scala用于yield设置值

发布于 2024-08-26 07:07:52 字数 399 浏览 7 评论 0原文

我想创建 GridBagPanel.Constraints 列表。我在 scala 编程书中读到，有一个很酷的 for-yield 构造，但我可能还没有理解它的正确工作方式，因为我的代码无法编译。如下：

        val d = for {
            i <- 0 until 4
            j <- 0 until 4
        } yield {
            c = new Constraints
            c.gridx = j
            c.gridy = i
        }

我想生成一个 List[Constraints] ，并为每个约束设置不同的 x,y 值，这样稍后当我添加组件时，它们将位于网格中。

原文

I want to create a list of GridBagPanel.Constraints.
I read it in the scala programming book, that there is a cool for-yield construction, but I probably haven't understood the way it works correctly, because my code doesn't compile. Here it is:

        val d = for {
            i <- 0 until 4
            j <- 0 until 4
        } yield {
            c = new Constraints
            c.gridx = j
            c.gridy = i
        }

I want to generate a List[Constraints] and for every constraint set different x,y values so later, when I later add the components, they're going to be in a grid.

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脸赞 2024-09-02 07:07:52

您只需在 yield 块末尾返回 c 即可获取 Constraints 集合。要让它返回 List，请使用 List 而不是 Range。就像这样：

val d = for {
            i <- List.range(0, 4)
            j <- List.range(0, 4)
        } yield {
            c = new Constraints
            c.gridx = j
            c.gridy = i
            c
        }

事实上，原始代码在 Scala 2.7 中不会执行您期望的操作，因为范围（如 Range 中）是不严格的。您可以在 Stack Overflow 或 Google 上查找它，但缺点是每次您在 d 上查找元素时，它都会创建一个新的 Constraint。 Scala 2.8 中此行为已更改。

You just need to return c at the end of the yield block to get a collection of Constraints. To get it to return a List, use a List instead of a Range. Like this:

val d = for {
            i <- List.range(0, 4)
            j <- List.range(0, 4)
        } yield {
            c = new Constraints
            c.gridx = j
            c.gridy = i
            c
        }

In fact, the original code would not do what you expected it to in Scala 2.7 because, there, ranges (as in Range) are non-strict. You may look it up on Stack Overflow or Google, but the short of it is that each time you looked up an element on d, it would create a new Constraint. This behavior has changed for Scala 2.8.

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哭泣的笑容 2024-09-02 07:07:52

试试这个：

def conCreate = { 
    val c = new Constraints
    c.gridx = j
    c.gridy = i
    c
}

val d = for( i <- 0 until 4;
             j <- 0 until 4 ) yield conCreate(i,j)

我已经用对函数的调用替换了您的调用。我已将until 替换为Iterator.range(0,4)，但我已将其返回到until。两者都是有效的代码，实际上含义相同。

Try this:

def conCreate = { 
    val c = new Constraints
    c.gridx = j
    c.gridy = i
    c
}

val d = for( i <- 0 until 4;
             j <- 0 until 4 ) yield conCreate(i,j)

I've replaced your call with a call to a function. I had replaced until with Iterator.range(0,4) but I've returned it to until. Both are valid code and actually mean the same thing.

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