after i print a list, it just despeared
In the following code, first create a list collection in the loadDataSet () function, and then use the map function to convert it into a set D ,after that,it can only print once and become enpty.Does anyone know what is going on here? thank you.
def loadDataSet():
return [ [ 1, 3, 4 ], [ 2, 3, 5 ], [ 1, 2, 3, 5 ], [ 2, 5 ] ]
if __name__ == '__main__':
myDat = loadDataSet()
D = map( set, myDat )
print("first print: ",list(D))
print("second print: ",list(D))
print("len of D: ",len(list(D)))
i use python 3.5.2 and the output is :
first print: [1, 3, 4, 2, 3, 5, 1, 2, 3, 5, 2, 5]
second print:
len of D: 0
python-3.x printing
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
add a comment |
In the following code, first create a list collection in the loadDataSet () function, and then use the map function to convert it into a set D ,after that,it can only print once and become enpty.Does anyone know what is going on here? thank you.
def loadDataSet():
return [ [ 1, 3, 4 ], [ 2, 3, 5 ], [ 1, 2, 3, 5 ], [ 2, 5 ] ]
if __name__ == '__main__':
myDat = loadDataSet()
D = map( set, myDat )
print("first print: ",list(D))
print("second print: ",list(D))
print("len of D: ",len(list(D)))
i use python 3.5.2 and the output is :
first print: [1, 3, 4, 2, 3, 5, 1, 2, 3, 5, 2, 5]
second print:
len of D: 0
python-3.x printing
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
add a comment |
In the following code, first create a list collection in the loadDataSet () function, and then use the map function to convert it into a set D ,after that,it can only print once and become enpty.Does anyone know what is going on here? thank you.
def loadDataSet():
return [ [ 1, 3, 4 ], [ 2, 3, 5 ], [ 1, 2, 3, 5 ], [ 2, 5 ] ]
if __name__ == '__main__':
myDat = loadDataSet()
D = map( set, myDat )
print("first print: ",list(D))
print("second print: ",list(D))
print("len of D: ",len(list(D)))
i use python 3.5.2 and the output is :
first print: [1, 3, 4, 2, 3, 5, 1, 2, 3, 5, 2, 5]
second print:
len of D: 0
python-3.x printing
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
In the following code, first create a list collection in the loadDataSet () function, and then use the map function to convert it into a set D ,after that,it can only print once and become enpty.Does anyone know what is going on here? thank you.
def loadDataSet():
return [ [ 1, 3, 4 ], [ 2, 3, 5 ], [ 1, 2, 3, 5 ], [ 2, 5 ] ]
if __name__ == '__main__':
myDat = loadDataSet()
D = map( set, myDat )
print("first print: ",list(D))
print("second print: ",list(D))
print("len of D: ",len(list(D)))
i use python 3.5.2 and the output is :
first print: [1, 3, 4, 2, 3, 5, 1, 2, 3, 5, 2, 5]
second print:
len of D: 0
python-3.x printing
python-3.x printing
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
asked Nov 15 '18 at 1:56
Hi ZengbiaoHi Zengbiao
11
11
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
Welcome to StackOverflow!
The reason is because of the behavior of map() function itself. It returns a generator, which can only be consumed once. This means, map can only execute the function to the list of given inputs once and return the resulting object. After that, the generator is exhausted and cannot be used to generate the same result. Therefore, the practice is to save the return value in a variable if you are going to use it more than once.
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
Please remember to accept the answer that suits you the most, so that StackOverflow can properly archive it.
– Andreas
Nov 15 '18 at 2:46
how to accept both two answers,I accepted one but the other was cancelled by the system.
– Hi Zengbiao
Nov 15 '18 at 2:55
I don't think you can accept more than 1 answers
– Andreas
Nov 15 '18 at 4:19
add a comment |
map creates an iterator, that can only be iterated once. The second time you call list, D is already empty since it has been iterated through already.
If you want to iterate through it multiple times, do D=list(map(set, myDat))
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
it works , thank you.
– Hi Zengbiao
Nov 15 '18 at 2:43
add a comment |
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2 Answers
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2 Answers
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active
oldest
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active
oldest
votes
Welcome to StackOverflow!
The reason is because of the behavior of map() function itself. It returns a generator, which can only be consumed once. This means, map can only execute the function to the list of given inputs once and return the resulting object. After that, the generator is exhausted and cannot be used to generate the same result. Therefore, the practice is to save the return value in a variable if you are going to use it more than once.
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
Please remember to accept the answer that suits you the most, so that StackOverflow can properly archive it.
