Javascript '>' 运算符如何将字符与空格进行比较？
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How does the Javascript '>' operator compare characters with a space?
提问by Guilherme Longo
I am trying to understand this expression:
我试图理解这个表达：
((ch = stream.getChar()) > ' ')
Here, getChar()gets a character. How does this greater-than comparision operator check if any char is greater than an empty space?
在这里，getChar()得到一个字符。这个大于比较运算符如何检查是否有任何字符大于空格？
Is this possible?
这可能吗？
回答by christopher
An empty space has a character code. Even though it doesn't look like much, it still has a value. So does the character taken from the stream. Comparing the character codes of these values is what produces the output.
空白处有一个字符代码。虽然看起来不多，但还是有一定的价值的。从流中提取的字符也是如此。比较这些值的字符代码是产生输出的原因。
回答by Zirak
Let's take a gander at the language specification(the algorithm itself is described in here) (do note that it defines <, but the >operator simply flips the resulting value).
让我们看一下语言规范（算法本身在此处描述）（请注意它定义了<，但>运算符只是翻转结果值）。
What the operator does is try to convert both operands to primitive types, with a preference for numbers:
运算符所做的是尝试将两个操作数转换为原始类型，并优先考虑数字：
2. a. Let py be the result of calling ToPrimitive(y, hint Number).
2. b. Let px be the result of calling ToPrimitive(x, hint Number).
In our case, x === stream.getChar()and y === ' '. Since both of the operands are primitive strings already, that results in the original values (px = x, py = y), and we move on to:
在我们的例子中，x === stream.getChar()和y === ' '。由于两个操作数都已经是原始字符串，因此会产生原始值 ( px = x, py = y)，我们继续：
4. Else, both px and py are Strings
Now it does checks to see if any of the operands are prefixes of the other, for example:
现在它会检查是否有任何操作数是另一个操作数的前缀，例如：
'abc' > 'abcd' // false
'foo' > 'foobar' // false
Which is relevant if getChar()results in a space, since the space is a prefix of itself:
如果getChar()结果为空格，这是相关的，因为空格是其自身的前缀：
' ' > ' ' // false
We move on, to finding the first character in xand ywho're on the same position in the strings, but are different characters:
我们继续寻找第一个字符 inx和ywho 在字符串中的相同位置，但不同的字符：
Let k be the smallest nonnegative integer such that the character at position k within px is different from the character at position k within py. (There must be such a k, for neither String is a prefix of the other.)
设 k 是最小的非负整数，使得 px 中位置 k 处的字符与 py 中位置 k 处的字符不同。（必须有这样的 ak，因为 String 都不是另一个的前缀。）
(e.g., 'efg'and 'efh', we want gand h)
(例如, 'efg'and 'efh', 我们想要gand h)
The characters we've found are then converted to their integer values:
我们找到的字符然后被转换为它们的整数值：
Let m be the integer that is the code unit value for the character at position k within px.
Let n be the integer that is the code unit value for the character at position k within py.
And finally, a comparison is made:
最后，进行比较：
If m < n, return true. Otherwise, return false.
And that's how it's compared to the space.
这就是它与空间相比的方式。
tl;dr It converts both arguments to their code-unit integer representations, and compares that.
tl;dr 它将两个参数转换为它们的代码单元整数表示，并进行比较。
回答by Alberto Segura
In Javascript strings are compared in alphabetical order. These expressions are true:
在 Javascript 中，字符串按字母顺序进行比较。这些表达是正确的：
'abacus' <= 'calculator'
'abacus' < 'abate'
回答by Jamiec
In most (if not all) programming languages, characters are represented internally by a number. When you do equality/greater-than/less-than checks what you're actually checking is the underlying number.
在大多数（如果不是全部）编程语言中，字符在内部由数字表示。当您执行相等/大于/小于检查时，您实际检查的是基础数字。
hence in JS:
因此在 JS 中：
alert('c' > 'b'); // alerts true
alert('a' > 'b'); // alerts false
A space character also has a numeric representation, therefore the check is a valid one.
空格字符也有数字表示，因此检查是有效的。
回答by simplyray
[string] > [string]will compare the character(s) by their representative values (see ASCII Table)
[string] > [string]将按字符的代表值比较字符（见ASCII 表）
回答by Gilad Naaman
Characters are stored in the computer's memory as a number (usually a byte or two).
字符以数字（通常是一两个字节）的形式存储在计算机的内存中。
Each character has a unique identifying number.
每个字符都有一个唯一的标识号。
By checking if a character is greater than space, you actually comapare their place in a table.
通过检查字符是否大于空格，您实际上将它们在表中的位置进行了比较。
See http://en.wikipedia.org/wiki/ASCIIfor more.
有关更多信息，请参阅http://en.wikipedia.org/wiki/ASCII。
回答by Deleteman
Check out this link, it'll explain how the comparison works on JS: http://javascript.about.com/od/decisionmaking/a/des02.htmBasically, you're comparing the ASCII value of each character to the ASCII value of the blank space, which is also, a character and therefore, has a corresponding ASCII value.
查看此链接，它将解释比较如何在 JS 上工作：http: //javascript.about.com/od/decisionmaking/a/des02.htm基本上，您将每个字符的 ASCII 值与 ASCII 进行比较空格的值，它也是一个字符，因此具有相应的 ASCII 值。
