Slices are a key data type in Go, giving a more powerful interface to sequences than arrays. Unlike arrays, slices are typed only by the elements they contain not the number of elements.
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To create an empty slice with non-zero length, use the builtin make. Here we make a slice of string s of length 3 initially zero-valued. In addition to these basic operations, slices support several more that make them richer than arrays. One is the builtin append , which returns a slice containing one or more new values. Note that we need to accept a return value from append as we may get a new slice value. Here we create an empty slice c of the same length as s and copy into c from s. For example, this gets a slice of the elements s , s , and s.
Slices can be composed into multi-dimensional data structures. The length of the inner slices can vary, unlike with multi-dimensional arrays. Note that while slices are different types than arrays, they are rendered similarly by fmt. However, there can be pitfalls you should watch out, and this part explains them.
The very first thing that confuses Python learners is that an index can be negative! Don't panic: a negative index means count backwards. A negative step means iterate the array backwards: from the end to start, with the end index included, and the start index excluded from the result.
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NOTE : when step is negative, the default value for start is len s while end does not equal to 0 , because s contains s. Be surprised: slice does not raise an IndexError when the index is out of range! If the index is out of range, Python will try its best to set the index to 0 or len s according to the situation. The previous answers don't discuss multi-dimensional array slicing which is possible using the famous NumPy package:. The " :2 " before the comma operates on the first dimension and the " " after the comma operates on the second dimension.
You can run this script and experiment with it, below is some samples that I got from the script. My brain seems happy to accept that lst[start:end] contains the start -th item. I might even say that it is a 'natural assumption'. But occasionally a doubt creeps in and my brain asks for reassurance that it does not contain the end -th element. This pretty property tells me that lst[start:end] does not contain the end -th item because it is in lst[end:]. When slicing from the start, you can omit the zero index, and when slicing to the end, you can omit the final index since it is redundant, so do not be verbose:.
Keep in mind that the result of slicing a collection is a whole new collection. In addition, when using slice notation in assignments, the length of the slice assignments do not need to be the same.
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The values before and after the assigned slice will be kept, and the collection will shrink or grow to contain the new values:. If the start and end indexes are omitted when performing an assignment operation, the entire content of the collection will be replaced with a copy of what is referenced:. However, using a negative value for step could become very confusing. Moreover, in order to be Pythonic , you should avoid using start , end , and step in a single slice. In case this is required, consider doing this in two assignments one to slice, and the other to stride. The extended indexing syntax used for slicing is aList[start:stop:step] , and basic examples are:.
More slicing examples: 15 Extended Slices. In my opinion, you will understand and memorize better the Python string slicing notation if you look at it the following way read on. For those who don't know, you can create any substring from azString using the notation azString[x:y]. Coming from other programming languages, that's when the common sense gets compromised. What are x and y? I had to sit down and run several scenarios in my quest for a memorization technique that will help me remember what x and y are and help me slice strings properly at the first attempt.
My conclusion is that x and y should be seen as the boundary indexes that are surrounding the strings that we want to extra.
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So all you have to do is setting index1 and index2 to the values that will surround the desired substring. For instance, to get the substring "cdefgh", you can use azString , because the index on the left side of "c" is 2 and the one on the right size of "h" is 8.
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Remember that we are setting the boundaries. And those boundaries are the positions where you could place some brackets that will be wrapped around the substring like this I want to add one Hello, World! It helped me a lot. Now the simplest slices of that list are its sublists. You made a cut before the element with index 2 and another cut before the element with index 5. So the result will be a slice between those two cuts, a list ['T', 'H', 'O']. If you feel negative indices in slicing is confusing, here's a very easy way to think about it: just replace the negative index with len - index.
So for example, replace -3 with len list - 3. The best way to illustrate what slicing does internally is just show it in code that implements this operation:. The basic slicing technique is to define the starting point, the stopping point, and the step size - also known as stride.
Create two lists to slice. The first is a numeric list from 1 to 9 List A. The second is also a numeric list, from 0 to 9 List B :. Extended indexing syntax used for slicing is aList[start:stop:step]. The start argument and the step argument both default to none - the only required argument is stop. Did you notice this is similar to how range was used to define lists A and B? This is because the slice object represents the set of indices specified by range start, stop, step.
Python 3. As you can see, defining only stop returns one element. Since the start defaults to none, this translates into retrieving only one element. It is important to note, the first element is index 0, not index 1. This is why we are using 2 lists for this exercise. List A's elements are numbered according to the ordinal position the first element is 1, the second element is 2, etc. With extended indexing syntax, we retrieve a range of values. For example, all values are retrieved with a colon. I don't think that the Python tutorial diagram cited in various other answers is good as this suggestion works for positive stride, but does not for a negative stride.
From the diagram, I expect a[-4,-6,-1] to be yP but it is ty. What always work is to think in characters or slots and use indexing as a half-open interval -- right-open if positive stride, left-open if negative stride. This way, I can think of a as a -6,-4] in interval terminology. It is easy to understand if we could relate slicing to range , which gives the indexes. We can categorize slicing into the following two categories:.
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