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printing and maps

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Rob Pike 2009-10-12 14:51:12 -07:00
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doc/effective_go.html
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@ -1,6 +1,6 @@
<!-- Effective Go -->
<!-- interfaces; slices; embedding; value vs. pointer receivers; methods on anything; errors; testing -->
<!-- interfaces; cast,conversion, type assertion; embedding; errors; testing; initialization -->
<h2 id="introduction">Introduction</h2>
@ -34,7 +34,7 @@ should read first.
<h3 id="read">Examples</h3>
<p>
The <a href="/src/pkg/">Go package sources</a>
The <a href="http://s2/?dir=//depot2/go/src/pkg">Go package sources</a>
are intended to serve not
only as the core library but also as examples of how to
use the language.
@ -97,7 +97,6 @@ type T struct {
<p>
All code in the libraries has been formatted with <code>gofmt</code>.
<font color=red>TODO</font>
</p>
@ -108,7 +107,7 @@ Some formatting details remain. Very briefly:
<dl>
<dt>Indentation</dt>
<dd>We use tabs for indentation and <code>gofmt</code> emits them by default.
Use spaces if you must.
Use spaces only if you must.
</dd>
<dt>Line length</dt>
<dd>
@ -118,7 +117,7 @@ Some formatting details remain. Very briefly:
<dt>Parentheses</dt>
<dd>
Go needs fewer parentheses: control structures (<code>if</code>,
<code>for</code>, <code>switch</code>) do not have parentheses in
<code>for</code>, <code>switch</code>) do not require parentheses in
their syntax.
Also, the operator precedence hierarchy is shorter and clearer, so
<pre>
@ -1059,8 +1058,231 @@ structure holding the pointer, length, and capacity) is passed by value.
<h3 id="maps">Maps</h3>
<p>
Maps are a convenient and powerful built-in data structure to associate
values of different types.
The key can be of type that implements equality, such as integers,
floats, strings, pointers, and interfaces (as long as the dynamic type
supports equality), but not structs, arrays or slices
because those types do not have equality defined upon them.
Like slices, maps are a reference type. If you pass a map to a function
that changes the contents of the map, the changes will be visible
in the caller.
</p>
<p>
Maps can be constructed using the usual composite literal syntax
with colon-separated key-value pairs,
so it's easy to build them during initialization.
</p>
<pre>
var timeZone = map[string] int {
"UTC": 0*60*60,
"EST": -5*60*60,
"CST": -6*60*60,
"MST": -7*60*60,
"PST": -8*60*60,
}
</pre>
<p>
Assigning and fetching map values looks syntactically just like
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
is not present in the map will cause the program to crash, but
there is a way to do so safely using a multiple assignment.
</p>
<pre>
var seconds int;
var ok bool;
seconds, ok = timeZone[tz]
</pre>
<p>
For obvious reasons this is called the &ldquo;comma ok&rdquo; idiom.
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,
<code>seconds</code> will be set to zero and <code>ok</code> will
be false.
Here's a function that puts it together:
</p>
<pre>
func offset(tz string) int {
if seconds, ok := timeZone[tz]; ok {
return seconds
}
log.Stderr("unknown time zone", tz);
return 0;
}
</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>).
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
identifier in place of the usual variable for the value.
</p>
<pre>
_, present := timeZone[tz];
</pre>
<p>
To delete a map entry, turn the multiple assignment around by placing
an extra boolean on the right; if the boolean is false, the entry
is deleted. It's safe to do this even if the key is already absent
from the map.
</p>
<pre>
timeZone["PDT"] = 0, false; // Now on Standard Time
</pre>
<h3 id="printing">Printing</h3>
<p>
Formatted printing in Go uses a style similar to C's <code>printf</code>
family but is richer and more general. The functions live in the <code>fmt</code>
package and have capitalized names: <code>fmt.Printf</code>, <code>fmt.Fprintf</code>,
<code>fmt.Sprintf</code> and so on. The string functions (<code>Sprintf</code> etc.)
return a string rather than filling in a provided buffer.
</p>
<p>
You don't need to provide a format string. For each of <code>Printf</code>,
<code>fmt.Fprintf</code> and <code>fmt.Sprintf</code> there is another pair
of functions, for instance <code>Print</code> and <code>Println</code>.
These functions do not take a format string but instead generate a default
format for each argument. The <code>ln</code> version also inserts a blank
between arguments if neither is a string and appends a newline to the output.
In this example each line produces the same output.
</p>
<pre>
fmt.Printf("Hello %d\n", 23);
fmt.Fprint(os.Stdout, "Hello ", 23, "\n");
fmt.Println(fmt.Sprint("Hello ", 23));
</pre>
<p>
Recall that <code>fmt.Fprint</code> and friends take as a first argument any object
that implements the <code>io.Writer</code> interface; the variables <code>os.Stdout</code>
and <code>os.Stderr</code> are familiar instances.
</p>
<p>
Here things start to diverge from C. First, the numeric formats such as <code>%d</code>
do not take flags for signedness or size; instead, the printing routines use the
type of the argument to decide these properties.
</p>
<pre>
var x uint64 = 1<<64 - 1;
fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x));
</pre>
<p>
prints
</p>
<pre>
18446744073709551615 ffffffffffffffff; -1 -1
</pre>
<p>
If you just want the default conversion, such as decimal for integers, you can use
the catchall format <code>%v</code> (for &ldquo;value&rdquo;); the result is exactly
what <code>Print</code> and <code>Println</code> would produce.
Moreover, that format can print <em>any</em> value, even arrays, structs, and
maps. Here is a print statement for the time zone map defined in the previous section.
</p>
<pre>
fmt.Printf("%v\n", timeZone); // or just fmt.Println(timeZone);
</pre>
<p>
which gives output
</p>
<pre>
map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200]
</pre>
<p>
For maps the keys may be output in any order, of course.
When printing a struct, the modified format <code>%+v</code> annotates the
fields of the structure with their names, and for any value the alternate
format <code>%#v</code> prints the value in full Go syntax.
</p>
<pre>
type T struct {
a int;
b float;
c string;
}
t := &amp;T{ 7, -2.35, "abc\tdef" };
fmt.Printf("%v\n", t);
fmt.Printf("%+v\n", t);
fmt.Printf("%#v\n", t);
fmt.Printf("%#v\n", timeZone);
</pre>
<p>
prints
</p>
<pre>
&amp;{7 -2.35 abc def}
&amp;{a:7 b:-2.35 c:abc def}
&amp;main.T{a:7, b:-2.35, c:"abc\tdef"}
map[string] int{"CST":-21600, "PST":-28800, "EST":-18000, "UTC":0, "MST":-25200}
</pre>
<p>
(Note the ampersands.)
That quoted string format is also available through <code>%q</code> when
applied to a value of type <code>string</code> or <code>[]byte</code>;
the alternate format <code>%#q</code> will use backquotes instead if possible.
Also, <code>%x</code> works on strings and arrays of bytes as well as on integers,
generating a long hexadecimal string, and with
a space in the format (<code>%&nbsp;x</code>) it puts spaces between the bytes.
</p>
<p>
Another handy format is <code>%T</code>, which prints the <em>type</em> of a value.
<pre>
fmt.Printf(&quot;%T\n&quot;, timeZone);
</pre>
<p>
prints
</p>
<pre>
map[string] int
</pre>
<p>
If you want to control the default format for a custom type, all that's required is to define
a method <code>String() string</code> on the type. (Methods are the subject of the next
section.) For our simple type <code>T</code>, that might look like this.
</p>
<pre>
func (t *T) String() string {
return fmt.Sprintf("%d/%g/%q", t.a, t.b, t.c);
}
fmt.Printf("%v\n", t);
</pre>
<p>
to print in the format
</p>
<pre>
7/-2.35/"abc\tdef"
</pre>
<p>
Our <code>String()</code> method is able to call <code>Sprintf</code> because the
print routines are fully reentrant and can be used recursively.
We can even go one step further and pass a print routine's arguments directly to another such routine.
The signature of <code>Printf</code> uses the <code>...</code>
type for its final argument to specify that an arbitrary number of parameters can appear
after the format.
</p>
<pre>
func Printf(format string, v ...) (n int, errno os.Error) {
</pre>
<p>
Within the function <code>Printf</code>, <code>v</code> is a variable that can be passed,
for instance, to another print routine. Here is the implementation of the
function <code>log.Stderr</code> we used above. It passes its arguments directly to
<code>fmt.Sprintln</code> for the actual formatting.
</p>
<pre>
// Stderr is a helper function for easy logging to stderr. It is analogous to Fprint(os.Stderr).
func Stderr(v ...) {
stderr.Output(2, fmt.Sprintln(v)); // Output takes parameters (int, string)
}
</pre>
<p>
There's even more to printing than we've covered here. See the <code>godoc</code> documentation
for package <code>fmt</code> for the details.
</p>
<h2>Methods</h2>
<h3 id="pointers_vs_values">Pointers vs. Values</h3>
@ -1112,7 +1334,7 @@ print into one:
</p>
<pre>
var b ByteSlice;
fmt.Fprintf(&amp;b, "This minute has %d seconds\n", 61);
fmt.Fprintf(&amp;b, "This hour has %d days\n", 7);
</pre>
<p>
Notice that we must pass the address of a <code>ByteSlice</code>
@ -1159,7 +1381,7 @@ Implementations of <code>os.Error</code> should
describe the error and provide context.
For example, <code>os.Open</code> returns an <code>os.PathError</code>:
<a href="/src/pkg/os/file.go">/src/pkg/os/file.go</a>:
<a href="http://go/godoc/src/pkg/os/file.go">http://go/godoc/src/pkg/os/file.go</a>:
<pre>
// PathError records an error and the operation and
// file path that caused it.