Python list loop from a index

Question

I have a list of dic

MyList=[
    {'Buy': 'Yes', 'date': monday, 'item': '1234'},
    {'Buy': 'Yes', 'date': monday, 'item': '4'},
    {'Buy': 'Yes', 'date': sunday, 'item': '134'},
    {'Buy': 'Yes', 'date': sunday, 'item': '124'},
    {'Buy': 'Yes', 'date': friday, 'item': '14'},
    {'Buy': 'Yes', 'date': friday, 'item': '234'}
]

I use a loop

for data in Mylist:

so it will go through all the items

if I want to do the same but I want to choose the begin and the end of le loop for example a loop start from 0 to 3 or a loop from 2 to end how can I do that?

thanks in advance

Community · Accepted Answer · 2017-05-23 10:25:54Z

3

Use slicing:

for data in Mylist[:3]:

or:

for data in Mylist[2:]:

Related: Python's slice notation

edited May 23, 2017 at 10:25

CommunityBot

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answered Jan 24, 2014 at 23:21

Ashwini Chaudhary

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1 Comment

aIKid Over a year ago

Can't get any simpler.

inspectorG4dget · Accepted Answer · 2014-01-24 23:32:07Z

1

To loop through the first n elements:

for i in itertools.islice(L, 0, n):
    # do stuff

To loop through the last n elements:

for i in itertools.islice(L, len(L)-n, len(L)):
    # do stuff

To loop through the first n and the last m elements:

for i in itertools.chain(itertools.islice(L, len(L)-n, len(L)), itertools.islice(L, len(L)-m, len(L))):
    # do stuff

answered Jan 24, 2014 at 23:32

inspectorG4dget

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4 Comments

abarnert Over a year ago

This is worth knowing about, but usually not worth doing with a sequence. If the slice is big enough that the copying cost matters, it's likely also big enough that the cost of manually skipping over the first n elements also matters. (For example, if a = list(range(100000)), a[50000:60000] is about 10x faster than list(islice(a, 50000, 60000)). Of course when you have an iterable that may not be a sequence, that's a different story…

inspectorG4dget Over a year ago

@abarnert: that's really cool. Out of sheer morbid curiosity: is there a timing difference in a[50000:60000] between python2.7 and python3.3?

abarnert Over a year ago

Good question. Python.org 64-bit CPython 3.3.2: 34.2µs slice, 291µs islice. Apple 64-bit CPython 2.7.5: 32.0µs slice, 344µs islice. So it looks like no significant change in slices, but islice got a little faster.

abarnert Over a year ago

Of course that isn't a totally fair test—putting the islice in a list like that means you don't save any of the list allocation/construction time that looping over an islice can avoid. But if I try with collections.deque(maxlen=0), which is about as close to consuming an iterator with no overhead as you can get, it just drops to 281µs/306µs. (Of course the real win is when you don't have enough memory, at least without swapping…)

Nicholas Flees · Accepted Answer · 2014-01-24 23:27:29Z

0

If you were interested in printing items 0 through 3, for example, you could do something like this:

for item in MyList[:3]:
    print item

answered Jan 24, 2014 at 23:27

Nicholas Flees

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