CODEWITHSUNDEEP
pythonrecursion
John
John

Reputation: 6065

Python recursion: nested list from flat list

I need figuring out how to create (what I think needs to be) a recursive function. My brain has never coped well with recursion.

I have a flat collection of items, that needs to be turned into a nested collection based on a value in each item. Each item has, among other attributes, a type. One possible item.type is "group header". Each "group header" will be closed by a "group footer". I need to nest the collection based on those two types.

The collection might look like this:

I want to make that collection look more like this:

There can be any depth of nesting, hence (I think) the need for recursion.

Any pointers on how to do it greatly appreciated. I simply cannot get my head around it, and I can't find an example online where a tag (in my case, "group footer") is used to jump back up a nest level.

Here are the beginnings of a python fiddle to work with: http://pythonfiddle.com/recursion-fiddle-ninety-nine

Example data from link:

test_data = [{"id":1, "type":"blurb", "info":"This is the blurb"},
            {"id":2, "type":"header", "info":"This is the first group header"},
            {"id":3, "type":"question", "info":"This is the first question"},
            {"id":4, "type":"question", "info":"This is the second question"},
            {"id":5, "type":"header", "info":"This is the second group header"},
            {"id":6, "type":"question", "info":"This is the third question"},
            {"id":7, "type":"question", "info":"This is the fourth question"},
            {"id":8, "type":"footer", "info":"This is the footer for the second header"},
            {"id":9, "type":"question", "info":"This is the fifth question"},
            {"id":10, "type":"footer", "info":"This is the footer for the first header"}]

thanks in advance

Jay

Upvotes: 1

Views: 663

Answers (1)

Aran-Fey
Aran-Fey

Reputation: 43196

I don't know how exactly you want the resulting list to be formatted, but here you go:

nested_data = []
stack= []
for item in test_data:
    if item['type']=='header': # if it's a header
        # add [item] to the list (the footer will be appended to this later)
        header= [[item]]
        nested_data.append(header)
        # push this list onto the stack so we can continue appending to it
        # after we've found the footer
        stack.append(nested_data)
        nested_data= header
    elif item['type']=='footer':
        # if it's a footer, pop the last list off the stack
        nested_data= stack.pop(-1)
        # and append the footer after the header so that
        # [header, footer] is the first item
        nested_data[-1][0].append(item)
    else:
        # if it's just a boring ol' item, all we need do is append it
        nested_data.append(item)

This produces (the nested_data variable holds the result):

[
  {
    "info": "This is the blurb", 
    "type": "blurb", 
    "id": 1
  }, 
  [
    [
      {
        "info": "This is the first group header", 
        "type": "header", 
        "id": 2
      }, 
      {
        "info": "This is the footer for the first header", 
        "type": "footer", 
        "id": 10
      }
    ], 
    {
      "info": "This is the first question", 
      "type": "question", 
      "id": 3
    }, 
    {
      "info": "This is the second question", 
      "type": "question", 
      "id": 4
    }, 
    [
      [
        {
          "info": "This is the second group header", 
          "type": "header", 
          "id": 5
        }, 
        {
          "info": "This is the footer for the second header", 
          "type": "footer", 
          "id": 8
        }
      ], 
      {
        "info": "This is the third question", 
        "type": "question", 
        "id": 6
      }, 
      {
        "info": "This is the fourth question", 
        "type": "question", 
        "id": 7
      }
    ], 
    {
      "info": "This is the fifth question", 
      "type": "question", 
      "id": 9
    }
  ]
]

Upvotes: 4

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