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Effective Go: update maps description regarding lookup of nonexistent entry.

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R=rsc, gri, iant
CC=golang-dev
https://golang.org/cl/821044
This commit is contained in:
Rob Pike 2010-03-30 11:21:50 -07:00
parent 6a71a4639e
commit 4710642d41
1 changed files with 20 additions and 5 deletions
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doc/effective_go.html
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@ -1121,9 +1121,23 @@ var timeZone = map[string] int {
<p> <p>
Assigning and fetching map values looks syntactically just like Assigning and fetching map values looks syntactically just like
doing the same for arrays except that the index doesn't need to doing the same for arrays except that the index doesn't need to
be an integer. An attempt to fetch a map value with a key that be an integer.
is not present in the map will cause the program to crash, but </p>
there is a way to do so safely using a multiple assignment. <pre>
offset := timeZone["EST"]
</pre>
<p>
An attempt to fetch a map value with a key that
is not present in the map will return the zero value for the type
of the entries
in the map. For instance, if the map contains integers, looking
up a non-existent key will return <code>0</code>.
</p>
<p>
Sometimes you need to distinguish a missing entry from
a zero value. Is there an entry for <code>"UTC"</code>
or is that zero value because it's not in the map at all?
You can discriminate with a form of multiple assignment.
</p> </p>
<pre> <pre>
var seconds int var seconds int
@ -1136,7 +1150,7 @@ In this example, if <code>tz</code> is present, <code>seconds</code>
will be set appropriately and <code>ok</code> will be true; if not, will be set appropriately and <code>ok</code> will be true; if not,
<code>seconds</code> will be set to zero and <code>ok</code> will <code>seconds</code> will be set to zero and <code>ok</code> will
be false. be false.
Here's a function that puts it together: Here's a function that puts it together with a nice error report:
</p> </p>
<pre> <pre>
func offset(tz string) int { func offset(tz string) int {
@ -1151,7 +1165,7 @@ func offset(tz string) int {
To test for presence in the map without worrying about the actual value, To test for presence in the map without worrying about the actual value,
you can use the <em>blank identifier</em>, a simple underscore (<code>_</code>). you can use the <em>blank identifier</em>, a simple underscore (<code>_</code>).
The blank identifier can be assigned or declared with any value of any type, with the The blank identifier can be assigned or declared with any value of any type, with the
value discarded harmlessly. For testing presence in a map, use the blank value discarded harmlessly. For testing just presence in a map, use the blank
identifier in place of the usual variable for the value. identifier in place of the usual variable for the value.
</p> </p>
<pre> <pre>
@ -1166,6 +1180,7 @@ from the map.
<pre> <pre>
timeZone["PDT"] = 0, false // Now on Standard Time timeZone["PDT"] = 0, false // Now on Standard Time
</pre> </pre>
<h3 id="printing">Printing</h3> <h3 id="printing">Printing</h3>
<p> <p>