标签：gopl

Go语言的比较运算符

以下摘自The Go Programming Language：

Two integers of the same type may be compared using the binary comparison operators below; the type of a comparison expression is a boolean.

== equal to
!= not equal to
< less than
<= less than or equal to
greater than
= greater than or equal to

In fact, all values of basic type—booleans, numbers, and strings—are comparable, meaning that two values of the same type may be compared using the == and != operators. Furthermore, integers, floating-point numbers, and strings are ordered by the comparison operators. The values of many other types are not comparable, and no other types are ordered.

 

Go语言的int，uint和uintptr类型

以下摘自The Go Programming Language：

There are also two types called just int and uint that are the natural or most efficient size for signed and unsigned integers on a particular platform; int is by far the most widely used numeric type. Both these types have the same size, either 32 or 64 bits, but one must not make assumptions about which; different compilers may make different choices even on identical hardware.

Finally, there is an unsigned integer type uintptr, whose width is not specified but is sufficient to hold all the bits of a pointer value. The uintptr type is used only for low-level programming, such as at the boundary of a Go program with a C library or an operating system.

Regardless of their size, int, uint, and uintptr are different types from their explicitly sized siblings. Thus int is not the same type as int32, even if the natural size of integers is 32 bits, and an explicit conversion is required to use an int value where an int32 is needed, and vice versa.

另外关于int和uint类型的使用：

Although Go provides unsigned numbers and arithmetic, we tend to use the signed int form even for quantities that can’t be negative, such as the length of an array, though uint might seem a more obvious choice. Indeed, the built-in len function returns a signed int, as in this loop which announces prize medals in reverse order:

medals := []string{“gold”, “silver”, “bronze”}
for i := len(medals) – 1; i >= 0; i– {
fmt.Println(medals[i]) // “bronze”, “silver”, “gold”
}

The alternative would be calamitous. If len returned an unsigned number, then i too would be a uint, and the condition i >= 0 would always be true by definition. After the third iteration, in which i == 0, the i– statement would cause i to become not −1, but the maximum uint value, and the evaluation of medals[i] would fail at run time, or panic, by attempting to access an element outside the bounds of the slice.

For this reason, unsigned numbers tend to be used only when their bitwise operators or peculiar arithmetic operators are required, as when implementing bit sets, parsing binary file formats, or for hashing and cryptography. They are typically not used for merely non-negative quantities.

多数时候，在Golang中应该使用int类型，即使有些变量不可能为负数。否则可能会出现严重的问题（就像上面的例子）。而uint仅仅是用在位运算，hash运算等等少数情况中。

 

Go语言的数据类型

以下摘自The Go Programming Language：

Go’s types fall into four categories: basic types, aggregate types, reference types, and interface types. Basic types, include numbers, strings, and booleans. Aggregate types—arrays and structs —form more complicated data types by combining values of several simpler ones. Reference types are a diverse group that includes pointers, slices, maps, functions, and channels, but what they have in common is that they all refer to program variables or state indirectly, so that the effect of an operation applied to one reference is observed by all copies of that reference.

 

Go语言中变量的scope

以下摘自The Go Programming Language：

A syntactic block is a sequence of statements enclosed in braces like those that surround the body of a function or loop. A name declared inside a syntactic block is not visible outside that block. The block encloses its declarations and determines their scope. We can generalize this notion of blocks to include other groupings of declarations that are not explicitly surrounded by braces in the source code; we’ll call them all lexical blocks. There is a lexical block for the entire source code, called the universe block; for each package; for each file; for each for, if, and switch statement; for each case in a switch or select statement; and, of course, for each explicit syntactic block.

A declaration’s lexical block determines its scope, which may be large or small. The declarations of built-in types, functions, and constants like int, len, and true are in the universe block and can be referred to throughout the entire program. Declarations outside any function, that is, at package level, can be referred to from any file in the same package. Imported packages, are declared at the file level, so they can be referred to from the same file, but not from another file in the same package without another import. Many declarations, like that of the variables in the function, are local, so they can be referred to only from within the same function or perhaps just a part of it.

The scope of a control-flow label, as used by break, continue, and goto statements, is the entire enclosing function.

需要注意universe block的定义：Go语言的built-in类型，函数和常量的scope都属于universe block。

Go语言的类型转换

以下摘自The Go Programming Language：

For every type T, there is a corresponding conversion operation T(x) that converts the value x to type T. A conversion from one type to another is allowed if both have the same underlying type, or if both are unnamed pointer types that point to variables of the same underlying type; these conversions change the type but not the representation of the value. If x is assignable to T, a conversion is permitted but is usually redundant.

Conversions are also allowed between numeric types, and between string and some slice types. These conversions may change the representation of the value. For instance, converting a floating-point number to an integer discards any fractional part, and converting a string to a []byte slice allocates a copy of the string data. In any case, a conversion never fails at run time.

 

Go语言的函数返回多个值

以下摘自The Go Programming Language：

Often, functions use these additional results to indicate some kind of error, either by returning an error as in the call to os.Open, or a bool, usually called ok. If a map lookup, type assertion, or channel receive appears in an assignment in which two results are expected, each produces an additional boolean result:
v, ok = m[key] // map lookup
v, ok = x.(T) // type assertion
v, ok = <-ch // channel receive
As with variable declarations, we can assign unwanted values to the blank identifier:
_, err = io.Copy(dst, src) // discard byte count
_, ok = x.(T)

map查找，类型断言和从channel接收数据都会返回两个值，其中一个是表示成功或失败的布尔值。同变量定义一样，不想要的值可以赋给_。

 

Go语言的new函数

以下摘自The Go Programming Language：

Each call to new returns a distinct variable with a unique address:
p := new(int)
q := new(int)
fmt.Println(p == q) // “false”
There is one exception to this rule: two variables whose type carries no information and is therefore of size zero, such as struct{} or [0]int, may, depending on the implementation, have the same address.
The new function is relatively rarely used because the most common unnamed variables are of struct types, for which the struct literal syntax is more flexible.

Since new is a predeclared function, not a keyword, it’s possible to redefine the name for something else within a function, for example:
func delta(old, new int) int { return new – old }
Of course, within delta, the built-in new function is unavailable.

 

Go语言的变量定义

以下摘自The Go Programming Language：

A var declaration creates a variable of a particular type, attaches a name to it, and sets its initial value. Each declaration has the general form
var name type = expression
Either the type or the = expression part may be omitted, but not both. If the type is omitted, it is determined by the initializer expression. If the expression is omitted, the initial value is the zero value for the type, which is 0 for numbers, false for booleans, “” for strings, and nil for interfaces and reference types (slice, pointer, map, channel, function). The zero value of an aggregate type like an array or a struct has the zero value of all of its elements or fields.

short variable declarations are used to declare and initialize the majority of local variables. A var declaration tends to be reserved for local variables that need an explicit type that differs from that of the initializer expression, or for when the variable will be assigned a value later and its initial value is unimportant:
i := 100 // an int
var boiling float64 = 100 // a float64
var names []string
var err error
var p Point

关于“A var declaration tends to be reserved for local variables that need an explicit type that differs from that of the initializer expression,”的理解：var boiling float64 = 100如果使用i := 100的形式来定义，则会认为i是int，而不是float。