Re: random selection from a list - C# Discussion Boards

5-Mar-12 14:57

hi there masters of c# code

i bow to your hard work and knowledge

i am a beginner and i would be very gratefull for full explainations and not just code snippets

my target is to create a web browser app in c# which load a difffrent website on each runtime

the string i want to run is posted below

i have about 70 webistes i wish the app to load a diffrent one from the 70 each runtime

the other thing is i want a way to make it load a diffrent user agent also each time i have also a list of about 120 user agents

thank you very much

i will post how far i got below

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            webBrowser1.Navigate("http://www.anyweb.com", "_self", null, "User-Agent: Mozilla/4.0 
        }
    }
}

PIEBALDconsult5-Mar-12 15:06

PIEBALDconsult

5-Mar-12 15:06

I would likely put the list of URLs in a database and select one at random -- but just how random do you want them?

james bradbery5-Mar-12 15:12

5-Mar-12 15:12

no need for real random , it could be one at a time as long as it doesnt keep useing same one i mean if the app runs 70 times i would expect it to choose a diffrent one each runtime same fo rteh user agents

RobCroll5-Mar-12 15:16

5-Mar-12 15:16

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        static List<string> urls;
        static Random random;

        public Form1()
        {
            InitializeComponent();
            if(urls ==  null)
            {
                urls = GetWebsiteUrls();  //Need to implement 
                random = new Random();
            }

            string url = urls[random.Next(urls.count)];
            webBrowser1.Navigate(url, "_self", null, "User-Agent: Mozilla/4.0 
        }
    }
}

cut this free hand so it may not compile but you get the idea.

"You get that on the big jobs."

james bradbery5-Mar-12 16:30

5-Mar-12 16:30

thank you that looks great

the part you said need to implement , could you elaborate or someone can ?

with regards to the user agents can the same method you have shown be used?

thanks again !

RobCroll5-Mar-12 17:08

5-Mar-12 17:08

Sure, you'll want to take the same approach for the webagents.

The GetWebsiteUrls method is a method that returns a collection of strings representing the the websites. I don't know how you are storing these values. Also the form shouldn't know anyway. In case you change the type of data source.

You could use a database if people are going to add delete entries. You could use a config file if the values rarely change or you could hard code it if they virtually never change

public List<string> GetWebsiteUrls()
{
    List<string> urls = new List<string>();
    url.Add("http://a.b.c");
    url.Add("http://a.c.c");
    ....
    return urls;
}

"You get that on the big jobs."

james bradbery5-Mar-12 17:47

5-Mar-12 17:47

yes it will be hard coded
not for user to edit

i am a bit of an amateur so i really apprcieate your help if you have any more would be great

more help with getting this working Wink | ;)

james bradbery5-Mar-12 17:56

5-Mar-12 17:56

ok i tried to put your code

and i get urls + random ( The name 'urls' does not exist in the current context

RobCroll6-Mar-12 20:07

6-Mar-12 20:07

Maybe try Form1.urls I don't have a Windows PC at hand at the moment. If that doesn't work, post the code

"You get that on the big jobs."

BobJanova5-Mar-12 22:17

BobJanova

5-Mar-12 22:17

Store the list and the current run number in state somewhere – I'd put them in a file, but you could use the registry or even a database if your app already needs one. On startup, read the run counter, increment it, re-save the state and read the appropriate URL and user agent combo. (You could read the whole list and just index into it, if it's small, which it is in your question.)

james bradbery10-Mar-12 18:22

Slope method optimisation

10-Mar-12 18:22

thanks guys great help

A*****5-Mar-12 14:51

A*****

5-Mar-12 14:51

I'm trying to optimise the following Slope method so that it runs in as few clock cycles as possible.

Any Suggestions?

public static double?[] slopeImpl(double?[] vals, int offset)
{
    double y = 0;

    int iterableLength = vals.Length;

    int numValues = vals.Length;
    double invNumValues = 1.0 / (vals.Length - offset);
    double x = (double)(((numValues * (numValues - 1)) >> 1) - ((offset * (offset - 1)) >> 1));

    for (int i = offset; i < iterableLength; i++) {

        y += (vals[i].GetType() == typeof(System.DBNull)) ? 0.0 : (double)vals[i];
    }

    y = y * invNumValues;

    double v1 = 0.0;
    double v2 = 0.0;
    double v2HalfResult = 0.0;

    for (int i = offset; i < iterableLength; i++) {
        v2HalfResult = (i - x);
        v1 += (v2HalfResult) * ((double)vals[i] - y);
        v2 += (v2HalfResult) * (v2HalfResult);
    }

    double slope = v1 / v2;
    double intercept = y - slope * x;

    double?[] result = new double?[2];
    result[0] = slope;
    result[1] = intercept;
    return result;
}

What I have considered:
1. There are two loops which are iterating the same number of times; however I am not sure if it is possible to use one loop. The second loop is dependant on the value of y which is calculated in the first loop and the line after.

2. Use a table to calculate x which is of the form
n(n-1) /2. Possible, however the values array can contain more than 365 elements.

Thanks for any help.

My blog:[^]

modified 5-Mar-12 21:35pm.

RobCroll5-Mar-12 18:24

5-Mar-12 18:24

Instead of

y += (vals[i].GetType() == typeof(System.DBNull)) ? 0.0 : (double)vals[i];

could be changed to something along the lines of

if (vals[i].HasValue) y += vals[i].Value;

But are there actually any null values? Changing the argument to just a straight out double[] would definitely help

"You get that on the big jobs."

A*****5-Mar-12 23:12

A*****

5-Mar-12 23:12

Thanks for the help, and yes the vals array can contain null values.

My blog:[^]

Luc Pattyn5-Mar-12 19:30

Luc Pattyn

5-Mar-12 19:30

No you can't avoid the two loops, you need the averages first, then the deviations, which you can't get without knowing the averages first.

IMO all you can do is clean up your code a bit, by:
- getting rid of the nullables, GetType() and typeof;
- not calling vals.Length three times;
- returning just the slope as a double;
- not calculating the intercept, unless you really need it, and then maybe provide the intercept as an out parameter (this saves the allocation of the double[2] array).

BTW: using >>1 to divide by 2 does not really help; compilers are smart enough to emit decent code to divide by 2.

BTW2: I have some doubts about the correctness of your line
double x = (double)(((numValues * (numValues - 1)) >> 1) - ((offset * (offset - 1)) >> 1));
it doesn't agree with my intuition, e.g. changing the value of offset from 0 to 1 would not change x at all??

Smile | :)

Luc Pattyn [My Articles] Nil Volentibus Arduum

A*****5-Mar-12 23:17

A*****

5-Mar-12 23:17

Thanks Luc. I was so caught up with optimising that I missed the case where offset was one will have to handle that. Good job spotting this.

My blog:[^]