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To extract only first element from a list, we can use sapply function and access the first element with double square brackets. For example, if we have a list called LIST that contains 5 elements each containing 20 elements then the first sub-element can be extracted by using the command sapply(LIST,"[[",1).
In the majority of programming languages when you need to access a nested data type (such as arrays, lists, or tuples), you append the brackets to get to the innermost item. The first bracket gives you the location of the tuple in your list. The second bracket gives you the location of the item in the tuple.
In Python lists are zero-indexed, so the first element is available at index 0 . Similarly, we can also use the slicing syntax [:1] to get the first element of a list in Python.
To extract the n-th elements from a list of tuples with Python, we can use list comprehension. We get the n-th element from each tuple and put them into a list with [x[n] for x in elements] . x is the tuple being retrieved. Therefore, e is [1, 3, 5] .
>>> a = [(1, u'abc'), (2, u'def')]
>>> [i[0] for i in a]
[1, 2]
Use the zip function to decouple elements:
>>> inpt = [(1, u'abc'), (2, u'def')]
>>> unzipped = zip(*inpt)
>>> print unzipped
[(1, 2), (u'abc', u'def')]
>>> print list(unzipped[0])
[1, 2]
Edit (@BradSolomon):
The above works for Python 2.x, where zip returns a list.
In Python 3.x, zip returns an iterator and the following is equivalent to the above:
>>> print(list(list(zip(*inpt))[0]))
[1, 2]
do you mean something like this?
new_list = [ seq[0] for seq in yourlist ]
What you actually have is a list of tuple objects, not a list of sets (as your original question implied). If it is actually a list of sets, then there is no first element because sets have no order.
Here I've created a flat list because generally that seems more useful than creating a list of 1 element tuples. However, you can easily create a list of 1 element tuples by just replacing seq[0] with (seq[0],).
This is what operator.itemgetter is for.
>>> a = [(1, u'abc'), (2, u'def')]
>>> import operator
>>> b = map(operator.itemgetter(0), a)
>>> b
[1, 2]
The itemgetter statement returns a function that returns the element at the index that you specify. It's exactly the same as writing
>>> b = map(lambda x: x[0], a)
But I find that itemgetter is a clearer and more explicit.
This is handy for making compact sort statements. For example,
>>> c = sorted(a, key=operator.itemgetter(0), reverse=True)
>>> c
[(2, u'def'), (1, u'abc')]
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