在python中填写列表

Question

I am trying to fill up a list of strings called lines with values from a dictionary called A in python.A is filled with integers.The pseudocode should be something like this.

 for i in xrange(0,256):
    for j in xrange(0,768):
        lines[j+4+(512*3)*i]= str(A[i,j])

As you can see I cannot insert the elements serially in lines.Is this possible? If it is then how should I do this? The list lines would be pretty large of size 786435. It depends on both i an jI would be greatly obliged if anyone can help me.BTW I am using python 2.7 .

Answer 1

Lists are contiguous sequences of elements. You could create the list up front, storing a reference to None at each index:

lines = [None] * (255 + 4 + (512 * 3) * 767)

but then you'd be creating an object with 1,178,371 (1 million plus) elements in it. That'll take a fair amount of memory just for the list object :

>>> import sys
>>> sys.getsizeof([None] * 1178371)
9427040

That's 9MB for the list object alone, where each reference points to the same None object (it is a singleton, there is only ever one in memory). Add strings, and memory requirements rapidly climb; that many empty strings requires 40+ MB:

>>> sys.getsizeof('') * 1178371
43599727

In Python 2, every additional character will add a byte, so add average string length times 1MB to the requirements for just this list . If your strings are only 3 characters, that's still doable; that'll be about 50MB in total, but if you are talking about 5000 character strings you easily end up in the gigabytes. 5000 characters isn't all that much.

Your indices are not contiguous, however. You only are producing 255 times 767 is 195585 indices; you appear to have a sparse data structure. You'd be better off using a dictionary in that case:

lines = {}
for i in xrange(0,256):
    for j in xrange(0,768):
        lines[j + 4 + (512 * 3) * i]= str(A[i, j])

This works as written, because now each index is a key in a dictionary instead.

If you do have to produce a continuous sequence, consider different techniques. If all your indices, from 0 through to the very last, can be computed like the above, you could produce each value by calculating ; given an index, calculate what string would be placed at that index. That way you don't have to produce all values up front.

The above loop indices follow a specific pattern, so you can simply derive the i and j from a given index:

def str_for_index(index):
    i = index // (512 * 3)
    j = (index % (512 * 3)) - 4
    if 0 <= i < 256 and 0 <= j < 768:
        return str(A[i, j])