Question

我在Racket中遇到了这个任务5天，有人知道我该如何处理它？

给定arity 2的函数和n元素的列表，返回所有元素的字符串函数的评估，例如：

>(reduce + '(1 2 3 4 5 6 7 8 9 10))

55

> (reduce zip '((1 2 3) (4 5 6) (7 8 9)))
'((1 (4 7)) (2 (5 8)) (3 (6 9)))

Answer 1

你走了。

(define (reduce func list)
  (assert (not (null? list)))
  (if (null? (cdr list))
      (car list)
      (func (car list) (reduce func (cdr list)))))

试验：

> (reduce + '(1 2 3 4 5 6 7 8 9 10))
55
> (reduce zip '((1 2 3) (4 5 6) (7 8 9)))
((1 (4 7)) (2 (5 8)) (3 (6 9)))

为了完整性，zip的实现（假设有两个列表并且列表长度相同）是：

(define (zip l1 l2) (map list l1 l2))

Answer 2

您可以使用foldl：

表达

(define (reduce f xs)
  (and (not (empty? xs)) (foldl f (first xs) (rest xs))))

Answer 3

import UIKit

class ViewController: UIViewController, UITableViewDataSource, UITableViewDelegate{
    @IBOutlet weak var tableView: UITableView!
    var checked = [Bool]()



    let section = ["Science", "Math"]

    let items = [["Physics", "Biology", "Chemistry"], ["Algebra I", "Algebra II", "Statistics"]]






    override func viewDidLoad(){
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
         self.tableView.allowsMultipleSelection = true

    }

    override func didReceiveMemoryWarning(){
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int{
        return self.items[section].count
    }

     func numberOfSectionsInTableView(tableView: UITableView) -> Int{
        // #warning Incomplete implementation, return the number of sections

        return self.section.count

    }
    func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String?{

        return self.section[section]

    }
    //All before.

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell
    {
        let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath) 

        cell.textLabel?.text = self.items[indexPath.section][indexPath.row]

        //MARK: -Checkmark and save support.
        cell.accessoryType = cell.isSelected ? .checkmark : .none
        cell.selectionStyle = .none // to prevent cells from being "highlighted"

        return cell
    }


    //MARK: - Checkmark and save functions.


     func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        tableView.cellForRow(at: indexPath)?.accessoryType = .checkmark

    }

     func tableView(_ tableView: UITableView, didDeselectRowAt indexPath: IndexPath) {
        tableView.cellForRow(at: indexPath)?.accessoryType = .none
    }



    //MARK: - Sections




}

减少Racket的功能？

3 个答案: