在Scala中，如何通过一对键值查找CSV中的elemein？

发布于 2024-09-18 05:52:02 字数 1034 浏览 5 评论 0原文

例如，从以下文件中：

Name,Surname,E-mail
John,Smith,[email protected]
Nancy,Smith,[email protected]
Jane,Doe,[email protected]
John,Doe,[email protected]

如何获取 John Doe 的电子邮件地址？

我现在使用以下代码，但现在只能指定一个关键字段：


val src = Source.fromFile(file)
val iter = src.getLines().drop(1).map(_.split(","))
var quote = ""
iter.find( _(1) == "Doe"  ) foreach (a => println(a(2)))
src.close()

我尝试编写 "iter.find( _(0) == "John" && _(1) == "Doe" )" ，但这会引发一个错误，指出只需要一个参数（将条件括在额外的一对括号中没有帮助）。

原文

For example, from a following file:

Name,Surname,E-mail
John,Smith,[email protected]
Nancy,Smith,[email protected]
Jane,Doe,[email protected]
John,Doe,[email protected]

how do I get e-mail address of John Doe?

I use the following code now, but can specify only one key field now:


val src = Source.fromFile(file)
val iter = src.getLines().drop(1).map(_.split(","))
var quote = ""
iter.find( _(1) == "Doe"  ) foreach (a => println(a(2)))
src.close()

I've tried writing "iter.find( _(0) == "John" && _(1) == "Doe" )", but this raises an error saying that only one parameter is expected (enclosing the condition into extra pair of parentheses does not help).

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淡写薰衣草的香 2024-09-25 05:52:02

下划线作为 lambda 参数的占位符并不像您想象的那样工作。

a => println(a)
// is equivalent to
println(_)

(a,b) => a + b
// is equivalent to 
_ + _

a => a + a
// is not equivalent to
_ + _

即第一个下划线表示第一个参数，第二个下划线表示第二个参数，以此类推。这就是您所看到的错误的原因——您使用了两个下划线，但只有一个参数。修复方法是使用显式版本：

iter.find( a=> a(0) == "John" && a(1) == "Doe" )

The underscore as a placeholder for a parameter to a lambda doesn't work the way that you think.

a => println(a)
// is equivalent to
println(_)

(a,b) => a + b
// is equivalent to 
_ + _

a => a + a
// is not equivalent to
_ + _

That is, the first underscore means the first parameter and the second one means the second parameter and so on. So that's the reason for the error that you're seeing -- you're using two underscores but have only one parameter. The fix is to use the explicit version:

iter.find( a=> a(0) == "John" && a(1) == "Doe" )

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做个ˇ局外人 2024-09-25 05:52:02

您可以使用正则表达式：

scala> def getRegex(v1: String, v2: String) = (v1 + "," + v2 +",(\\S+)").r
getRegex: (v1: String,v2: String)scala.util.matching.Regex

scala> val src = """John,Smith,[email protected]
     | Nancy,Smith,[email protected]
     | Jane,Doe,[email protected]
     | John,Doe,[email protected]
     | """
src: java.lang.String =
John,Smith,[email protected]
Nancy,Smith,[email protected]
Jane,Doe,[email protected]
John,Doe,[email protected]


scala> val MAIL = getRegex("John","Doe")
MAIL: scala.util.matching.Regex = John,Doe,(\S+)

scala> val itr = src.lines
itr: Iterator[String] = non-empty iterator

scala> for(MAIL(address) <- itr) println(address)
[email protected]

scala>

You can use Regex:

scala> def getRegex(v1: String, v2: String) = (v1 + "," + v2 +",(\\S+)").r
getRegex: (v1: String,v2: String)scala.util.matching.Regex

scala> val src = """John,Smith,[email protected]
     | Nancy,Smith,[email protected]
     | Jane,Doe,[email protected]
     | John,Doe,[email protected]
     | """
src: java.lang.String =
John,Smith,[email protected]
Nancy,Smith,[email protected]
Jane,Doe,[email protected]
John,Doe,[email protected]


scala> val MAIL = getRegex("John","Doe")
MAIL: scala.util.matching.Regex = John,Doe,(\S+)

scala> val itr = src.lines
itr: Iterator[String] = non-empty iterator

scala> for(MAIL(address) <- itr) println(address)
[email protected]

scala>

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·深蓝 2024-09-25 05:52:02

您还可以在 for 理解中对 split 的结果进行模式匹配。

val firstName = "John"
val surName = "Doe"
val emails = for {
  Array(`firstName`, `surName`, email) <- 
    src.getLines().drop(1) map { _ split ',' }
} yield { email }

println(emails.mkString(","))

请注意模式中的反引号：这意味着我们匹配 firstName 的值，而不是引入一个与任何内容匹配的新变量并隐藏 val firstname。

You could also do a pattern match on the result of split in a for comprehension.

val firstName = "John"
val surName = "Doe"
val emails = for {
  Array(`firstName`, `surName`, email) <- 
    src.getLines().drop(1) map { _ split ',' }
} yield { email }

println(emails.mkString(","))

Note the backticks in the pattern: this means we match on the value of firstName instead of introducing a new variable matching anything and shadowing the val firstname.

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