具有固定数量的元素的python列表 [英] python list with fixed number of elements

问题描述

我想在Python中创建一个大小为6的元组的固定大小列表.请注意，在下面的代码中，我总是在重新初始化外部for循环中的值，以重置先前创建的列表，该列表已添加到globalList中.这是一个代码段:

I want to create a fixed size list of size 6 of tuples in Python. Note that in the code below i am always re-initializing the values in the outer for loop in order to reset the previously created list which was already added to globalList. Here is a snippet:

for i,j in dictCaseId.iteritems():
    listItems=[None]*6
    for x,y in j:
        if x=='cond':
            tuppo = y
            listItems.insert(0,tuppo)
        if x=='act':
            tuppo = y
            listItems.insert(1,tuppo)
        if x=='correc':
            tuppo = y
            listItems.insert(2,tuppo)
            ...
            ...
    globalList.append(listItems)

但是当我尝试运行上面的代码(上面仅显示了代码段)时，它会增加列表的大小.我的意思是，添加了东西，但我还看到列表包含更多数量的元素.我不希望我的列表大小增加，并且我的列表是6个元组的列表.

But when I try to run the above (snippet only shown above) it increases the list size. I mean, stuff gets added but I also see the list contains more number of elements. I dont want my list size to increase and my list is a list of 6 tuples.

例如:

Initially: [None,None,None,None,None,None]
What I desire: [Mark,None,Jon,None,None,None]
What I get: [Mark,None,Jon,None,None,None,None,None]

推荐答案

而不是插入，您应该分配这些值. list.insert在传递给它的索引处插入一个新元素，因此列表长度在每次插入操作后增加1. 另一方面，赋值会修改特定索引处的值，因此长度保持不变.

Instead of inserting you should assign those values. list.insert inserts a new element at the index passed to it, so length of list increases by 1 after each insert operation. On the other hand assignment modifies the value at a particular index, so length remains constant.

for i,j in dictCaseId.iteritems():
    listItems=[None]*6
    for x,y in j:
        if x=='cond':
            tuppo = y
            listItems[0]=tuppo
        if x=='act':
            tuppo = y
            listItems[1]=tuppo
        if x=='correc':
            tuppo = y
            listItems[2]=tuppo

示例:

>>> lis = [None]*6
>>> lis[1] = "foo"   #change the 2nd element
>>> lis[4] = "bar"   #change the fifth element
>>> lis
[None, 'foo', None, None, 'bar', None]

更新:

>>> lis = [[] for _ in xrange(6)] # don't use  [[]]*6
>>> lis[1].append("foo")
>>> lis[4].append("bar")
>>> lis[1].append("py")
>>> lis
[[], ['foo', 'py'], [], [], ['bar'], []]

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