想要调用指针类型方法(*Point).ScaleBy，只要提供一个Point类型的指针即可，像下面这样。

r := &Point{1, 2}
r.ScaleBy(2)
fmt.Println(*r) // "{2, 4}"

或者这样：

p := Point{1, 2}
pptr := &p
pptr.ScaleBy(2)
fmt.Println(p) // "{2, 4}"

或者这样:

p := Point{1, 2}
(&p).ScaleBy(2)
fmt.Println(p) // "{2, 4}"

不过后面两种方法有些笨拙。幸运的是，go语言本身在这种地方会帮到我们。如果接收器p是一个Point类型的变量，并且其方法需要一个Point指针作为接收器，我们可以用下面这种简短的写法：

p.ScaleBy(2)

编译器会隐式地帮我们用&p去调用ScaleBy这个方法。这种简写方法只适用于“变量”，包括struct里的字段比如p.X，以及array和slice内的元素比如perim[0]。我们不能通过一个无法取到地址的接收器来调用指针方法，比如临时变量的内存地址就无法获取得到：

Point{1, 2}.ScaleBy(2) // compile error: can't take address of Point literal

但是我们可以用一个Point这样的接收器来调用Point的方法，因为我们可以通过地址来找到这个变量，只要用解引用符号来取到该变量即可。编译器在这里也会给我们隐式地插入*这个操作符，所以下面这两种写法等价的：

pptr.Distance(q)
(*pptr).Distance(q)

这里的几个例子可能让你有些困惑，所以我们总结一下：在每一个合法的方法调用表达式中，也就是下面三种情况里的任意一种情况都是可以的：

不论接收器的实际参数和其形式参数是相同，比如两者都是类型T或者都是类型*T：

Point{1, 2}.Distance(q) //  Point
pptr.ScaleBy(2)         // *Point

或者接收器实参是类型T，但接收器形参是类型*T，这种情况下编译器会隐式地为我们取变量的地址：

p.ScaleBy(2) // implicit (&p)

或者接收器实参是类型*T，形参是类型T。编译器会隐式地为我们解引用，取到指针指向的实际变量：

pptr.Distance(q) // implicit (*pptr)

如果命名类型T（译注：用type xxx定义的类型）的所有方法都是用T类型自己来做接收器（而不是*T），那么拷贝这种类型的实例就是安全的；调用他的任何一个方法也就会产生一个值的拷贝。比如time.Duration的这个类型，在调用其方法时就会被全部拷贝一份，包括在作为参数传入函数的时候。但是如果一个方法使用指针作为接收器，你需要避免对其进行拷贝，因为这样可能会破坏掉该类型内部的不变性。比如你对bytes.Buffer对象进行了拷贝，那么可能会引起原始对象和拷贝对象只是别名而已，实际上它们指向的对象是一样的。紧接着对拷贝后的变量进行修改可能会有让你有意外的结果。

译注： 作者这里说的比较绕，其实有两点：

不管你的method的receiver是指针类型还是非指针类型，都是可以通过指针/非指针类型进行调用的，编译器会帮你做类型转换。
在声明一个method的receiver该是指针还是非指针类型时，你需要考虑两方面的因素，第一方面是这个对象本身是不是特别大，如果声明为非指针变量时，调用会产生一次拷贝；第二方面是如果你用指针类型作为receiver，那么你一定要注意，这种指针类型指向的始终是一块内存地址，就算你对其进行了拷贝。熟悉C或者C++的人这里应该很快能明白。

autocomplete="off"

经常不管用

请用一下设置试一试！

autocomplete="new-password"

参考：https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion

1.2. 命令行参数

os包以跨平台的方式，提供了一些与操作系统交互的函数和变量。程序的命令行参数可从os包的Args变量获取；os包外部使用os.Args访问该变量。

os.Args变量是一个字符串（string）的切片（slice）

和大多数编程语言类似，区间索引时，Go言里也采用左闭右开形式, 即，区间包括第一个索引元素，不包括最后一个, 因为这样可以简化逻辑。

比如s[m:n]这个切片，0 ≤ m ≤ n ≤ len(s)，包含n-m个元素。

os.Args的第一个元素，os.Args[0], 是命令本身的名字；其它的元素则是程序启动时传给它的参数。s[m:n]形式的切片表达式，产生从第m个元素到第n-1个元素的切片，下个例子用到的元素包含在os.Args[1:len(os.Args)]切片中。如果省略切片表达式的m或n，会默认传入0或len(s)，因此前面的切片可以简写成os.Args[1:]。

In JavaScript, computations with numbers don’t always produce precise results. For example:

0.1 + 0.2
0.30000000000000004
To understand why, we need to explore how JavaScript represents floating point numbers internally. It uses three integers to do so, as described in tbl. 5.

