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• Code Golf: Evaluating Mathematical Expressions

  - by Noldorin

  Challenge Here is the challenge (of my own invention, though I wouldn't be surprised if it has previously appeared elsewhere on the web). Write a function that takes a single argument that is a string representation of a simple mathematical expression and evaluates it as a floating point value. A "simple expression" may include any of the following: positive or negative decimal numbers, +, -, *, /, (, ). Expressions use (normal) infix notation. Operators should be evaluated in the order they appear, i.e. not as in BODMAS, though brackets should be correctly observed, of course. The function should return the correct result for any possible expression of this form. However, the function does not have to handle malformed expressions (i.e. ones with bad syntax). Examples of expressions: 1 + 3 / -8 = -0.5 (No BODMAS) 2*3*4*5+99 = 219 4 * (9 - 4) / (2 * 6 - 2) + 8 = 10 1 + ((123 * 3 - 69) / 100) = 4 2.45/8.5*9.27+(5*0.0023) = 2.68... Rules I anticipate some form of "cheating"/craftiness here, so please let me forewarn against it! By cheating, I refer to the use of the eval or equivalent function in dynamic languages such as JavaScript or PHP, or equally compiling and executing code on the fly. (I think my specification of "no BODMAS" has pretty much guaranteed this however.) Apart from that, there are no restrictions. I anticipate a few Regex solutions here, but it would be nice to see more than just that. Now, I'm mainly interested in a C#/.NET solution here, but any other language would be perfectly acceptable too (in particular, F# and Python for the functional/mixed approaches). I haven't yet decided whether I'm going to accept the shortest or most ingenious solution (at least for the language) as the answer, but I would welcome any form of solution in any language, except what I've just prohibited above! My Solution I've now posted my C# solution here (403 chars). Update: My new solution has beaten the old one significantly at 294 chars, with the help of a bit of lovely regex! I suspected that this will get easily beaten by some of the languages out there with lighter syntax (particularly the funcional/dynamic ones), and have been proved right, but I'd be curious if someone could beat this in C# still. Update I've seen some very crafty solutions already. Thanks to everyone who has posted one. Although I haven't tested any of them yet, I'm going to trust people and assume they at least work with all of the given examples. Just for the note, re-entrancy (i.e. thread-safety) is not a requirement for the function, though it is a bonus. Format Please post all answers in the following format for the purpose of easy comparison: Language Number of characters: ??? Fully obfuscated function: (code here) Clear/semi-obfuscated function: (code here) Any notes on the algorithm/clever shortcuts it takes.

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• How can I combine several Expressions into a fast method?

  - by chillitom

  Suppose I have the following expressions: Expression<Action<T, StringBuilder>> expr1 = (t, sb) => sb.Append(t.Name); Expression<Action<T, StringBuilder>> expr2 = (t, sb) => sb.Append(", "); Expression<Action<T, StringBuilder>> expr3 = (t, sb) => sb.Append(t.Description); I'd like to be able to compile these into a method/delegate equivalent to the following: void Method(T t, StringBuilder sb) { sb.Append(t.Name); sb.Append(", "); sb.Append(t.Description); } What is the best way to approach this? I'd like it to perform well, ideally with performance equivalent to the above method. UPDATE So, whilst it appears that there is no way to do this directly in C#3 is there a way to convert an expression to IL so that I can use it with System.Reflection.Emit?

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• Getting Dynamic in SSIS Queries

  - by ejohnson2010

  When you start working with SQL Server and SSIS, it isn’t long before you find yourself wishing you could change bits of SQL queries dynamically. Most commonly, I see people that want to change the date portion of a query so that you can limit your query to the last 30 days, for example. This can be done using a combination of expressions and variables. I will do this in two parts, first I will build a variable that will always contain the 1 st day of the previous month and then I will dynamically...(read more)

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• How do I test complex expressions with bash?

  - by router

  I can only do simply expressions like 1=1 and 0=1 test 1 = 1 -a 0 = 1 But how to do complex expressions like 1=1 and (0=1 or 1=1) ?

