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  • MySQL table organization and optimization (Rails)

    - by aguynamedloren
    I've been learning Ruby on Rails over the past few months with no prior programming experience. Lately, I've been thinking about database optimization and table organization. I know there are great books on the subject, but I typically learn by example / as I go. Here's a hypothetical situation: Let's say I am building a social network for a niche community with 250,000 members (users). The users have the ability to attend events. Let's say there are 50,000 past/present/future events. Much like Facebook events, a user can attend any number of events and an event can have any number of attendees. In the database, there would be a table for users and a table for events. Somehow I would have to create an association between the users and events. I could create an "events" column in the users table such that each user row would contain a hash of event IDs, or I could create an "attendees" column in the events table such that each event row would contain a hash of user IDs. Neither of these solutions seem ideal, however. On a users profile page, I want to display the list of events they are associated with, which would require scanning the 50,000 event rows for the user ID of said user if I include an "attendees" column in the events table. Likewise, on an event page, I want to display a list of attendees for the event, which would require scanning the 250,000 user rows for the event ID of said event if I include an "events" column in the users table. Option 3 would be to create a third table that contains the attendee information for each and every event - but I don't see how this would solve any problems. Are these non-issues? Rails makes accessing all of this information easy, but I guess I'm worried about scale. It is entirely possible that I am under-estimating the speed and processing power of modern databases / servers / etc. How long would it take to scan 250,000 user rows for specific event IDs - 10ms? 100ms? 1,000ms? I guess that's not that bad. Am I just over-thinking this?

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  • Regular Expression for Decimal or Blank

    - by Phil P
    Sorry for the potentially dumb question but I am trying to pull together a regular expression that will allow: A number with 1 or 2 numbers before a decimal point, and 0-6 numbers after the decimal point. However I also need to allow the field to be blank if so required. Valid Examples 0.952321 1.20394 12.12 25 Blank Invalid Examples 123.45678 1.1234567 Please can anyone help?

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  • LINQ aggregate left join on SQL CE

    - by P Daddy
    What I need is such a simple, easy query, it blows me away how much work I've done just trying to do it in LINQ. In T-SQL, it would be: SELECT I.InvoiceID, I.CustomerID, I.Amount AS AmountInvoiced, I.Date AS InvoiceDate, ISNULL(SUM(P.Amount), 0) AS AmountPaid, I.Amount - ISNULL(SUM(P.Amount), 0) AS AmountDue FROM Invoices I LEFT JOIN Payments P ON I.InvoiceID = P.InvoiceID WHERE I.Date between @start and @end GROUP BY I.InvoiceID, I.CustomerID, I.Amount, I.Date ORDER BY AmountDue DESC The best equivalent LINQ expression I've come up with, took me much longer to do: var invoices = ( from I in Invoices where I.Date >= start && I.Date <= end join P in Payments on I.InvoiceID equals P.InvoiceID into payments select new{ I.InvoiceID, I.CustomerID, AmountInvoiced = I.Amount, InvoiceDate = I.Date, AmountPaid = ((decimal?)payments.Select(P=>P.Amount).Sum()).GetValueOrDefault(), AmountDue = I.Amount - ((decimal?)payments.Select(P=>P.Amount).Sum()).GetValueOrDefault() } ).OrderByDescending(row=>row.AmountDue); This gets an equivalent result set when run against SQL Server. Using a SQL CE database, however, changes things. The T-SQL stays almost the same. I only have to change ISNULL to COALESCE. Using the same LINQ expression, however, results in an error: There was an error parsing the query. [ Token line number = 4, Token line offset = 9,Token in error = SELECT ] So we look at the generated SQL code: SELECT [t3].[InvoiceID], [t3].[CustomerID], [t3].[Amount] AS [AmountInvoiced], [t3].[Date] AS [InvoiceDate], [t3].[value] AS [AmountPaid], [t3].[value2] AS [AmountDue] FROM ( SELECT [t0].[InvoiceID], [t0].[CustomerID], [t0].[Amount], [t0].[Date], COALESCE(( SELECT SUM([t1].[Amount]) FROM [Payments] AS [t1] WHERE [t0].[InvoiceID] = [t1].[InvoiceID] ),0) AS [value], [t0].[Amount] - (COALESCE(( SELECT SUM([t2].[Amount]) FROM [Payments] AS [t2] WHERE [t0].[InvoiceID] = [t2].[InvoiceID] ),0)) AS [value2] FROM [Invoices] AS [t0] ) AS [t3] WHERE ([t3].[Date] >= @p0) AND ([t3].[Date] <= @p1) ORDER BY [t3].[value2] DESC Ugh! Okay, so it's ugly and inefficient when run against SQL Server, but we're not supposed to care, since it's supposed to be quicker to write, and the performance difference shouldn't be that large. But it just doesn't work against SQL CE, which apparently doesn't support subqueries within the SELECT list. In fact, I've tried several different left join queries in LINQ, and they all seem to have the same problem. Even: from I in Invoices join P in Payments on I.InvoiceID equals P.InvoiceID into payments select new{I, payments} generates: SELECT [t0].[InvoiceID], [t0].[CustomerID], [t0].[Amount], [t0].[Date], [t1].[InvoiceID] AS [InvoiceID2], [t1].[Amount] AS [Amount2], [t1].[Date] AS [Date2], ( SELECT COUNT(*) FROM [Payments] AS [t2] WHERE [t0].[InvoiceID] = [t2].[InvoiceID] ) AS [value] FROM [Invoices] AS [t0] LEFT OUTER JOIN [Payments] AS [t1] ON [t0].[InvoiceID] = [t1].[InvoiceID] ORDER BY [t0].[InvoiceID] which also results in the error: There was an error parsing the query. [ Token line number = 2, Token line offset = 5,Token in error = SELECT ] So how can I do a simple left join on a SQL CE database using LINQ? Am I wasting my time?

