) for any func.

So what's the best way to make it valid?

The variable is bound to the name `i` when the lambda function is
created not to the value that `i` had at that time. The idiomatic way is
to use a default value for an argument because those are evaluated at
definition time of functions::

F.append(lambda x, i=i: x + i)

Ciao,
Marc 'BlackJack' Rintsch

[Bruno Desthuilliers]

dm************* @scipy.org a écrit :

hi all,
I need to create a Python list of lambda-funcs that are dependent on
the number of the ones, for example

F = []
for i in xrange(N):
F.append(lambda x: x + i)

however, the example don't work - since i in end is N-1 it yields x+
(N-1) for any func.

So what's the best way to make it valid?

It's a FAQ. The answer is (using list-comp instead of a for loop):

funcs = [lambda x, i=i : x + i for i in xrange(n)]

[Ryan Ginstrom]

On Behalf Of dm************* @scipy.org

F = []
for i in xrange(N):
F.append(lambda x: x + i)

however, the example don't work - since i in end is N-1 it yields x+
(N-1) for any func.

How about:

>>def make_adder(i):

def adder(x):
return x+i
return adder

>>funcs = [make_adder(i) for i in xrange(10)]
print [func(10) for func in funcs]

[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

>>>

Regards,
Ryan Ginstrom

[=?utf-8?b?U3TDqXBoYW5l?= Larouche]

Ryan Ginstrom <software <atginstrom.com writes:

How about:

>def make_adder(i):

def adder(x):
return x+i
return adder

>funcs = [make_adder(i) for i in xrange(10)]
print [func(10) for func in funcs]

[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

>>

Or if you want a one liner:

funcs = [(lambda i: lambda x: x+i)(i) for i in xrange(10)]

Stéphane

[Bruno Desthuilliers]

Stéphane Larouche a écrit :
(snip)

>
funcs = [(lambda i: lambda x: x+i)(i) for i in xrange(10)]

A bit more complex than necessary... The canonical solution is
funcs = [lambda x, i=i: x+i for i in xrange(10)]