Question

我无法弄清楚两个递归（myMaximumBy）是如何协同工作的，而我正在尝试在纸上绘制图表但是我被卡住了。例如，myMaximumBy比较[1,5,2,4,3]

myMaximumBy :: (a -> a -> Ordering) -> [a] -> a
myMaximumBy _ (x:[]) = x
myMaximumBy f (x:xs) = if (f x (myMaximumBy f xs)) == GT then x else (myMaximumBy f xs)

Answer 1

基本上你遍历整个列表，直到你击中一个元素x（第一行）。由于x是唯一的元素，因此必须是maxium。

现在你向后检查每个元素y与x：如果y大于x（第一种情况），那么你继续用y作为最大值，否则你保持x（第二种情况）。

我不会将你的定义与if子句一起使用，而是使用maxBy来说明这一点：

maximumBy f [x] = x
maximumBy f (x:xs) = maxBy f x (maximumBy f xs)

maxBy f x y | f x y == GT = x
            | otherwise   = y

此定义与您的定义相同。

示例：

maximumBy (comparing abs) [2,5,-3,1]
== maxBy (comparing abs) 2 (maxBy (comparing abs) 5 (maxBy (comparing abs) -3 (maximumBy (comparing abs) [1])))
== maxBy (comparing abs) 2 (maxBy (comparing abs) 5 (maxBy (comparing abs) -3 1))
== maxBy (comparing abs) 2 (maxBy (comparing abs) 5 -3)
== maxBy (comparing abs) 2 5
== 5

有人可以解释下面的代码如何工作？

1 个答案: