What is a list compression in Python?

[Kit]

Hello Everyone, I am not sure if I have posted this question in a
correct board. Can anyone please teach me:

What is a list compression in Python?

Would you mind give me some list compression examples?


Thanks & really appreciate that.
Kit

 

[Simon Brunning]

Hello Everyone, I am not sure if I have posted this question in a
correct board. Can anyone please teach me:

What is a list compression in Python?

Perhaps you mean a list comprehension? If so, see
<http://diveintopython.org/native_data_types/mapping_lists.html>.

 

[Steven D'Aprano]

Hello Everyone, I am not sure if I have posted this question in a
correct board. Can anyone please teach me:

What is a list compression in Python?

Google "python list comprehension".

If Google is broken for you, try Yahoo, or any other search engine.

Would you mind give me some list compression examples?

Instead of this:

L = []
for x in range(10):
L.append(x**2)


you can write:

L = [x**2 for x in range(10)]


Instead of this example:


L = []
for x in range(10):
if x % 2 == 0:
L.append(x**2)



you can write:

L = [x**2 for x in range(10) if x % 2 == 0]

 

[Kit]

Thank you so much guys.

Just out of curiosity: can I do something like this to "square all
even numbers in the range 1-10"?
print [x^2 for x in range (1,11) if x % 2 == 0]

Or is there a better way of doing it? Thanks for the help, and I am
really appreciate your help.

Kit.

Hello Everyone, I am not sure if I have posted this question in a
correct board. Can anyone please teach me:

What is a list compression in Python?

Google "python list comprehension".

If Google is broken for you, try Yahoo, or any other search engine.

Would you mind give me some list compression examples?

Instead of this:

L = []
for x in range(10):
    L.append(x**2)

you can write:

L = [x**2 for x in range(10)]

Instead of this example:

L = []
for x in range(10):
    if x % 2 == 0:
        L.append(x**2)

you can write:

L = [x**2 for x in range(10) if x % 2 == 0]

 

[Kit]

Oops...

print [x^2 for x in range (1,11) if x % 2 == 0]

print [x^2 for x in range (1,10) if x % 2 == 0]

Thank you so much guys.

Just out of curiosity: can I do something like this to "square all
even numbers in the range 1-10"?
print [x^2 for x in range (1,11) if x % 2 == 0]

Or is there a better way of doing it? Thanks for the help, and I am
really appreciate your help.

Kit.

Google "python list comprehension".

If Google is broken for you, try Yahoo, or any other search engine.

Instead of this:

L = []
for x in range(10):
    L.append(x**2)

you can write:

L = [x**2 for x in range(10)]

Instead of this example:

L = []
for x in range(10):
    if x % 2 == 0:
        L.append(x**2)

you can write:

L = [x**2 for x in range(10) if x % 2 == 0]

 

[MRAB]

Thank you so much guys.

Just out of curiosity: can I do something like this to "square all
even numbers in the range 1-10"?
print [x^2 for x in range (1,11) if x % 2 == 0]

Or is there a better way of doing it? Thanks for the help, and I am
really appreciate your help.

That would be:

print [x ** 2 for x in range(1, 11) if x % 2 == 0]

Note that Python uses "**" for raising to a power and "^" (like in C)
for bitwise exclusive-or.

 

[Gary Herron]

Thank you so much guys.

Just out of curiosity: can I do something like this to "square all
even numbers in the range 1-10"?
print [x^2 for x in range (1,11) if x % 2 == 0]

Or is there a better way of doing it? Thanks for the help, and I am
really appreciate your help.

That's a fine way to do it, although:

xrange is slightly more efficient than range for large
sets of values. (This isn't such a large set for it to matter.)

You can get range and xrange to count by two's, eliminating
the need for the "if" portion in this case.

The exponent operator is ** not ^, and x*x is more efficient than x**2.

So.

print [x**2 for x in xrange(1,11,2)]

Gary Herron

Kit.

Hello Everyone, I am not sure if I have posted this question in a
correct board. Can anyone please teach me:

What is a list compression in Python?

Google "python list comprehension".

If Google is broken for you, try Yahoo, or any other search engine.

Would you mind give me some list compression examples?

Instead of this:

L = []
for x in range(10):
L.append(x**2)

you can write:

L = [x**2 for x in range(10)]

Instead of this example:

L = []
for x in range(10):
if x % 2 == 0:
L.append(x**2)

you can write:

L = [x**2 for x in range(10) if x % 2 == 0]

 

[Kit]

Cool! Thank you very much. You mean this instead right?

print [x*x for x in xrange(1,11,2)]

Kit

Thank you so much guys.

Just out of curiosity: can I do something like this to "square all
even numbers in the range 1-10"?
print [x^2 for x in range (1,11) if x % 2 == 0]

Or is there a better way of doing it? Thanks for the help, and I am
really appreciate your help.

That's a fine way to do it, although:

  xrange is slightly more efficient than range for large
  sets of values.  (This isn't such a large set for it to matter.)

  You can get range and xrange to count by two's, eliminating
  the need for the "if" portion in this case.

  The exponent operator is ** not ^, and  x*x is more efficient than x**2.

So.

print [x**2 for x in xrange(1,11,2)]

Gary Herron

Kit.

On 1月19æ—¥, 上åˆ12時30分, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.au> wrote:

On Mon, 18 Jan 2010 08:07:41 -0800, Kit wrote:
Hello Everyone, I am not sure if I have posted this question in a
correct board. Can anyone please teach me:
What is a list compression in Python?
Google "python list comprehension".
If Google is broken for you, try Yahoo, or any other search engine.
Would you mind give me some list compression examples?
Instead of this:
L = []
for x in range(10):
    L.append(x**2)
you can write:
L = [x**2 for x in range(10)]
Instead of this example:
L = []
for x in range(10):
    if x % 2 == 0:
        L.append(x**2)
you can write:
L = [x**2 for x in range(10) if x % 2 == 0]

 

[MRAB]

Oops...

print [x^2 for x in range (1,11) if x % 2 == 0]

print [x^2 for x in range (1,10) if x % 2 == 0]

The start is inclusive and the end is exclusive. This is a general rule
in Python. In fact, there's only one exception that I know of:
randint(a, b) in the 'random' module.

 

[Gib Bogle]

Thank you so much guys.

Just out of curiosity: can I do something like this to "square all
even numbers in the range 1-10"?
print [x^2 for x in range (1,11) if x % 2 == 0]

Why not try it?

 

[Steven D'Aprano]

Thank you so much guys.

Just out of curiosity: can I do something like this to "square all even
numbers in the range 1-10"?
print [x^2 for x in range (1,11) if x % 2 == 0]

^ is the XOR operator in Python. You want:


[x**2 for x in range (1,11) if x % 2 == 0]

or even better:

[x**2 for x in range (2, 11, 2)]

 

[Rainer Grimm]

Hallo,
you can also look at list comprehension as syntactic sugar for the
functions map and filter. The two functions from the functional world
can be expressed in a comprehensive way with list comprehension.

[x**2 for x in range(10) ] == map ( lambda x: x*x, range(10)) True
[ x for x in range(10) if x%2 == 0 ] == filter ( lambda x: x%2 == 0 , range(10))

True

Greetings

 

[Luis M. González]

Hello Everyone, I am not sure if I have posted this question in a
correct board. Can anyone please teach me:

What is a list compression in Python?

Would you mind give me some list compression examples?

Thanks & really appreciate that.
Kit

It's also worth noting that from Python 3.0 on, you have also dict and
set comprehensions.