blocks|key|669707|text|您可以使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|669708|>>>+lst+=+[None]+*+5
>>>+lst
[None,+None,+None,+None,+None]|code-block|syntax|javascript|669709|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can use:

<pre><code>&gt;&gt;&gt; lst = [None] * 5
&gt;&gt;&gt; lst
[None, None, None, None, None]
</code></pre>

blocks|key|669845|text|为什么这些问题不能用显而易见的答案来回答呢？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|669846|a+=+numpy.empty(n,+dtype=object)|code-block|syntax|javascript|669847|这将创建一个长度为n的数组，该数组可以存储对象。它不能调整大小或追加。特别是，它不会通过填充长度来浪费空间。这是与Java的Python等价物|669848|Object[]+a+=+new+Object[n];|669849|如果您确实对性能和空间感兴趣，并且知道您的数组将只存储某些数值类型，那么您可以将dtype参数更改为其他值，如int。然后numpy会将这些元素直接打包到数组中，而不是让数组引用int对象。|669850|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Why don't these questions get answered with the obvious answer?

<pre><code>a = numpy.empty(n, dtype=object)
</code></pre>

This creates an array of length n that can store objects. It can't be resized or appended to. In particular, it doesn't waste space by padding its length. This is the Python equivalent of Java's

<pre><code>Object[] a = new Object[n];
</code></pre>

If you're really interested in performance and space and know that your array will only store certain numeric types then you can change the dtype argument to some other value like int. Then numpy will pack these elements directly into the array rather than making the array reference int objects.

blocks|key|670175|text|执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|670176|>>>+d+=+[+[+None+for+y+in+range(+2+)+]+for+x+in+range(+2+)+]
>>>+d
[[None,+None],+[None,+None]]
>>>+d[0][0]+=+1
>>>+d
[[1,+None],+[None,+None]]|code-block|syntax|javascript|670177|其他解决方案将导致此类问题：|670178|>>>+d+=+[+[+None+]+*+2+]+*+2
>>>+d
[[None,+None],+[None,+None]]
>>>+d[0][0]+=+1
>>>+d
[[1,+None],+[1,+None]]|670179|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Do this:

<pre><code>&gt;&gt;&gt; d = [ [ None for y in range( 2 ) ] for x in range( 2 ) ]
&gt;&gt;&gt; d
[[None, None], [None, None]]
&gt;&gt;&gt; d[0][0] = 1
&gt;&gt;&gt; d
[[1, None], [None, None]]
</code></pre>

The other solutions will lead to this kind of problem:

<pre><code>&gt;&gt;&gt; d = [ [ None ] * 2 ] * 2
&gt;&gt;&gt; d
[[None, None], [None, None]]
&gt;&gt;&gt; d[0][0] = 1
&gt;&gt;&gt; d
[[1, None], [1, None]]
</code></pre>

blocks|key|669733|text|最好的选择是使用numpy库。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|669734|from+numpy+import+ndarray

a+=+ndarray((5,),int)|code-block|syntax|javascript|669735|entityMap|0|LINK|mutability|MUTABLE|url|http://numpy.scipy.org/^0|8|5|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

The best bet is to use the <a href="http://numpy.scipy.org/" rel="noreferrer">numpy</a> library.

<pre><code>from numpy import ndarray

a = ndarray((5,),int)
</code></pre>

blocks|key|670052|text|>>>+import+numpy
>>>+x+=+numpy.zeros((3,4))
>>>+x
array([[+0.,++0.,++0.,++0.],
+++++++[+0.,++0.,++0.,++0.],
+++++++[+0.,++0.,++0.,++0.]])
>>>+y+=+numpy.zeros(5)+++
>>>+y
array([+0.,++0.,++0.,++0.,++0.])|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|670053|X是2维数组，y是1维数组。它们都被初始化为零。|unstyled|670054|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>&gt;&gt;&gt; import numpy
&gt;&gt;&gt; x = numpy.zeros((3,4))
&gt;&gt;&gt; x
array([[ 0., 0., 0., 0.],
 [ 0., 0., 0., 0.],
 [ 0., 0., 0., 0.]])
&gt;&gt;&gt; y = numpy.zeros(5) 
&gt;&gt;&gt; y
array([ 0., 0., 0., 0., 0.])
</code></pre>

x is a 2-d array, and y is a 1-d array. They are both initialized with zeros.