Table 5: Internally, JavaScript uses three integers to represent floating point numbers. They take up a total of 64 bits of storage (double precision).

The floating point number represented by these integers is computed as follows:

(–1)sign × 0b1.fraction × 2exponent

To make further discussions easier, we simplify this representation:

• Instead of base 2 (binary), we use base 10 (decimal), because that’s what most people are more familiar with.
• The fraction is a natural number that is interpreted as a fraction (digits after a point). We switch to a mantissa, an integer that is interpreted as itself. As a consequence, the exponent is used differently, but its fundamental role doesn’t change.
• As the mantissa is an integer (with its own sign), we don’t need a separate sign, anymore.

The new representation works like this:

mantissa × 10exponent

Let’s try out this representation for a few floating point numbers.

• For the integer −123, we mainly need the mantissa:

-123 * (10 ** 0)
-123

• For the number 1.5, we imagine there being a point after the mantissa. We use a negative exponent to move that point one digit to the left:

15 * (10 ** -1)
1.5

• For the number 0.25, we move the point two digits to the left:

25 * (10 ** -2)
0.25

Representations with negative exponents can also be written as fractions with positive exponents in the denominators:

15 * (10 ** -1) === 15 / 10
true
25 * (10 ** -2) === 25 / 100
true

These fractions help with understanding why there are numbers that our encoding cannot represent:

• 1/10 can be represented. It already has the required format: a power of 10 in the denominator.
• 1/2 can be represented as 5/10. We turned the 2 in the denominator into a power of 10, by multiplying numerator and denominator with 5.
• 1/4 can be represented as 25/100. We turned the 4 in the denominator into a power of 10, by multiplying numerator and denominator with 25.
• 1/3 cannot be represented. There is no way to turn the denominator into a power of 10. (The prime factors of 10 are 2 and 5. Therefore, any denominator that only has these prime factors can be converted to a power of 10, by multiplying both numerator and denominator with enough twos and fives. If a denominator has a different prime factor, then there’s nothing we can do.)

To conclude out excursion, we switch back to base 2:

• 0.5 = 1/2 can be represented with base 2, because the denominator is already a power of 2.
• 0.25 = 1/4 can be represented with base 2, because the denominator is already a power of 2.
• 0.1 = 1/10 cannot be represented, because the denominator cannot be converted to a power of 2.
• 0.2 = 2/10 cannot be represented, because the denominator cannot be converted to a power of 2.

Now we can see why 0.1 + 0.2 doesn’t produce a precise result: Internally, neither of the two operands can be represented precisely.

The only way to compute precisely with decimal fractions is by internally switching to base 10. For many programming languages, base 2 is the default and base 10 an option. For example, Java has the class BigDecimal and Python has the module decimal. There are tentative plans to add something similar to JavaScript: The ECMAScript proposal “Decimal” is currently at stage 0.

reference

http://exploringjs.com/impatient-js/ch_numbers.html#background-floating-point-precision

Functions for converting numbers to integers. Note how things change with negative numbers, because “larger” always means “closer to positive infinity”.

当一个goroutine尝试在一个channel上做send或者receive操作时，这个goroutine会阻塞在调用处，直到另一个goroutine从这个channel里接收或者写入值，这样两个goroutine才会继续执行channel操作之后的逻辑。在这个例子中，每一个fetch函数在执行时都会往channel里发送一个值(ch <- expression)，主函数负责接收这些值(<-ch)。这个程序中我们用main函数来接收所有fetch函数传回的字符串，可以避免在goroutine异步执行还没有完成时main函数提前退出。