小编dij*_*jam的帖子

我是一个学习C#的初学者，我正在制作一个模拟购物清单收据程序来管理您的购物。我生成 .txt 收据，但是右对齐字符串有问题，这是我的代码；

public static void GenerateNewReciept(Customer customer)
{
    List<ShoppingItems> customerPurchaedItems;
    try
    {
        sw = File.AppendText("receipt.txt");

        //Customer object
        Customer customers = customer;
        //List of all items in list
        List<ShoppingList> customerItems = customer.CustomerShoppingList;
        DateTime todayDandT = DateTime.Now;

        //Making reciept layout
        sw.WriteLine("Date Generated: " + todayDandT.ToString("g", CultureInfo.CreateSpecificCulture("en-us")));
        sw.WriteLine("Customer: " + customer.FName + " " + customer.SName1);

        for (int i = 0; i < customerItems.Count; i++)
        {
            customerPurchaedItems = customerItems[i].ShoppingItems;
            foreach (ShoppingItems item in customerPurchaedItems)
            {
                sw.WriteLine(String.Format("{0,0} {1,25:C2}", item.ItemName, item.Price));  
            }
        }

        sw.WriteLine("Total …

我已设法实现模拟过滤器功能(经过多次尝试)

filter' :: (Num a, Eq a) => (a -> Bool) -> [a] -> [a]
filter' a [] = []
filter' a (x:xs) = if a x
  then x : filter' a xs
  else filter' a xs

我不清楚的是类型声明

filter' :: (Num a, Eq a) => (a -> Bool) -> [a] -> [a]
-- filter' (<10) [1,2,3] 
-- output = []

我们传入(<10) [1,2,3].但是在类型声明中,(a -> Bool)我们传递一个来自列表的递归,输出为true或false.然而,表达测试(<10)?怎么样我们为什么不添加另一个Bool？

我有一个函数addIndex [1,2,3] = [(0,1),(1,2),(2,3)]应该为每一对添加一个索引我在实现它时遇到了麻烦.这就是我所做的;

addIndexs :: [Int] -> [(Int,Int)]
addIndexs [] = []
addIndexs x = zip x [1..length x]

我能够使用zip put实现它我感觉它的作弊,我希望能够在不使用任何预构建函数的情况下实现它.

我没有使用预构建功能的最接近的是

addindex x = [(i,z) | z <- x, i <-[1..length x]]

但是这会为每个元组添加所有索引

addIndex [1,2,3] = [(0,1),(1,1),(2,1),(0,2),(1,2),(2,2)...]

我只是个初学者.

我正在努力学习haskell我坚持以下问题

记下一个函数split :: RawText - > [Word],它将给定的字符串拆分为单词列表.

为了简单起见,我们假设原始文本只是一串字母和空格,可能有不规则的间距.例如:split"one two three ff"= ["one","two","three","ff"]

我一直得到["一个"***例外:recursion.hs:30:1-14:函数中的非详尽模式溢出'所以递归永远不会结束,但我有一个基本情况.

我的功能

type RawText = String
-- [Char] 'a','b','c','d','e'
type SingleWord = String
type Line = [SingleWord]
type Page = [Line]

--        [-------CHAR-----]
-- split " one two three ff " = ["one","two","three","ff"]
--                              [char],[char],[char],[char]
-- " " is delimiter for a []

split' :: RawText -> [SingleWord]
spilt' [] = []
split' xs
    | xs == [] = []
    | isBlank (head xs) = split' xs
    | …