# Python | Binary element list grouping

Sometimes while working with the databases, we need to perform certain list operations that are more like query language, for instance, grouping of nested list element with respect to its other index elements. This article deals with binary nested list and group each nested list element with respect to its other index elements

Method #1 : Using list comprehension
List comprehension can perform the task that would usually take 4-5 lines in 1-2 lines. This groups the element by assigning to each number values according to the match with another element of list.

 `# Python3 code to demonstrate  ` `# to perform binary list grouping ` `# using list comprehension ` ` `  `# initializing list  ` `test_list ``=` `[[``"G"``, ``0``], [``"F"``, ``0``], [``"B"``, ``2``], ` `             ``[``"E"``, ``2``], [``'I'``, ``1``], [``'S'``, ``1``], ` `             ``[``'S'``, ``2``], [``'T'``, ``2``], [``'G'``, ``0``]] ` ` `  `# using list comprehension ` `# to perform binary list grouping ` `temp ``=` `set``(``map``(``lambda` `i : i[``1``], test_list)) ` `res ``=`  `[[j[``0``] ``for` `j ``in` `test_list ``if` `j[``1``] ``=``=` `i] ``for` `i ``in` `temp] ` ` `  `# printing result ` `print` `(``"The grouped list is : "` `+`  `str``(res)) `

Output:

```The grouped list is : [['G', 'F', 'G'], ['I', 'S'], ['B', 'E', 'S', 'T']]
```

Method #2 : Using `itertools.groupby() + itemgetter()`
We can also use groupby function to perform this particular task. This method follows 2-3 steps. First, the sequence is sort with respect to second element, now this can be fed to get grouped and is then grouped. Then lastly, we print the first element as required by the result.

 `# Python3 code to demonstrate  ` `# to perform binary list grouping ` `# using itertools.groupby() + itemgetter() ` `from` `itertools ``import` `groupby ` `from` `operator ``import` `itemgetter ` ` `  `# initializing list  ` `test_list ``=` `[[``"G"``, ``0``], [``"F"``, ``0``], [``"B"``, ``2``], ` `             ``[``"E"``, ``2``], [``'I'``, ``1``], [``'S'``, ``1``],  ` `             ``[``'S'``, ``2``], [``'T'``, ``2``], [``'G'``, ``0``]] ` ` `  `# using itertools.groupby() + itemgetter() ` `# to perform binary list grouping ` `test_list.sort(key ``=` `itemgetter(``1``)) ` `groups ``=` `groupby(test_list, itemgetter(``1``)) ` `res ``=` `[[i[``0``] ``for` `i ``in` `val] ``for` `(key, val) ``in` `groups] ` ` `  `# printing result ` `print` `(``"The grouped list is : "` `+`  `str``(res)) `

Output:

```The grouped list is : [['G', 'F', 'G'], ['I', 'S'], ['B', 'E', 'S', 'T']]
```

Method #3 : Using `collections.defaultdict()`

This is conventional method of performing hashing of all the keys with corresponding value in sequence and then printing the values corresponding to the hashed list.

 `# Python3 code to demonstrate  ` `# to perform binary list grouping ` `# using collections.defaultdict() ` `import` `collections ` ` `  `# initializing list  ` `test_list ``=` `[[``"G"``, ``0``], [``"F"``, ``0``], [``"B"``, ``2``], ` `             ``[``"E"``, ``2``], [``'I'``, ``1``], [``'S'``, ``1``], ` `             ``[``'S'``, ``2``], [``'T'``, ``2``], [``'G'``, ``0``]] ` ` `  `# using collections.defaultdict() ` `# to perform binary list grouping ` `res ``=` `collections.defaultdict(``list``) ` `for` `val ``in` `test_list: ` `    ``res[val[``1``]].append(val[``0``]) ` ` `  `# printing result ` `print` `(``"The grouped list is : "` `+`  `str``(res.values())) `

Output:

```The grouped list is : dict_values([['G', 'F', 'G'], ['I', 'S'], ['B', 'E', 'S', 'T']])
```

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