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• rm command and regular expressions via Linux BASH shell

  - by PeanutsMonkey

  I am attempting to use regular expressions to remove set of files however the bash shell returns the message rm: cannot remove `[0-99]+ -': No such file or directory rm: cannot remove `[a-zA-Z': No such file or directory rm: cannot remove `]+.[a-z]+': No such file or directory The command is [0-99]+\ - [a-zA-Z ]+\.[a-z]+ Questions Can I use regular expressions? If yes, how do I use them with commands such as rm, mkdir, etc

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• String manipulation of type String substitution in mathematical expression

  - by Peterstone

  Imagine something like exp(49/200)+(x-49/200) I want to pass as argument of the function "roundn" whatever operation that is not a addtion or a subtraction So my expresion became roundn(exp(roundn(49/200,n)),n)+(x - roundn(49/200,n) Well the expression I want to manipulate is this: exp(49/200)+exp(49/200)*(x-49/200)+1/2*exp(49/200)*(x-49/200)^2+1/6*exp(49/200)*(x- 49/200)^3+1/24*exp(49/200)*(x-49/200)^4+1/120*exp(49/200)*(x-49/200)^5+1/720*exp(49/200)*(x-49/200)^6+1/5040*exp(49/200)*(x-49/200)^7+1/40320*exp(49/200)*(x-49/200)^8+1/362880*exp(49/200)*(x-49/200)^9+1/3628800*exp(49/200)*(x-49/200)^10+1/39916800*exp(49/200)*(x-49/200)^11 I´m looking for a method (That include whatever program) not based in lenguage programming, as much batch or somithing like that... Thank you!

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• Mathematical modulus in c#

  - by penguat

  Is there a library function in c# for the mathematical modulus of a number - by this I specifically mean that a negative integer modulo a positive integer should yield a positive result.

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• How To Use Regular Expressions for Data Validation and Cleanup

  You need to provide data validation at the server level for complex strings like phone numbers, email addresses, etc. You may also need to do data cleanup / standardization before moving it from source to target. Although SQL Server provides a fair number of string functions, the code developed with these built-in functions can become complex and hard to maintain or reuse. The Future of SQL Server Monitoring "Being web-based, SQL Monitor 2.0 enables you to check on your servers from almost any location" Jonathan Allen.Try SQL Monitor now.

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• Using LINQ Lambda Expressions to Design Customizable Generic Components

  LINQ makes code easier to write and maintain by abstracting the data source. It provides a uniform way to handle widely diverse data structures within an application. LINQ’s Lambda syntax is clever enough to even allow you to create generic building blocks with hooks, into which you can inject arbitrary functions. Michael Sorens explains, and demonstrates with examples. span.fullpost {display:none;}

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• Using LINQ Lambda Expressions to Design Customizable Generic Components

  LINQ makes code easier to write and maintain by abstracting the data source. It provides a uniform way to handle widely diverse data structures within an application. LINQ’s Lambda syntax is clever enough even to allow you to create generic building blocks with hooks into which you can inject arbitrary functions. Michael Sorens explains, and demonstrates with examples.

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• Databinder.Eval using Lamda Expressions

  Using lambda expression to help with compile time checking of Eval statements...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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• List querying with Lamda Expressions in C#.NET

