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All,

I've recently picked up Python and currently in the process of dealing with lists. I'm using a test file containing several lines of characters indented by a tab and then passing this into my python program. The aim of my python script is to insert each line into a list using the length as the index which means that the list would be automatically sorted. I am considering the most basic case and am not concerned about any complex cases.

My python code below;

newList = []

for line in sys.stdin:
    data = line.strip().split('\t')
    size = len(data)
    newList.insert(size, data)
for i in range(len(newList)):
    print ( newList[i])

My 'test' file below;

2   2   2   2
1
3   2
2   3   3   3   3
3   3   3

My expectation of the output of the python script is to print the contents of the list in the following order sorted by length;

['1']
['3', '2']
['3', '3', '3']
['2', '2', '2', '2']
['2', '3', '3', '3', '3']

However, when I pass in my test file to my python script, I get the following;

cat test | ./listSort.py 
['2', '2', '2', '2']
['1']
['3', '2']
['3', '3', '3']
['2', '3', '3', '3', '3']

The first line of the output ['2', '2', '2', '2'] is incorrect. I'm trying to figure out why it isn't being printed at the 4th line (because of length 4 which would mean that it would have been inserted into the 4th index of the list). Could someone please provide some insight into why this is? My understanding is that I am inserting each 'data' into the list using 'size' as the index which means when I print out the contents of the list, they would be printed in sorted order.

Thanks in advance!

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4
  • 3
    Try to replay your algorithm using pen & paper and you will see why the result is wrong. Commented Oct 12, 2017 at 10:52
  • 1
    Also note the "useless use of cat": cat filename | program is the same as program <filename. Commented Oct 12, 2017 at 10:54
  • 2
    Or if your understanding of lists is wrong, then using pen & paper might not help you... The thing is that if you have a list of length N then inserting something at index n > N will just append it to the end of the list. E.g. inserting x into an empty list (N = 0) at index "4" still just results in a list [x], not something like [–, –, –, –, x]. Commented Oct 12, 2017 at 11:02
  • Thank you very much, that explained it better. My understanding of lists was incorrect. Commented Oct 12, 2017 at 11:10

2 Answers 2

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4

Inserting into lists work quite differently than what you think:

>>> newList = []
>>> newList.insert(4, 4)
>>> newList
[4]
>>> newList.insert(1, 1)
>>> newList
[4, 1]
>>> newList.insert(2, 2)
>>> newList
[4, 1, 2]
>>> newList.insert(5, 5)
>>> newList
[4, 1, 2, 5]
>>> newList.insert(3, 3)
>>> newList
[4, 1, 2, 3, 5]
>>> newList.insert(0, 0)
>>> newList
[0, 4, 1, 2, 3, 5]

Hopefully you can see two things from this example:

  • The list indices are 0-based. That is to say, the first entry has index 0, the second has index 1, etc.
  • list.insert(idx, val) inserts things into the position which currently has index idx, and bumps everything after that down a position. If idx is larger than the current length of the list, the new item is silently added in the last position.

There are several ways to implement the functionality you want:

  1. If you can predict the number of lines, you can allocate the list beforehand, and simply assign to the elements of the list instead of inserting:

    newList = [None] * 5

for line in sys.stdin:
    data = line.strip().split('\t')
    size = len(data)
    newList[size - 1] = data
for i in range(len(newList)):
    print ( newList[i])

    If you can predict a reasonable upper bound of the number of lines, you can also do this, but you need to have some way to remove the None entries afterwards.

  2. Use a dictionary:

    newList = {}

for line in sys.stdin:
    data = line.strip().split('\t')
    size = len(data)
    newList[size - 1] = data
for i in range(len(newList)):
    print ( newList[i])

  3. Add elements to the list as necessary, which is probably a little bit more involved:

    newList = []

for line in sys.stdin:
    data = line.strip().split('\t')
    size = len(data)
    if len(newList) < size: newList.extend([None] * (size - len(newList)))
    newList[size - 1] = data
for i in range(len(newList)):
    print ( newList[i])
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Comments

1

I believe I've figured out the answer to my question, thanks to mkrieger1. I append to the list and then sort it using the length as the key;

newList = []

for line in sys.stdin:
    data = line.strip().split('\t')
    newList.append(data)
newList.sort(key=len)
for i in range(len(newList)):
    print (newList[i])

I got the output I wanted;

/listSort.py < test
['1']
['3', '2']
['3', '3', '3']
['2', '2', '2', '2']
['2', '3', '3', '3', '3']
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2 Comments

You don't need to sort the list every time, only after you've appended all the data.
Yep, I accidentally indented the sort. Edited the answer so that the sort occurs after appending all the data.

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