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  • php codeigniter mail function not working

    - by Hmwd
    Hi, i wa trying to do mail with php codeigner, but unfortunately its not working. the error which i got was the following; somebody please help me.. A PHP Error was encountered Severity: Warning Message: fsockopen() [function.fsockopen]: unable to connect to localhost:8025 (Connection refused) Filename: libraries/smtp.php Line Number: 105 Thanks in advance Ahamed

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  • Python Expand Tabs Length Calculation

    - by Mithrill
    I'm confused by how the length of a string is calculated when expandtabs is used. I thought expandtabs replaces tabs with the appropriate number of spaces (with the default number of spaces per tab being 8). However, when I ran the commands using strings of varying lengths and varying numbers of tabs, the length calculation was different than I thought it would be (i.e., each tab didn't always result in the string length being increased by 8 for each instance of "/t"). Below is a detailed script output with comments explaining what I thought should be the result of the command executed above. Would someone please explain the how the length is calculated when expand tabs is used? IDLE 2.6.5 >>> s = '\t' >>> print len(s) 1 >>> #the length of the string without expandtabs was one (1 tab counted as a single space), as expected. >>> print len(s.expandtabs()) 8 >>> #the length of the string with expandtabs was eight (1 tab counted as eight spaces). >>> s = '\t\t' >>> print len(s) 2 >>> #the length of the string without expandtabs was 2 (2 tabs, each counted as a single space). >>> print len(s.expandtabs()) 16 >>> #the length of the string with expandtabs was 16 (2 tabs counted as 8 spaces each). >>> s = 'abc\tabc' >>> print len(s) 7 >>> #the length of the string without expandtabs was seven (6 characters and 1 tab counted as a single space). >>> print len(s.expandtabs()) 11 >>> #the length of the string with expandtabs was NOT 14 (6 characters and one 8 space tabs). >>> s = 'abc\tabc\tabc' >>> print len(s) 11 >>> #the length of the string without expandtabs was 11 (9 characters and 2 tabs counted as a single space). >>> print len(s.expandtabs()) 19 >>> #the length of the string with expandtabs was NOT 25 (9 characters and two 8 space tabs). >>>

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  • Using Jquery.Form Plugin + MultiFile to automatically upload a single file

    - by Alan Neal
    I wanted to find a way to upload a single file*, in the background, have it start automatically after file selection, and not require a flash uploader, so I am trying to use two great mechanisms (jQuery.Form and JQuery MultiFile) together. I haven't succeeded, but I'm pretty sure it's because I'm missing something fundamental. Just using MultiFile, I define the form as follows... <form id="photoForm" action="image.php" method="post" enctype="multipart/form-data"> The file input button is defined as... <input id="photoButton" "name="sourceFile" class="photoButton max-1 accept-jpg" type="file"> And the Javascript is... $('#photoButton').MultiFile({ afterFileSelect: function(){ document.getElementById("photoForm").submit(); } }); This works perfectly. As soon as the user selects a single file, MultiFile submits the form to the server. If instead of using MultiFile, as shown above, let's say I include a Submit button along with the JQuery Form plugin defined as follows... var options = { success: respondToUpload }; $('#photoForm').ajaxForm(options); ... this also works perfectly. When the Submit button is clicked, the form is uploaded in the background. What I don't know how to do is get these two to work together. If I use Javascript to submit the form (as shown in the MultiFile example above), the form is submitted but the JQuery.Form function is not called, so the form does not get submitted in the background. I thought that maybe I needed to change the form registration as follows... $('#photoForm').submit(function() { $('#photoForm').ajaxForm(options); }); ...but that didn't solve the problem. The same is true when I tried .ajaxSubmit instead of .ajaxForm. What am I missing? BTW: I know it might sound strange to use MultiFile for single-file uploads, but the idea is that the number of files will be dynamic based on the user's account. So, I'm starting with one but the number changes depending on conditions.

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  • Passcode functionality for iPhone app?

    - by David Liu
    I was thinking about putting in PIN number functionality within my iPhone app (various examples can be seen here http://forums.mint.com/showthread.php?t=5956 ), but I was kinda stumped how to go about it. Should I just go with a hidden textfield and simulate the inputs with images?

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  • Derivative of a Higher-Order Function

    - by Claudiu
    This is in the context of Automatic Differentiation - what would such a system do with a function like map, or filter - or even one of the SKI Combinators? Example: I have the following function: def func(x): return sum(map(lambda a: a**x, range(20))) What would its derivative be? What will an AD system yield as a result? (This function is well-defined on real-number inputs).

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  • scripts on Cshell

    - by lego69
    hello, I've got some problem, I have list of data in the file: 053-37878 03828008 Moskovitch James 500 052-34363 01234567 Mendelson Kippi 450 053-32322 03828008 Jameson Shula 350 054-39238 03333333 Merden Moshe 300 is it possible rewrite this list in the same file (without using temporary file) but without last number thanks in advance for any help (I'm talking about C-Shell scripts)

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  • Are hash collisions with different file sizes just as likely as same file size?

    - by rwmnau
    I'm hashing a large number of files, and to avoid hash collisions, I'm also storing a file's original size - that way, even if there's a hash collision, it's extremely unlikely that the file sizes will also be identical. Is this sound (a hash collision is equally likely to be of any size), or do I need another piece of information (if a collision is more likely to also be the same length as the original). Or, more generally: Is every file just as likely to produce a particular hash, regardless of original file size?

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  • Java - Highest, Lowest and Average

    - by Emily
    Hello, I've just started studying and I need help on one of my exercises. I need the end user to input a rain fall number for each month. I then need to out put the average rainfall, highest month and lowest month and the months which rainfall was above average. I keep getting the same number in the highest and lowest and I have no idea why. I am seriously pulling my hair out. Any help would be greatly appreciated. This is what I have so far: public class rainfall { /** * @param args */ public static void main(String[] args) { int[] numgroup; numgroup = new int [13]; ConsoleReader console = new ConsoleReader(); int highest; int lowest; int index; int tempVal; int minMonth; int minIndex; int maxMonth; int maxIndex; System.out.println("Welcome to Rainfall"); for(index = 1; index < 13; index = index + 1) { System.out.println("Please enter the rainfall for month " + index); tempVal = console.readInt(); while (tempVal>100 || tempVal<0) { System.out.println("The rating must be within 0...100. Try again"); tempVal = console.readInt(); } numgroup[index] = tempVal; } lowest = numgroup[0]; for(minIndex = 0; minIndex < numgroup.length; minIndex = minIndex + 1); { if (numgroup[0] < lowest) { lowest = numgroup[0]; minMonth = minIndex; } } highest = numgroup[1]; for(maxIndex = 0; maxIndex < numgroup.length; maxIndex = maxIndex + 1); { if (numgroup[1] > highest) { highest = numgroup[1]; maxMonth = maxIndex; } } System.out.println("The average monthly rainfall was "); System.out.println("The lowest monthly rainfall was month " + minIndex); System.out.println("The highest monthly rainfall was month " + maxIndex); System.out.println("Thank you for using Rainfall"); } private static ConsoleReader ConsoleReader() { return null; } } Thanks, Emily

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  • Maven archetype for basic spring hibernate struts2 application

    - by Shekhar
    Hello I am looking for a basic maven archetype which can help me create a spring hibernate struts2 application. I have tried using appfuse but it sucks. It took more than half an hour to download n number of dependencies and the project it created was having errors also. if you know anyother maven archetype please tell me. Thanks Shekhar

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  • Error using `loess.smooth` but not `loess` or `lowess`