blocks|key|670010|text|>>>+n+=+5+++++++++++++++++++++#length+of+list
>>>+list+=+[None]+*+n+++++++++#populate+list,+length+n+with+n+entries+"None"
>>>+print(list)
[None,+None,+None,+None,+None]

>>>+list.append(1)++++++++++++#append+1+to+right+side+of+list
>>>+list+=+list[-n:]++++++++++#redefine+list+as+the+last+n+elements+of+list
>>>+print(list)
[None,+None,+None,+None,+1]

>>>+list.append(1)++++++++++++#append+1+to+right+side+of+list
>>>+list+=+list[-n:]++++++++++#redefine+list+as+the+last+n+elements+of+list
>>>+print(list)
[None,+None,+None,+1,+1]

>>>+list.append(1)++++++++++++#append+1+to+right+side+of+list
>>>+list+=+list[-n:]++++++++++#redefine+list+as+the+last+n+elements+of+list
>>>+print(list)
[None,+None,+1,+1,+1]|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|670011|或者在列表中实际上什么都没有：|unstyled|670012|>>>+n+=+5+++++++++++++++++++++#length+of+list
>>>+list+=+[]+++++++++++++++++#+create+list
>>>+print(list)
[]

>>>+list.append(1)++++++++++++#append+1+to+right+side+of+list
>>>+list+=+list[-n:]++++++++++#redefine+list+as+the+last+n+elements+of+list
>>>+print(list)
[1]|670013|在append的第4次迭代中：|670014|>>>+list.append(1)++++++++++++#append+1+to+right+side+of+list
>>>+list+=+list[-n:]++++++++++#redefine+list+as+the+last+n+elements+of+list
>>>+print(list)
[1,1,1,1]|670015|5和所有后续的：|670016|>>>+list.append(1)++++++++++++#append+1+to+right+side+of+list
>>>+list+=+list[-n:]++++++++++#redefine+list+as+the+last+n+elements+of+list
>>>+print(list)
[1,1,1,1,1]|670017|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|T|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$B|C]]|$1|M|3|N|5|F|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$B|C]]|$1|Q|3|-4|5|F|7|Z|8|@]|9|@]|A|$]]]|R|$]]

<pre><code>&gt;&gt;&gt; n = 5 #length of list
&gt;&gt;&gt; list = [None] * n #populate list, length n with n entries "None"
&gt;&gt;&gt; print(list)
[None, None, None, None, None]

&gt;&gt;&gt; list.append(1) #append 1 to right side of list
&gt;&gt;&gt; list = list[-n:] #redefine list as the last n elements of list
&gt;&gt;&gt; print(list)
[None, None, None, None, 1]

&gt;&gt;&gt; list.append(1) #append 1 to right side of list
&gt;&gt;&gt; list = list[-n:] #redefine list as the last n elements of list
&gt;&gt;&gt; print(list)
[None, None, None, 1, 1]

&gt;&gt;&gt; list.append(1) #append 1 to right side of list
&gt;&gt;&gt; list = list[-n:] #redefine list as the last n elements of list
&gt;&gt;&gt; print(list)
[None, None, 1, 1, 1]
</code></pre>

or with really nothing in the list to begin with:

<pre><code>&gt;&gt;&gt; n = 5 #length of list
&gt;&gt;&gt; list = [] # create list
&gt;&gt;&gt; print(list)
[]

&gt;&gt;&gt; list.append(1) #append 1 to right side of list
&gt;&gt;&gt; list = list[-n:] #redefine list as the last n elements of list
&gt;&gt;&gt; print(list)
[1]
</code></pre>

on the 4th iteration of append:

