如何在 Scala 中使用 foreach 循环来改变对象？
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How to use a foreach-loop in Scala that mutates an object?
提问by Jonas
I use Scala and JDBC, now I want to reuse an PreparedStatementto do a multi-insert. I want to use a foreach-loop over an array, but I can't get it right with Scala.
我使用 Scala 和 JDBC，现在我想重用 anPreparedStatement来做一个多插入。我想在数组上使用 foreach 循环，但我无法在 Scala 中正确使用。
val stmt = conn.prepareStatement(insertStatement)
// wrong Scala
items.foreach(item : MyItem =>
stmt.setInt(1, item.id)
stmt.setInt(2, item.value)
stmt.executeUpdate()
)
itemsis an array containing multiple MyItem.
items是一个包含多个MyItem.
How can I write this foreach-loop in Scala and reuse the PreparedStatement?
如何在 Scala 中编写这个 foreach 循环并重用PreparedStatement?
回答by Andrzej Doyle
You need to use curly braces for the argument to foreachif you want it to be interpreted as a multi-statement block(which you do).
foreach如果您希望将其解释为多语句块（您这样做），则需要使用大括号作为参数。
Asides from that, what you're doing looks fine. Here's a REPL session where I'm mutating an object in a foreach block in a similar way (using a StringBuilder for simplicity):
除此之外，你在做什么看起来不错。这是一个 REPL 会话，其中我以类似的方式改变 foreach 块中的对象（为简单起见，使用 StringBuilder）：
scala> val sb = new java.lang.StringBuilder
sb: java.lang.StringBuilder =
scala> val items = List("tinker", "tailor", "soldier", "spy")
items: List[java.lang.String] = List(tinker, tailor, soldier, spy)
scala> items.foreach { item =>
| sb.append(item)
| sb.append("; ")
| println(sb)
| }
tinker;
tinker; tailor;
tinker; tailor; soldier;
tinker; tailor; soldier; spy;
(And using parentheses for the foreach block leads to the error <console>:3: error: ')' expected but '.' found.)
（并且为 foreach 块使用括号会导致错误<console>:3: error: ')' expected but '.' found.）
回答by Kevin Wright
A small point, but Andrzej's answer can be cleaned up by using infix notation more liberally:
一个小问题，但 Andrzej 的答案可以通过更自由地使用中缀符号来清理：
val sb = new java.lang.StringBuilder
val items = List("tinker", "tailor", "soldier", "spy")
items foreach { item =>
sb append item
sb append "; "
println(sb)
}
Generally, it's considered more idiomatic to use the infix form for collection operations such as map, flatMap, foreachand filter
一般情况下，它被认为是更地道用于收集操作，如缀形式map，flatMap，foreach和filter
