What was the main difference between df[:] and df[::] ? python3

Question

What is the main difference between df[:] and df[::] and could you please give me one sample example for regarding this.i was unable to understand.

Can't test right now but there may be differences in regards to slices and views (the former making a silent copy) in pandas — roganjosh, Oct 31 '18 at 08:27

score 0 · Accepted Answer · answered Oct 31 '18 at 08:51

Assuming you are talking about python lists and not pandas dataframes:

Consider a list l:

In [301]: l = range(20,30)

In [302]: l
Out[302]: [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]

So, if you do something like :

In [303]: l[3:6]
Out[303]: [23, 24, 25]

This means, you want to extract a list of elements from l from index=3 to index=6(6 not included). So, it returned 23 which is l[3], 24 which is l[4] and 25 which is l[5].

Note: 26(l[6]) was not returned as 6 is not included in l[3:6]

So, l[:] -- Would return the all the elements as no range was specified.

In [305]: l[:]
Out[305]: [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]

For extended slicing:

l[1:6:2] -- Would mean, you want elements from index=1 to index=6 with a step of 2. Step=2 means, increment every index by 2

So, you should get below elements:
 l[1], 
 l[1 + 2](because the step we defined is 2), 
 l[1 + 2 + 2]( keep adding `2` to the previous index )

Now, previous index here is 5 and the limit we gave was 6 where 6 is not included. Hence, we get only 3 elements as mentioned above. Check below:

In [307]: l[1:6:2]
Out[307]: [21, 23, 25]

So, l[::] would also return the entire list, as we haven;t specified any range here.

In [309]: l[::]
Out[309]: [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]

You would need to practice slicing of strings in python to get a hang of it.

Let me know if this helps.

score 0 · Answer 2 · answered Oct 31 '18 at 08:53

There is no difference.

sequence[start:end:step]
sequence[start:end]

':' and '::' represent slices of sequences. If start, end and/or step are omitted, then the defaults are used instead: 0 is the default for start, len(sequence) is the default for end, and 1 is the default for step.

So if, df = [1,2,3,4,5,6,7,8] then,

[df[::]==df[:], df[:] == df[0: len(df): 1], df[::] == df[0: len(df): 1]]

Outputs: [True, True, True]

What was the main difference between df[:] and df[::] ? python3

2 Answers2