<pre><code>&gt;&gt;&gt; list.append(1) #append 1 to right side of list
&gt;&gt;&gt; list = list[-n:] #redefine list as the last n elements of list
&gt;&gt;&gt; print(list)
[1,1,1,1]
</code></pre>

5 and all subsequent:

<pre><code>&gt;&gt;&gt; list.append(1) #append 1 to right side of list
&gt;&gt;&gt; list = list[-n:] #redefine list as the last n elements of list
&gt;&gt;&gt; print(list)
[1,1,1,1,1]
</code></pre>

blocks|key|887063|text|您可以尝试使用Descriptor来限制大小|type|unstyled|depth|inlineStyleRanges|entityRanges|data|887064|class+fixedSizeArray(object):
++++def+__init__(self,+arraySize=5):
++++++++self.arraySize+=+arraySize
++++++++self.array+=+[None]+*+self.arraySize

++++def+__repr__(self):
++++++++return+str(self.array)

++++def+__get__(self,+instance,+owner):
++++++++return+self.array

++++def+append(self,+index=None,+value=None):
++++++++print+"Append+Operation+cannot+be+performed+on+fixed+size+array"
++++++++return

++++def+insert(self,+index=None,+value=None):
++++++++if+not+index+and+index+-+1+not+in+xrange(self.arraySize):
++++++++++++print+'invalid+Index+or+Array+Size+Exceeded'
++++++++++++return
++++++++try:
++++++++++++self.array[index]+=+value
++++++++except:
++++++++++++print+'This+is+Fixed+Size+Array:+Please+Use+the+available+Indices'


arr+=+fixedSizeArray(5)
print+arr
arr.append(100)
print+arr
arr.insert(1,+200)
print+arr
arr.insert(5,+300)
print+arr|code-block|syntax|javascript|887065|输出：|887066|[None,+None,+None,+None,+None]
Append+Operation+cannot+be+performed+on+fixed+size+array
[None,+None,+None,+None,+None]
[None,+200,+None,+None,+None]
This+is+Fixed+Size+Array:+Please+Use+the+available+Indices
[None,+200,+None,+None,+None]|887067|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

You can try using Descriptor, to limit the size

<pre><code>class fixedSizeArray(object):
 def __init__(self, arraySize=5):
 self.arraySize = arraySize
 self.array = [None] * self.arraySize

 def __repr__(self):
 return str(self.array)

 def __get__(self, instance, owner):
 return self.array

 def append(self, index=None, value=None):
 print "Append Operation cannot be performed on fixed size array"
 return

 def insert(self, index=None, value=None):
 if not index and index - 1 not in xrange(self.arraySize):
 print 'invalid Index or Array Size Exceeded'
 return
 try:
 self.array[index] = value
 except:
 print 'This is Fixed Size Array: Please Use the available Indices'


arr = fixedSizeArray(5)
print arr
arr.append(100)
print arr
arr.insert(1, 200)
print arr
arr.insert(5, 300)
print arr
</code></pre>

OUTPUT:

<pre><code>[None, None, None, None, None]
Append Operation cannot be performed on fixed size array
[None, None, None, None, None]
[None, 200, None, None, None]
This is Fixed Size Array: Please Use the available Indices
[None, 200, None, None, None]
</code></pre>

blocks|key|669964|text|我发现容易做的一件事是设置一个空字符串数组来指定我喜欢的大小，例如|type|unstyled|depth|inlineStyleRanges|entityRanges|data|669965|代码：|669966|import+numpy+as+np

x=+np.zeros(5,str)
print+x|code-block|syntax|javascript|669967|输出：|669968|[''+''+''+''+'']|669969|希望这能对您有所帮助:)|669970|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

One thing I find easy to do is i set an array
of empty strings for the size I prefer, for example

Code:

<pre><code>import numpy as np

x= np.zeros(5,str)
print x
</code></pre>

Output:

<pre><code>['' '' '' '' '']
</code></pre>

Hope this is helpful :)

blocks|key|886976|text|好吧，我想通过发布一个示例程序及其输出来帮助您|type|unstyled|depth|inlineStyleRanges|entityRanges|data|886977|程序：|886978|t+=+input("")
x+=+[None]*t
y+=+[[None]*t]*t

for+i+in+range(1,+t%2B1):
++++x[i-1]+=+i;

++++for+j+in+range(1,+t%2B1):
++++++++y[i-1][j-1]+=+j;

print+x
print+y|code-block|syntax|javascript|886979|输出:-|886980|2
[1,+2]
[[1,+2],+[1,+2]]|886981|我希望这澄清了你关于他们的声明的一些非常基本的概念。用其他一些特定值初始化它们，比如用0初始化它们。您可以将它们声明为：|offset|length|style|CODE|886982|x+=+[0]*10|886983|希望它能有所帮助..！！;)|886984|entityMap^0|0|0|0|0|0|17|1|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|13|8|@$O|14|P|15|Q|R]]|9|@]|A|$]]|$1|S|3|T|5|F|7|16|8|@]|9|@]|A|$G|H]]|$1|U|3|V|5|6|7|17|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|X|$]]

Well I would like to help you by posting a sample program and its output

Program:

<pre><code>t = input("")
x = [None]*t
y = [[None]*t]*t

for i in range(1, t+1):
 x[i-1] = i;

 for j in range(1, t+1):
 y[i-1][j-1] = j;

print x
print y
</code></pre>

Output :-

<pre><code>2
[1, 2]
[[1, 2], [1, 2]]
</code></pre>

I hope this clears some very basic concept of yours regarding their declaration.
To initialize them with some other specific values, like initializing them with <code>0</code>.. you can declare them as:

<pre><code>x = [0]*10
</code></pre>

Hope it helps..!! ;)

blocks|key|670105|text|如果你正在处理字节，你可以使用内置的bytearray。如果您正在使用其他整数类型，请查看内置的array。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|670106|特别要理解的是，list不是array。|670107|例如，如果您正在尝试创建一个缓冲区来读取文件内容，您可以使用bytearray，如下所示(有更好的方法可以做到这一点，但这个示例是有效的)：|670108|with+open(FILENAME,+'rb')+as+f:
++++data+=+bytearray(os.path.getsize(FILENAME))
++++f.readinto(data)|code-block|syntax|javascript|670109|在这段代码中，为bytearray内存预先分配了固定长度的FILENAME大小，单位为字节。这种预分配允许使用缓冲区协议来更有效地将文件读取到可变缓冲区中，而无需数组复制。还有更好的方法可以做到这一点，但我相信这为你的问题提供了一个答案。|670110|entityMap^0|I|9|1C|5|0|8|4|E|5|0|0|0|8|9|T|8|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|12|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|13|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|R|$]]

If you are working with bytes you could use the builtin <code>bytearray</code>. If you are working with other integral types look at the builtin <code>array</code>.

Specifically understand that a <code>list</code> is not an <code>array</code>.

If, for example, you are trying to create a buffer for reading file contents into you could use bytearray as follows (there are better ways to do this but the example is valid):

<pre><code>with open(FILENAME, 'rb') as f:
 data = bytearray(os.path.getsize(FILENAME))
 f.readinto(data)
</code></pre>

In this snippet the <code>bytearray</code> memory is preallocated with the fixed length of <code>FILENAME</code>s size in bytes. This preallocation allows the use of the buffer protocol to more efficiently read the file into a mutable buffer without an array copy. There are yet better ways to do this but I believe this provides one answer to your question.

I would like to know how i can initialize an array(or list), yet to be populated with values, to have a defined size.
For example in C:
<pre class="lang-c prettyprint-override"><code>int x[5]; /* declared without adding elements*/
</code></pre>
How do I do that in Python?

Initialising an array of fixed size in Python

Python

Java

我想知道如何初始化一个数组(或列表)，使其具有定义的大小。例如，在C中：int x[5]; /* declared without adding elements*/我如何在Python中做到这一点？

问在Python中初始化固定大小的数组
EN

回答 10

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