Understanding Slice Symbols

For me, this notation requires some attention.

It looks very powerful, but I don't fully understand it yet.

#1st floor

In Python 2.7

Slice with Python

[a:b:c] len = length of string, tuple or list c -- default is +1. The sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward. a -- When c is positive or blank, default is 0. When c is negative, default is -1. b -- When c is positive or blank, default is len. When c is negative, default is -(len+1).

Understanding index allocation is important.

In forward direction, starts at 0 and ends at len-1 In backward direction, starts at -1 and ends at -len

When you say [a:b:c], what you are saying is based on the symbol of C (forward or backward), starting with a and ending with B (excluding the element at the bth index).Using the index rules above, keep in mind that you will only find elements in this range:

-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1

But this range can continue indefinitely in both directions:

...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....

For example:

0 1 2 3 4 5 6 7 8 9 10 11 a s t r i n g -9 -8 -7 -6 -5 -4 -3 -2 -1

If your choice of a, b, and c allows you to use the rules of a, b, and c above to iterate over the range above, you will get a list of elements (touched during the iteration), or you will get an empty list.

Last thing: If a and b are equal, you will also get an empty list:

>>> l1 [2, 3, 4] >>> l1[:] [2, 3, 4] >>> l1[::-1] # a default is -1 , b default is -(len+1) [4, 3, 2] >>> l1[:-4:-1] # a default is -1 [4, 3, 2] >>> l1[:-3:-1] # a default is -1 [4, 3] >>> l1[::] # c default is +1, so a default is 0, b default is len [2, 3, 4] >>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1) [4, 3, 2] >>> l1[-100:-200:-1] # Interesting [] >>> l1[-1:-200:-1] # Interesting [4, 3, 2] >>> l1[-1:-1:1] [] >>> l1[-1:5:1] # Interesting [4] >>> l1[1:-7:1] [] >>> l1[1:-7:-1] # Interesting [3, 2] >>> l1[:-2:-2] # a default is -1, stop(b) at -2 , step(c) by 2 in reverse direction [4]

#2nd floor

Index: ------------> 0 1 2 3 4 +---+---+---+---+---+ | a | b | c | d | e | +---+---+---+---+---+ 0 -4 -3 -2 -1 <------------ Slice: <---------------| |---------------> : 1 2 3 4 : +---+---+---+---+---+ | a | b | c | d | e | +---+---+---+---+---+ : -4 -3 -2 -1 : |---------------> <---------------|

I hope this will help you model lists in Python.

Reference resources: http : //wiki.python.org/moin/MovingToPythonFromOtherLanguages

#3rd floor

You can also use slice assignment to remove one or more elements from the list:

r = [1, 'blah', 9, 8, 2, 3, 4] >>> r[1:4] = [] >>> r [1, 2, 3, 4]

#4th floor

Python Slice Symbol:

a[start:end:step]

• For start and end, negative values are interpreted as relative to the end of the sequence.
• The positive end index indicates the position after the last element to be included.
• The default values for blank values are as follows: [+0:-0:1].
• Using negative steps reverses the interpretation of start and end

The symbol extends to (numpy) matrices and multidimensional arrays.For example, to slice an entire column, you can use:

m[::,0:2:] ## slice the first two columns

Slices contain references to array elements, not copies.If you want to create a separate copy of the array, you can use deepcopy() .

#5th floor

Often, writing code with many hard-coded index values can lead to confusion in readability and maintenance.For example, if you return the code a year later, you'll look at it and want to know what you think when you write it.The solution shown is just a way to get a clearer picture of how the code actually works.Typically, a built-in slice () creates a slice object that can be used anywhere a slice is allowed.For example:

>>> items = [0, 1, 2, 3, 4, 5, 6] >>> a = slice(2, 4) >>> items[2:4] [2, 3] >>> items[a] [2, 3] >>> items[a] = [10,11] >>> items [0, 1, 10, 11, 4, 5, 6] >>> del items[a] >>> items [0, 1, 4, 5, 6]

If you have a slice instance s, you can get more information about it by looking at its s.start, s.stop, and s.step properties, respectively.For example:

>>> a = slice(10, 50, 2) >>> a.start 10 >>> a.stop 50 >>> a.step 2 >>>

3 December 2019, 18:23 | Views: 8103