– Andreas
Nov 15 '18 at 2:46
how to accept both two answers,I accepted one but the other was cancelled by the system.
– Hi Zengbiao
Nov 15 '18 at 2:55
I don't think you can accept more than 1 answers
– Andreas
Nov 15 '18 at 4:19
add a comment |
Welcome to StackOverflow!
The reason is because of the behavior of map() function itself. It returns a generator, which can only be consumed once. This means, map can only execute the function to the list of given inputs once and return the resulting object. After that, the generator is exhausted and cannot be used to generate the same result. Therefore, the practice is to save the return value in a variable if you are going to use it more than once.
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
Please remember to accept the answer that suits you the most, so that StackOverflow can properly archive it.
– Andreas
Nov 15 '18 at 2:46
how to accept both two answers,I accepted one but the other was cancelled by the system.
– Hi Zengbiao
Nov 15 '18 at 2:55
I don't think you can accept more than 1 answers
– Andreas
Nov 15 '18 at 4:19
add a comment |
Welcome to StackOverflow!
The reason is because of the behavior of map() function itself. It returns a generator, which can only be consumed once. This means, map can only execute the function to the list of given inputs once and return the resulting object. After that, the generator is exhausted and cannot be used to generate the same result. Therefore, the practice is to save the return value in a variable if you are going to use it more than once.
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
Welcome to StackOverflow!
The reason is because of the behavior of map() function itself. It returns a generator, which can only be consumed once. This means, map can only execute the function to the list of given inputs once and return the resulting object. After that, the generator is exhausted and cannot be used to generate the same result. Therefore, the practice is to save the return value in a variable if you are going to use it more than once.
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
answered Nov 15 '18 at 2:08
AndreasAndreas
2,05431423
2,05431423
Please remember to accept the answer that suits you the most, so that StackOverflow can properly archive it.
– Andreas
Nov 15 '18 at 2:46
how to accept both two answers,I accepted one but the other was cancelled by the system.
– Hi Zengbiao
Nov 15 '18 at 2:55
I don't think you can accept more than 1 answers
– Andreas
Nov 15 '18 at 4:19
add a comment |
Please remember to accept the answer that suits you the most, so that StackOverflow can properly archive it.
– Andreas
Nov 15 '18 at 2:46
how to accept both two answers,I accepted one but the other was cancelled by the system.
– Hi Zengbiao
Nov 15 '18 at 2:55
I don't think you can accept more than 1 answers
– Andreas
Nov 15 '18 at 4:19
Please remember to accept the answer that suits you the most, so that StackOverflow can properly archive it.
– Andreas
Nov 15 '18 at 2:46
Please remember to accept the answer that suits you the most, so that StackOverflow can properly archive it.
– Andreas
Nov 15 '18 at 2:46
how to accept both two answers,I accepted one but the other was cancelled by the system.
– Hi Zengbiao
Nov 15 '18 at 2:55
how to accept both two answers,I accepted one but the other was cancelled by the system.
– Hi Zengbiao
Nov 15 '18 at 2:55
I don't think you can accept more than 1 answers
– Andreas
Nov 15 '18 at 4:19
I don't think you can accept more than 1 answers
– Andreas
Nov 15 '18 at 4:19
add a comment |
map creates an iterator, that can only be iterated once. The second time you call list, D is already empty since it has been iterated through already.
If you want to iterate through it multiple times, do D=list(map(set, myDat))
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
it works , thank you.
– Hi Zengbiao
Nov 15 '18 at 2:43
add a comment |
map creates an iterator, that can only be iterated once. The second time you call list, D is already empty since it has been iterated through already.
If you want to iterate through it multiple times, do D=list(map(set, myDat))
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
it works , thank you.
– Hi Zengbiao
Nov 15 '18 at 2:43
add a comment |
map creates an iterator, that can only be iterated once. The second time you call list, D is already empty since it has been iterated through already.
If you want to iterate through it multiple times, do D=list(map(set, myDat))
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
map creates an iterator, that can only be iterated once. The second time you call list, D is already empty since it has been iterated through already.
If you want to iterate through it multiple times, do D=list(map(set, myDat))
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
answered Nov 15 '18 at 2:07
SuperStormerSuperStormer
272614
272614
it works , thank you.
– Hi Zengbiao
Nov 15 '18 at 2:43
add a comment |
it works , thank you.
– Hi Zengbiao
Nov 15 '18 at 2:43
it works , thank you.
– Hi Zengbiao
Nov 15 '18 at 2:43
it works , thank you.
– Hi Zengbiao
Nov 15 '18 at 2:43
add a comment |
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