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Clarify the use of the backwards index operator (^N) in tut1 (#14681)

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* Clarify the use of the backwards index operator (^N) in tut1

For consistency:
- Do `[a .. ^b]` (notice spaces on both sides of `..`)
- Do `[c ..< d]` (notice spaces on both sides of `..<`)

Fixes https://github.com/nim-lang/Nim/issues/14671.

* tut1: Add a note that ^ template calls can be saved to consts
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Kaushal Modi 2020-06-19 10:22:48 -04:00 • committed by GitHub
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@ -395,20 +395,29 @@ Since counting up occurs so often in programs, Nim also has a `..
<system.html#...i,T,T>`_ iterator that does the same: <system.html#...i,T,T>`_ iterator that does the same:
.. code-block:: nim .. code-block:: nim
for i in 1..10: for i in 1 .. 10:
... ...
Zero-indexed counting have two shortcuts ``..<`` and ``..^`` to simplify counting to one less than the higher index: Zero-indexed counting has two shortcuts ``..<`` and ``.. ^1``
(`backwards index operator <system.html#^.t%2Cint>`_) to simplify
counting to one less than the higher index:
.. code-block:: nim .. code-block:: nim
for i in 0..<10: for i in 0 ..< 10:
... # 0..9 ... # 0 .. 9
or or
.. code-block:: nim .. code-block:: nim
var s = "some string" var s = "some string"
for i in 0..<s.len: for i in 0 ..< s.len:
...
or
.. code-block:: nim
var s = "some string"
for idx, c in s[0 .. ^1]:
... ...
Other useful iterators for collections (like arrays and sequences) are Other useful iterators for collections (like arrays and sequences) are
@ -1402,8 +1411,8 @@ define operators which accept Slice objects to define ranges.
a = "Nim is a programming language" a = "Nim is a programming language"
b = "Slices are useless." b = "Slices are useless."
echo a[7..12] # --> 'a prog' echo a[7 .. 12] # --> 'a prog'
b[11..^2] = "useful" b[11 .. ^2] = "useful"
echo b # --> 'Slices are useful.' echo b # --> 'Slices are useful.'
In the previous example slices are used to modify a part of a string. The In the previous example slices are used to modify a part of a string. The
@ -1425,17 +1434,22 @@ indices are
0 11 17 using indices 0 11 17 using indices
^19 ^8 ^2 using ^ syntax ^19 ^8 ^2 using ^ syntax
where ``b[0..^1]`` is equivalent to ``b[0..b.len-1]`` and ``b[0..<b.len]``, and it where ``b[0 .. ^1]`` is equivalent to ``b[0 .. b.len-1]`` and ``b[0 ..< b.len]``, and it
can be seen that the ``^1`` provides a short-hand way of specifying the ``b.len-1``. can be seen that the ``^1`` provides a short-hand way of specifying the ``b.len-1``. See
the `backwards index operator <system.html#^.t%2Cint>`_.
In the above example, because the string ends in a period, to get the portion of the In the above example, because the string ends in a period, to get the portion of the
string that is "useless" and replace it with "useful". string that is "useless" and replace it with "useful".
``b[11..^2]`` is the portion "useless", and ``b[11..^2] = "useful"`` replaces the ``b[11 .. ^2]`` is the portion "useless", and ``b[11 .. ^2] = "useful"`` replaces the
"useless" portion with "useful", giving the result "Slices are useful." "useless" portion with "useful", giving the result "Slices are useful."
Note: alternate ways of writing this are ``b[^8..^2] = "useful"`` or Note 1: alternate ways of writing this are ``b[^8 .. ^2] = "useful"`` or
as ``b[11..b.len-2] = "useful"`` or as ``b[11..<b.len-1] = "useful"``. as ``b[11 .. b.len-2] = "useful"`` or as ``b[11 ..< b.len-1] = "useful"``.
Note 2: As the ``^`` template returns a `distinct int <manual.html#types-distinct-type>`_
of type ``BackwardsIndex``, we can have a ``lastIndex`` constant defined as ``const lastIndex = ^1``,
and later used as ``b[0 .. lastIndex]``.
Objects Objects
------- -------