Split list python by index

How do I split a list into sub-lists based on index ranges?

e.g. original list:

list1 = [x,y,z,a,b,c,d,e,f,g]

using index ranges 0–4:

list1a = [x,y,z,a,b]

using index ranges 5–9:

list1b = [c,d,e,f,g]

I already known the (variable) indices of list elements which contain certain string and want to split the list based on these index values.

Also need to split into variable number of sub-lists, i.e.:

list1a
list1b
.
.
list1[x]

mkrieger1

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asked Sep 2, 2013 at 10:15

1

In python, it's called slicing. Here is an example of python's slice notation:

>>> list1 = ['a','b','c','d','e','f','g','h', 'i', 'j', 'k', 'l']
>>> print list1[:5]
['a', 'b', 'c', 'd', 'e']
>>> print list1[-7:]
['f', 'g', 'h', 'i', 'j', 'k', 'l']

Note how you can slice either positively or negatively. When you use a negative number, it means we slice from right to left.

answered Sep 2, 2013 at 10:20

TerryATerryA

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1

Note that you can use a variable in a slice:

l = ['a',' b',' c',' d',' e']
c_index = l.index("c")
l2 = l[:c_index]

This would put the first two entries of l in l2

answered Sep 2, 2013 at 10:33

0

If you already know the indices:

list1 = ['x','y','z','a','b','c','d','e','f','g']
indices = [(0, 4), (5, 9)]
print [list1[s:e+1] for s,e in indices]

Note that we're adding +1 to the end to make the range inclusive...

answered Sep 2, 2013 at 10:31

Jon Clements♦Jon Clements

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list1a=list[:5]
list1b=list[5:]

answered Sep 2, 2013 at 10:17

no1no1

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5

One of the ways to do it if you have multiple indexes or know the range of indexes you need to get:

split_points - points where you will split your string or list

k - the range you need to split, example = 3

split_points = [i for i in range(0, len(string), k)]

parts = [string[ind:ind + k] for ind in split_points]

answered Apr 11, 2021 at 18:32

list1=['x','y','z','a','b','c','d','e','f','g']
find=raw_input("Enter string to be found")
l=list1.index(find)
list1a=[:l]
list1b=[l:]

answered Sep 2, 2013 at 10:28

no1no1

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0

Consider the core pesudocode of the following example:

def slice_it(list_2be_sliced, indices):
    """Slices a list at specific indices into constituent lists.
    """
    indices.append(len(list_2be_sliced))
    return [list_2be_sliced[indices[i]:indices[i+1]] for i in range(len(indices)-1)]

answered Mar 4, 2021 at 21:17

ShadyShady

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This is the way I do it, if the input is a list of indices on which to split an array:

#input
list1 = ['x','y','z','a','b','c','d','e','f','g']
split_points = [2,5,8] 

#split array on indices:
s = split_points+[len(list1)]  #must contain index beyond last element, alternatively use directly split_points.append(len(list1))
print([list1[i1:i2] for i1,i2 in zip([0]+s[:-1],s)])

>>> [['x', 'y'], ['z', 'a', 'b'], ['c', 'd', 'e'], ['f', 'g']]

answered Jun 27 at 16:37

VincenzoooVincenzooo

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