primes = [2,3,5,7..] (prime numbers)
map(lambda x:print(x),primes)
It does not print anything. Why is that? I've tried
sys.stdout.write(x)
too, but doesn't work either.
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4That's a syntax error in Python 2, so I'd assume Python 3.Greg Hewgill– Greg Hewgill2011-10-11 19:15:52 +00:00Commented Oct 11, 2011 at 19:15
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4 Answers
Since lambda x: print(x) is a syntax error in Python < 3, I'm assuming Python 3. That means map returns a generator, meaning to get map to actually call the function on every element of a list, you need to iterate through the resultant generator.
Fortunately, this can be done easily:
list(map(lambda x:print(x),primes))
Oh, and you can get rid of the lambda too, if you like:
list(map(print,primes))
But, at that point you are better off with letting print handle it:
print(*primes, sep='\n')
NOTE: I said earlier that '\n'.join would be a good idea. That is only true for a list of str's.
7 Comments
utdemir
print(x) isn't a syntax error in Python < 3. Edit: Sorry, it's syntax error in this question.
matiit
list(map(lambda x:print(x) if x>=a and isIntPalindrome(x),primes)) Ok, but why that doesn't work (syntax)
agf
If you don't want to iterate over all of
primes at once, or for some other reason you want separate print statements, the Pythonic way is a normal for loop, not list on a map which accumulates the Nones. If you need to use map for some reason, you can avoid None accumulation with next((x for x in map(print, primes) if x != None), None), but there is no reason to do that. |
This works for me:
>>> from __future__ import print_function
>>> map(lambda x: print(x), primes)
2
3
5
7
17: [None, None, None, None]
Are you using Python 2.x where print is a statement, not a function?
2 Comments
hitzg
you can actually do
map(print, primes) directly, without defining a lambda function.Alternatively, you can unpack it by putting * before map(...) like the following
[*map(...)]
or
{*map(...)}
Choose the output you desire, a list or a dictionary.
1 Comment
matiit
Thanks, I am long gone from python, but that's an elegant way imho.
Another reason why you could be seeing this is that you're not evaluating the results of the map function. It returns a generator (an iterable) that evaluates your function lazily and not an actual list.
primes = [2,3,5,7]
map(print, primes) # no output, because it returns a generator
primes = [2,3,5,7]
for i in map(print, primes):
pass # prints 2,3,5,7
Alternately, you can do list(map(print, primes)) which will also force the generator to be evaluated and call the print function on each member of your list.
answered May 25, 2019 at 23:21
Anirudh Ramanathan
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