else condition in list comprehension

[Luis M. Gonzalez]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

 

[Reinhold Birkenfeld]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

You'll have to add the condition at the front:

z = [(i+2, i-2)[i%2] for i in range(10)]

should do what you need.

Reinhold

 

[Matteo Dell'Amico]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

You could use

[(i-2, i+2)[bool(i%2 == 0)] for i in range(10)]

or, in a less general but shorter way

[(i+2, i-2)[i%2] for i in range(10)]

or even

[i%2 and i-2 or i+2 for i in range(10)]

The "if" clause in comprehensions is used as a filter condition.

 

[Luis M. Gonzalez]

Thank you guys!

 

[Reinhold Birkenfeld]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

You could use

[(i-2, i+2)[bool(i%2 == 0)] for i in range(10)]

or, in a less general but shorter way

[(i+2, i-2)[i%2] for i in range(10)]

or even

[i%2 and i-2 or i+2 for i in range(10)]

One should note that the (cond and X or Y) construct only works if X can
never produce a false value (such as 0, "", []). In this example, it is
okay, but replace 2 with 1 and you will run into trouble for i = 1.

Reinhold

 

[Dan Bishop]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i [sic] to be "i-2" if i%2 is not equal to 0?

z = [i + (2, -2)[i % 2] for i in range(10)]

In general, the expression "T if C is true, or F if C is false" can be
written as (F, T)[bool(C)]. (If you know that C will always be either
0 or 1, as is the case here, the "bool" is redundant.)

Unless, of course, either F or T has side effects. For a side-effect
free expression, you can use (C and [T] or [F])[0] or one of the many
other ternary operator substitutes. (Search for PEP 308.)

 

[It's me]

z = [i + (2, -2)[i % 2] for i in range(10)]

But then why would you want to use such feature? Wouldn't that make the
code much harder to understand then simply:

z=[]
for i in range(10):
if i%2:
z.append(i-2)
else:
z.append(i+2)

Or are we trying to write a book on "Puzzles in Python"?

 

[Luis M. Gonzalez]

z = [i + (2, -2)[i % 2] for i in range(10)]

But then why would you want to use such feature? Wouldn't that make the
code much harder to understand then simply:

z=[]
for i in range(10):
if i%2:
z.append(i-2)
else:
z.append(i+2)

Or are we trying to write a book on "Puzzles in Python"?

Once you get used to list comprehensions (and it doesn't take long),
they are a more concise and compact way to express these operations.
I think that writing 6 lines instead of 1 could be more readable of you
are a beginner, but after playing a little bit with listcomps for the
first time, you'll see they are very practical yet readable.

 

[Steven Bethard]

z = [i + (2, -2)[i % 2] for i in range(10)]

But then why would you want to use such feature? Wouldn't that make
the code much harder to understand then simply:

z=[]
for i in range(10):
if i%2:
z.append(i-2)
else:
z.append(i+2)

Or are we trying to write a book on "Puzzles in Python"?

Once you get used to list comprehensions (and it doesn't take long),
they are a more concise and compact way to express these operations.

After looking the two suggestions over a couple of times, I'm still
undecided as to which one is more readable for me. The problem is not
the list comprehensions (which I love and use extensively). The problem
is the odd syntax that has to be used for an if/then/else expression in
Python. I think I would have less trouble reading something like:

z = [i + (if i % 2 then -2 else 2) for i in range(10)]

but, of course, adding a if/then/else expression to Python is unlikely
to ever happen -- see the rejected PEP 308[1].

Steve

[1] http://www.python.org/peps/pep-0308.html

 

[Nick Coghlan]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i [sic] to be "i-2" if i%2 is not equal to 0?

z = [i + (2, -2)[i % 2] for i in range(10)]

For the specific case of +/- a number, (-1) ** x works, too:

z = [i + 2 * ((-1) ** i) for i in range(10)]

Not that I'm claiming it's particularly readable or anything. . . just that it
works

Cheers,
Nick.

 

[Anthony]

z = [i + (2, -2)[i % 2] for i in range(10)]

But then why would you want to use such feature? Wouldn't that make
the code much harder to understand ...
Or are we trying to write a book on "Puzzles in Python"?

Once you get used to list comprehensions (and it doesn't take long),
they are a more concise and compact way to express these operations.

After looking the two suggestions over a couple of times, I'm still
undecided as to which one is more readable for me. The problem is not
the list comprehensions (which I love and use extensively). The problem
is the odd syntax that has to be used for an if/then/else expression in
Python.

They're both pretty unreadable, IMHO. Why not just factor out the
if/then/else function like this:

..def plusMinusTwo(i):
.. if i%2 == 0:
.. return i-2
.. else:
.. return i+2
..
..z = [plusMinusTwo(i) for i in range(10)]

Then you can add whatever you like into the function.

Anthony

 

[Serhiy Storchaka1659322541]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i [sic] to be "i-2" if i%2 is not equal to 0?

z = [i + (2, -2)[i % 2] for i in range(10)]

For the specific case of +/- a number, (-1) ** x works, too:

z = [i + 2 * ((-1) ** i) for i in range(10)]

Not that I'm claiming it's particularly readable or anything. . . just
that it works

Yet another variant:
z = [i + ( (i % 2) and -2 or 2 ) for i in range(10)]

 

[Nick Coghlan]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

Hmm:

z = [newval(i) for i in range(10)] using:
def newval(x):
if x % 2:
return x - 2
else:
return x + 2

Just some more mental twiddling relating to the thread on statement local
namespaces.

Cheers,
Nick.

 

[Steve Holden]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

Hmm:

z = [newval(i) for i in range(10)] using:
def newval(x):
if x % 2:
return x - 2
else:
return x + 2

Just some more mental twiddling relating to the thread on statement
local namespaces.

I presume the point of this is to avoid polluting the local namespace
with "newval". I further presume you also have plans to do something
about "i"? ;-)

regards
Steve

 

[Stephen Thorne]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

z = [i+2-(i%2)*4 for i in range(10)]

C'mon, who needs an 'if' statement when we have maths!

Stephen.

 

[Steven Bethard]

z = [newval(i) for i in range(10)] using:
def newval(x):
if x % 2:
return x - 2
else:
return x + 2

Just some more mental twiddling relating to the thread on statement
local namespaces.

I presume the point of this is to avoid polluting the local namespace
with "newval". I further presume you also have plans to do something
about "i"? ;-)

Well, while I'm not at all a fan of the "using" syntax, getting rid of
'i' is simple:

z = list(newval(i) for i in range(10))

=)

Steve

 

[Andrey Tatarinov]

Hi there,

I'd like to know if there is a way to add and else condition into a
list comprehension. I'm sure that I read somewhere an easy way to do
it, but I forgot it and now I can't find it...

for example:
z=[i+2 for i in range(10) if i%2==0]
what if I want i to be "i-2" if i%2 is not equal to 0?

Hmm:

z = [newval(i) for i in range(10)] using:
def newval(x):
if x % 2:
return x - 2
else:
return x + 2

Just some more mental twiddling relating to the thread on statement
local namespaces.

I presume the point of this is to avoid polluting the local namespace
with "newval". I further presume you also have plans to do something
about "i"? ;-)

no, the point is in grouping definition of newval() with place where it
is used.

 

[Nick Coghlan]

no, the point is in grouping definition of newval() with place where it
is used.

I'd have said the point was both

But yeah, unfortunately the 'leaking list comp' problem won't be fixed in the
2.x series due to the compatibility problem. Fortunately, generator expressions
didn't inherit the issue.

Cheers,
Nick.