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spec: define s[i:j:k]

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R=rsc, r, iant, ken
CC=golang-dev
https://golang.org/cl/10243046
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Robert Griesemer 2013-09-11 17:18:52 -07:00
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commit e333b96529
1 changed files with 61 additions and 4 deletions
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of Aug 15, 2013",
"Subtitle": "Version of Sep 12, 2013",
"Path": "/ref/spec"
}-->
@ -869,7 +869,7 @@ The array underlying a slice may extend past the end of the slice.
The <i>capacity</i> is a measure of that extent: it is the sum of
the length of the slice and the length of the array beyond the slice;
a slice of length up to that capacity can be created by
<a href="#Slices"><i>slicing</i></a> a new one from the original slice.
<a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
The capacity of a slice <code>a</code> can be discovered using the
built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
</p>
@ -2359,7 +2359,9 @@ PrimaryExpr =
Selector = "." identifier .
Index = "[" Expression "]" .
Slice = "[" [ Expression ] ":" [ Expression ] "]" .
Slice = "[" ( [ Expression ] ":" [ Expression ] ) |
( [ Expression ] ":" Expression ":" Expression )
"]" .
TypeAssertion = "." "(" Type ")" .
Call = "(" [ ArgumentList [ "," ] ] ")" .
ArgumentList = ExpressionList [ "..." ] .
@ -2621,7 +2623,15 @@ Assigning to an element of a <code>nil</code> map causes a
</p>
<h3 id="Slices">Slices</h3>
<h3 id="Slice_expressions">Slice expressions</h3>
<p>
Slice expressions construct a substring or slice from a string, array, pointer
to array, or slice. There are two variants: a simple form that specifies a low
and high bound, and a full form that also specifies a bound on the capacity.
</p>
<h4>Simple slice expressions</h4>
<p>
For a string, array, pointer to array, or slice <code>a</code>, the primary expression
@ -2695,6 +2705,53 @@ If the sliced operand of a valid slice expression is a <code>nil</code> slice, t
is a <code>nil</code> slice.
</p>
<h4>Full slice expressions</h4>
<p>
For an array, pointer to array, or slice <code>a</code> (but not a string), the primary expression
</p>
<pre>
a[low : high : max]
</pre>
<p>
constructs a slice of the same type, and with the same length and elements as the simple slice
expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
After slicing the array <code>a</code>
</p>
<pre>
a := [5]int{1, 2, 3, 4, 5}
t := a[1:3:5]
</pre>
<p>
the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
</p>
<pre>
t[0] == 2
t[1] == 3
</pre>
<p>
As for simple slice expressions, if <code>a</code> is a pointer to an array,
<code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
</p>
<p>
The indices are <i>in range</i> if <code>0 &lt;= low &lt;= high &lt;= max &lt;= cap(a)</code>,
otherwise they are <i>out of range</i>.
A <a href="#Constants">constant</a> index must be non-negative and representable by a value of type
<code>int</code>.
If multiple indices are constant, the constants that are present must be in range relative to each
other.
If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
</p>
<h3 id="Type_assertions">Type assertions</h3>
<p>