Question

我正在寻找一种更聪明，更有希望更快的方法来遍历数据帧的行：

鉴于此类数据框：

d <- data.frame(x=1:10, y=rep(NA,10))

我希望第i行的列“y”是x的第i和第（i-1）个值的总和：

y[i] = x[i] + x[i-1]

R中的

for( i in 1:nrow(d)) d$y[i] <- ifelse( i>1, d$x[i] + d$x[i-1], d$x[i])

但是在R中循环数据帧绝不是最好的主意，可以对这种情况进行处理或解决吗？

Answer 1

以下是使用sapply

的一种方法

sapply(1:nrow(d), function(i) sum(d$x[(i-1):i]))
# [1]  1  3  5  7  9 11 13 15 17 19

另一个使用rollsum zoo个包

library(zoo)
rollsum(x = d$x, k = 2, align = 'right', fill = d$x[1])
# [1]  1  3  5  7  9 11 13 15 17 19

Answer 2

我的经验是Ulrich的TTR包快速完成这项任务

import UIKit
import Foundation
import Firebase
import FirebaseDatabase
import FirebaseStorage

struct postStruct {
    let username : String!
    let message : String!
    let photoURL : String!
}

class GeneralChatroom: UIViewController, UITableViewDataSource, UITableViewDelegate, UITextFieldDelegate {

    @IBOutlet weak var messageTextField: UITextField!

    var generalRoomDataArr = [postStruct]()

    @IBOutlet weak var tableView: UITableView!

    override func viewDidLoad() {
        super.viewDidLoad()

        tableView.rowHeight = UITableViewAutomaticDimension
        tableView.estimatedRowHeight = 140



        let ref = FIRDatabase.database().reference()
        let userID = FIRAuth.auth()?.currentUser?.uid




        ref.child("general_room").child("chat").child(userID!).queryOrderedByKey().observe(.childAdded, with: {snapshot in

            let snapDict = snapshot.value as? NSDictionary
            let username = snapDict?["Username"] as? String ?? ""
            let message = snapDict?["Message"] as? String ?? ""
            let firebaseUserPhotoURL = snapDict?["photo_url"] as? String ?? ""

            self.generalRoomDataArr.insert(postStruct(username: username, message: message, photoURL: firebaseUserPhotoURL), at: 0)
            self.tableView.reloadData()

        })

    }



    @IBAction func backButtonPressed(_ sender: UIButton) {
        self.performSegue(withIdentifier: "BackToRoom", sender: nil)
    }










    //Message Send button is pressed data uploaded to firebase
    @IBAction func sendButtonPressed(_ sender: UIButton) {

        let message : String = self.messageTextField.text!

        UploadGeneralChatRoom(message: message) //upload to general_room

        self.messageTextField.text = nil
        messageTextField.resignFirstResponder()//Quit keyboard
        self.tableView.reloadData() //Reload tableView
        //UploadUserData() //Update Rank in database
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return generalRoomDataArr.count // your number of cell here
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        let cell = tableView.dequeueReusableCell(withIdentifier: "cell")



        let usernameLabel = cell?.viewWithTag(1) as! UILabel
        usernameLabel.text = generalRoomDataArr[indexPath.row].username

        let messageLabel = cell?.viewWithTag(2) as! UILabel
        messageLabel.numberOfLines=0 // line wrap
        messageLabel.lineBreakMode = NSLineBreakMode.byWordWrapping
        messageLabel.text = generalRoomDataArr[indexPath.row].message



        //initialize UI Profile Image
        let imageView = cell?.viewWithTag(3) as! UIImageView

        //Make Porfile Image Cirlce
        imageView.layer.cornerRadius = imageView.frame.size.width/2
        imageView.clipsToBounds = true

        //User Profile image in tableview
        if generalRoomDataArr[indexPath.row].photoURL != nil
        {
            //let imageView = cell?.viewWithTag(3) as! UIImageView

            if let url = NSURL(string: generalRoomDataArr[indexPath.row].photoURL) {

                if let data = NSData(contentsOf: url as URL) {

                    imageView.image = UIImage(data: data as Data)
                }
            }
        }







        // your cell coding
        return cell!
    }


}//END CLASS

更聪明的方法是避免循环数据帧？

2 个答案: