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effective_go: cleanups and fixes

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Also explain the situation with recursive String methods more accurately,
and clean up the code now that the fmt package is more careful.

R=golang-dev, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/5907047
This commit is contained in:
Rob Pike 2012-03-25 11:34:51 +11:00
parent 73b5eb38c1
commit 4074795e15
2 changed files with 48 additions and 22 deletions
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50
doc/effective_go.html
View File

@ -625,9 +625,28 @@ or reading from a channel, a <code>range</code> clause can
manage the loop.
</p>
<pre>
var m map[string]int
for key, value := range oldMap {
newMap[key] = value
}
</pre>
<p>
If you only need the first item in the range (the key or index), drop the second:
</p>
<pre>
for key := range m {
if expired(key) {
delete(m, key)
}
}
</pre>
<p>
If you only need the second item in the range (the value), use the <em>blank identifier</em>, an underscore, to discard the first:
</p>
<pre>
sum := 0
for _, value := range m { // key is unused
for _, value := range array {
sum += value
}
</pre>
@ -709,7 +728,7 @@ func shouldEscape(c byte) bool {
Here's a comparison routine for byte arrays that uses two
<code>switch</code> statements:
<pre>
// Compare returns an integer comparing the two byte arrays
// Compare returns an integer comparing the two byte arrays,
// lexicographically.
// The result will be 0 if a == b, -1 if a &lt; b, and +1 if a &gt; b
func Compare(a, b []byte) int {
@ -1003,7 +1022,7 @@ but the rules are simple.
Let's talk about <code>new</code> first.
It's a built-in function that allocates memory, but unlike its namesakes
in some other languages it does not <em>initialize</em> the memory,
it only <em>zeroes</em> it.
it only <em>zeros</em> it.
That is,
<code>new(T)</code> allocates zeroed storage for a new item of type
<code>T</code> and returns its address, a value of type <code>*T</code>.
@ -1265,7 +1284,7 @@ any. To read into the first 32 bytes of a larger buffer
</pre>
<p>
Such slicing is common and efficient. In fact, leaving efficiency aside for
the moment, this snippet would also read the first 32 bytes of the buffer.
the moment, the following snippet would also read the first 32 bytes of the buffer.
</p>
<pre>
var n int
@ -1407,7 +1426,7 @@ func offset(tz string) int {
</pre>
<p>
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 blank identifier (<code>_</code>).
The blank identifier can be assigned or declared with any value of any type, with the
value discarded harmlessly. For testing just presence in a map, use the blank
identifier in place of the usual variable for the value.
@ -1697,13 +1716,20 @@ automatically for printing, even as part of a general type.
</p>
{{code "/doc/progs/eff_bytesize.go" `/^func.*ByteSize.*String/` `/^}/`}}
<p>
(The <code>float64</code> conversions prevent <code>Sprintf</code>
from recurring back through the <code>String</code> method for
<code>ByteSize</code>.)
The expression <code>YB</code> prints as <code>1.00YB</code>,
while <code>ByteSize(1e13)</code> prints as <code>9.09TB</code>.
</p>
<p>
Note that it's fine to call <code>Sprintf</code> and friends in the
implementation of <code>String</code> methods, but beware of
recurring into the <code>String</code> method through the nested
<code>Sprintf</code> call using a string format
(<code>%s</code>, <code>%q</code>, <code>%v</code>, <code>%x</code> or <code>%X</code>).
The <code>ByteSize</code> implementation of <code>String</code> is safe
because it calls <code>Sprintf</code> with <code>%f</code>.
</p>
<h3 id="variables">Variables</h3>
<p>
@ -2520,8 +2546,8 @@ system, and there's not a mutex in sight.
<p>
Another application of these ideas is to parallelize a calculation
across multiple CPU cores. If the calculation can be broken into
separate pieces, it can be parallelized, with a channel to signal
when each piece completes.
separate pieces that can execute independently, it can be parallelized,
with a channel to signal when each piece completes.
</p>
<p>
Let's say we have an expensive operation to perform on a vector of items,
@ -2563,7 +2589,7 @@ func (v Vector) DoAll(u Vector) {
</pre>
<p>
The current implementation of <code>gc</code> (<code>6g</code>, etc.)
The current implementation of the Go runtime
will not parallelize this code by default.
It dedicates only a single core to user-level processing. An
arbitrary number of goroutines can be blocked in system calls, but

18
doc/progs/eff_bytesize.go
View File

@ -23,23 +23,23 @@ const (
func (b ByteSize) String() string {
switch {
case b >= YB:
return fmt.Sprintf("%.2fYB", float64(b/YB))
return fmt.Sprintf("%.2fYB", b/YB)
case b >= ZB:
return fmt.Sprintf("%.2fZB", float64(b/ZB))
return fmt.Sprintf("%.2fZB", b/ZB)
case b >= EB:
return fmt.Sprintf("%.2fEB", float64(b/EB))
return fmt.Sprintf("%.2fEB", b/EB)
case b >= PB:
return fmt.Sprintf("%.2fPB", float64(b/PB))
return fmt.Sprintf("%.2fPB", b/PB)
case b >= TB:
return fmt.Sprintf("%.2fTB", float64(b/TB))
return fmt.Sprintf("%.2fTB", b/TB)
case b >= GB:
return fmt.Sprintf("%.2fGB", float64(b/GB))
return fmt.Sprintf("%.2fGB", b/GB)
case b >= MB:
return fmt.Sprintf("%.2fMB", float64(b/MB))
return fmt.Sprintf("%.2fMB", b/MB)
case b >= KB:
return fmt.Sprintf("%.2fKB", float64(b/KB))
return fmt.Sprintf("%.2fKB", b/KB)
}
return fmt.Sprintf("%.2fB", float64(b))
return fmt.Sprintf("%.2fB", b)
}
func main() {