Haskell：检查字符串是否是有效的数字 [英] Haskell: check if string is valid number

问题描述

如何在检查字符串时检查小数点是否是有效数字？

我在想的是我使用类似下面的内容，但添加代码检查小数点！

  isNumber :: String  - > Bool 
 isNumber xs = all isDigit xs ||在这里添加额外的代码
  

在EBNF中定义的有效数字为：

 数字 - > .digit + |数字+ [.digit *] 
  

例如，.5,1.5,1,1都是有效的数字。 +
表示一个或多个匹配项，*表示零个或多个匹配项。

解决方案

这里有一个简单的策略： p>

1. 删除字符串开头的所有数字。



2. 剩余的字符串应该现在是
   
   

   a）空字符串，或者
   
   小数点后跟所有数字。

好的，差不多。这也会匹配空字符串和。，但我们可以将它们视为特殊情况。 p>

转换为Haskell：

  isNumber :: String  - > Bool 
 isNumber= False 
 isNumber。 = False 
 isNumber xs = 
 case dropWhile isDigit xs of 
 - > True 
（'。'：ys） - > all isDigit ys 
 _  - >假
  

How do I check for a decimal point when checking a string is a valid number?

What I am thinking is that I use something like the following, but add code to check for the decimal point!

isNumber :: String -> Bool
isNumber xs = all isDigit xs || add extra code here

Where a valid number is defined in EBNF as:

number -> .digit+ | digit+ [ .digit*]

For example, .5, 1.5, 1, 1. are all valid numbers. + signifies one or more occurrences, and * denotes zero or more.

解决方案

Here's a simple strategy:

1. Strip off all the digits at the beginning of the string.

2. The remaining string should now be either

   a) the empty string, or

   b) a decimal point followed by all digits.

Well, almost. This would also match the empty string "" and "." but we can treat those as special cases.

Translated to Haskell:

isNumber :: String -> Bool
isNumber ""  = False
isNumber "." = False
isNumber xs  =
  case dropWhile isDigit xs of
    ""       -> True
    ('.':ys) -> all isDigit ys
    _        -> False

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