    - by Sandy
    I need to smooth some simulated data, but occasionally run into problems when the simulated ordinates to be smoothed are mostly the same value. Here is a small reproducible example of the simplest case. > x <- 0:50 > y <- rep(0,51) > loess.smooth(x,y) Error in simpleLoess(y, x, w, span, degree, FALSE, FALSE, normalize = FALSE, : NA/NaN/Inf in foreign function call (arg 1) loess(y~x), lowess(x,y), and their analogue in MATLAB produce the expected results without error on this example. I am using loess.smooth here because I need the estimates evaluated at a set number of points. According to the documentation, I believe loess.smooth and loess are using the same estimation functions, but the former is an "auxiliary function" to handle the evaluation points. The error seems to come from a C function: > traceback() 3: .C(R_loess_raw, as.double(pseudovalues), as.double(x), as.double(weights), as.double(weights), as.integer(D), as.integer(N), as.double(span), as.integer(degree), as.integer(nonparametric), as.integer(order.drop.sqr), as.integer(sum.drop.sqr), as.double(span * cell), as.character(surf.stat), temp = double(N), parameter = integer(7), a = integer(max.kd), xi = double(max.kd), vert = double(2 * D), vval = double((D + 1) * max.kd), diagonal = double(N), trL = double(1), delta1 = double(1), delta2 = double(1), as.integer(0L)) 2: simpleLoess(y, x, w, span, degree, FALSE, FALSE, normalize = FALSE, "none", "interpolate", control$cell, iterations, control$trace.hat) 1: loess.smooth(x, y) loess also calls simpleLoess, but with what appears to be different arguments. Of course, if you vary enough of the y values to be nonzero, loess.smooth runs without error, but I need the program to run in even the most extreme case. Hopefully, someone can help me with one and/or all of the following: Understand why only loess.smooth, and not the other functions, produces this error and find a solution for this problem. Find a work-around using loess but still evaluating the estimate at a specified number of points that can differ from the vector x. For example, I might want to use only x <- seq(0,50,10) in the smoothing, but evaluate the estimate at x <- 0:50. As far as I know, using predict with a new data frame will not properly handle this situation, but please let me know if I am missing something there. Handle the error in a way that doesn't stop the program from moving onto the next simulated data set. Thanks in advance for any help on this problem.

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  • monitor table in data base

    - by hatem gamil
    hi all i want to make a web site to act as a monitor to a certain table in data base or act as a listenter on that table eg lets say i have table employees i want to make a web page that listen to changes occurs on that table (all DML operations) whenever a record is inserted i want this page alert me that a "1 row is inserted in table employee",or updated i want to get an alert " row number xx is updated" and so on so what is the best practise to do so thnx

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  • BST insert operation. don't insert a node if a duplicate exists already

    - by jeev
    the following code reads an input array, and constructs a BST from it. if the current arr[i] is a duplicate, of a node in the tree, then arr[i] is discarded. count in the struct node refers to the number of times a number appears in the array. fi refers to the first index of the element found in the array. after the insertion, i am doing a post-order traversal of the tree and printing the data, count and index (in this order). the output i am getting when i run this code is: 0 0 7 0 0 6 thank you for your help. Jeev struct node{ int data; struct node *left; struct node *right; int fi; int count; }; struct node* binSearchTree(int arr[], int size); int setdata(struct node**node, int data, int index); void insert(int data, struct node **root, int index); void sortOnCount(struct node* root); void main(){ int arr[] = {2,5,2,8,5,6,8,8}; int size = sizeof(arr)/sizeof(arr[0]); struct node* temp = binSearchTree(arr, size); sortOnCount(temp); } struct node* binSearchTree(int arr[], int size){ struct node* root = (struct node*)malloc(sizeof(struct node)); if(!setdata(&root, arr[0], 0)) fprintf(stderr, "root couldn't be initialized"); int i = 1; for(;i<size;i++){ insert(arr[i], &root, i); } return root; } int setdata(struct node** nod, int data, int index){ if(*nod!=NULL){ (*nod)->fi = index; (*nod)->left = NULL; (*nod)->right = NULL; return 1; } return 0; } void insert(int data, struct node **root, int index){ struct node* new = (struct node*)malloc(sizeof(struct node)); setdata(&new, data, index); struct node** temp = root; while(1){ if(data<=(*temp)->data){ if((*temp)->left!=NULL) *temp=(*temp)->left; else{ (*temp)->left = new; break; } } else if(data>(*temp)->data){ if((*temp)->right!=NULL) *temp=(*temp)->right; else{ (*temp)->right = new; break; } } else{ (*temp)->count++; free(new); break; } } } void sortOnCount(struct node* root){ if(root!=NULL){ sortOnCount(root->left); sortOnCount(root->right); printf("%d %d %d\n", (root)->data, (root)->count, (root)->fi); } }

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  • How to write "Hello World" Program for MediaTek SDK?

    - by Mediatek Beginner
    Hi Friends, I know these days number of mobile handsets are increasing and Vendors are trying to produce low cost handsets and some how I got to know that MediaTek SDK is the right one to write program for these kind of handsets. Is there any one who knows which IDE should I use to write programs for these kind of handsets? Is there any source code and sample available? I know that Fly, Karbonn, Lava Mobiles are Mediatek sdk based. Please Help Me. Regards, Mediatek Beginner

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  • Run all SQL files in a directory

    - by Khalil Dahab
    I have a number of .sql files which I have to run in order to apply changes made by other developers on an SQL Server 2005 database. The files are named according to the following pattern: 0001 - abc.sql 0002 - abcef.sql 0003 - abc.sql ... Is there a way to run all of them in one go?

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