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