问题
I have a list L of elements, say natural numbers. I want to print them in one line with a single space as a separator. But I don't want a space after the last element of the list (or before the first).
In Python 2, this can easily be done with the following code. The implementation of the print statement (mysteriously, I must confess) avoids to print an extra space before the newline.
L = [1, 2, 3, 4, 5]
for x in L:
print x,
print
However, in Python 3 it seems that the (supposedly) equivalent code using the print function produces a space after the last number:
L = [1, 2, 3, 4, 5]
for x in L:
print(x, end=" ")
print()
Of course there are easy answers to my question. I know I can use string concatenation:
L = [1, 2, 3, 4, 5]
print(" ".join(str(x) for x in L))
This is a quite good solution, but compared to the Python 2 code I find it counter-intuitive and definitely slower. Also, I know I can choose whether to print a space or not myself, like:
L = [1, 2, 3, 4, 5]
for i, x in enumerate(L):
print(" " if i>0 else "", x, sep="", end="")
print()
but again this is worse than what I had in Python 2.
So, my question is, am I missing something in Python 3? Is the behavior I'm looking for supported by the print function?
回答1:
You can apply the list as separate arguments:
print(*L)
and let print() take care of converting each element to a string. You can, as always, control the separator by setting the sep keyword argument:
>>> L = [1, 2, 3, 4, 5]
>>> print(*L)
1 2 3 4 5
>>> print(*L, sep=', ')
1, 2, 3, 4, 5
>>> print(*L, sep=' -> ')
1 -> 2 -> 3 -> 4 -> 5
Unless you need the joined string for something else, this is the easiest method. Otherwise, use str.join():
joined_string = ' '.join([str(v) for v in L])
print(joined_string)
# do other things with joined_string
Note that this requires manual conversion to strings for any non-string values in L!
回答2:
Although the accepted answer is absolutely clear, I just wanted to check efficiency in terms of time.
The best way is to print joined string of numbers converted to strings.
print(" ".join(list(map(str,l))))
Note that I used map instead of loop. I wrote a little code of all 4 different ways to compare time:
import time as t
a, b = 10, 210000
l = list(range(10, 21000))
tic = t.time()
for i in l:
print(i, end=" ")
print()
tac = t.time()
t1 = (tac - tic) * 1000
print(*l)
toe = t.time()
t2 = (toe - tac) * 1000
print(" ".join([str(i) for i in l]))
joe = t.time()
t3 = (joe - toe) * 1000
print(" ".join(list(map(str, l))))
toy = t.time()
t4 = (toy - joe) * 1000
print("Time,t1,t2,t3,t4")
Result:
Time 74344.76 71790.83 196.99 153.99
The output was quite surprising to me. Huge difference of time in cases of 'loop method' and 'joined-string method'.
Conclusion: Do not use loops for printing list if size is too large( in order of 10**5 or more).
回答3:
list = [1, 2, 3, 4, 5]
for i in list[0:-1]:
print(i, end=', ')
print(list[-1])
do for loops really take that much longer to run?
was trying to make something that printed all str values in a list separated by commas, inserting "and" before the last entry and came up with this:
spam = ['apples', 'bananas', 'tofu', 'cats']
for i in spam[0:-1]:
print(i, end=', ')
print('and ' + spam[-1])
来源：https://stackoverflow.com/questions/22556449/print-a-list-of-space-separated-elements-in-python-3