  - by Pavan Kumar Pabothu

  public class Employees {     public int EmployeeId { get; set; }     public string Name { get; set; }     public decimal Salary { get; set; } } List<Employees> employeeList = new List<Employees>(); List<Employees> resultList = new List<Employees>(); decimal maxSalary; List<string> employeeNames = new List<string>(); protected void Page_Load(object sender, EventArgs e) {     if (!IsPostBack)     {         FillEmployees();     }     // Getting a max salary     maxSalary = employeeList.Max((emp) => emp.Salary);     // Filtering a List     resultList = employeeList.Where((emp) => emp.Salary > 50000).ToList();     // Sorting a List     // To get a descending order replace OrderBy with OrderByDescending     resultList = employeeList.OrderBy<Employees, decimal>((emp) => emp.Salary).ToList();     // Get the List of employee names only     employeeNames = employeeList.Select<Employees, string>(emp => emp.Name).ToList();        // Getting a customized object with a given list     var employeeResultSet = employeeList.Select((emp) => new { Name = emp.Name, BigSalary = emp.Salary > 50000 }).ToList(); } private void FillEmployees() {     employeeList.Add(new Employees { EmployeeId = 1, Name = "Shankar", Salary = 125000 });     employeeList.Add(new Employees { EmployeeId = 2, Name = "Prasad", Salary = 90000 });     employeeList.Add(new Employees { EmployeeId = 3, Name = "Mahesh", Salary = 36000 }); }

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• Databinder.Eval using Lamda Expressions

  Using lambda expression to help with compile time checking of Eval statements

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• Query-generated Column Expressions

  A technique from Bill Nicolich that allows you to target columns by data type for the same custom expression and easily build complex queries.

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• Use Expressions with LINQ to Entities

  - by EltonStoneman

  [Source: http://geekswithblogs.net/EltonStoneman] Recently I've been putting together a generic approach for paging the response from a WCF service. Paging changes the service signature, so it's not as simple as adding a behavior to an existing service in config, but the complexity of the paging is isolated in a generic base class. We're using the Entity Framework talking to SQL Server, so when we ask for a page using LINQ's .Take() method we get a nice efficient SQL query for just the rows we want, with minimal impact on SQL Server and network traffic. We use the maximum ID of the record returned as a high-water mark (rather than using .Skip() to go to the next record), so the approach caters for records being deleted between page requests. In the paged response we include a HasMorePages indicator, computed by comparing the max ID in the page of results to the max ID for the whole resultset - if the latter is bigger, then there are more pages. In some quick performance testing, the paged version of the service performed much more slowly than the unpaged version, which was unexpected. We narrowed it down to the code which gets the max ID for the full resultset - instead of building an efficient MAX() SQL query, EF was returning the whole resultset and then computing the max ID in the service layer. It's easy to reproduce - take this AdventureWorks query:             var context = new AdventureWorksEntities();             var query = from od in context.SalesOrderDetail                         where od.ModifiedDate >= modified                          && od.SalesOrderDetailID.CompareTo(id) > 0                         orderby od.SalesOrderDetailID                         select od;   We can find the maximum SalesOrderDetailID like this:             var maxIdEfficiently = query.Max(od => od.SalesOrderDetailID);   which produces our efficient MAX() SQL query. If we're doing this generically and we already have the ID function in a Func:             Func<SalesOrderDetail, int> idFunc = od => od.SalesOrderDetailID;             var maxIdInefficiently = query.Max(idFunc);   This fetches all the results from the query and then runs the Max() function in code. If you look at the difference in Reflector, the first call passes an Expression to the Max(), while the second call passes a Func. So it's an easy fix - wrap the Func in an Expression:             Expression<Func<SalesOrderDetail, int>> idExpression = od => od.SalesOrderDetailID;             var maxIdEfficientlyAgain = query.Max(idExpression);   - and we're back to running an efficient MAX() statement. Evidently the EF provider can dissect an Expression and build its equivalent in SQL, but it can't do that with Funcs.

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• Expressions that are idiomatic in one language but not used or impossible in another

  - by Tungsten

  I often find myself working in unfamiliar languages. I like to read code written by others and then jump in and write something myself before going back and learning the corners of each language. To speed up this process, it really helps to know a few of the idioms you'll encounter ahead of time. Some of these, I've found are fairly unique. In Python you might do something like this: '\n'.join(listOfThings) Not all languages allow you to call methods on string literals like this. In C, you can write a loop like this: int i = 50; while(i--) { /* do something 50 times */ } C lets you decrement in the loop condition expression. Most more modern languages disallow this. Do you have any other good examples? I'm interested in often used constructions not odd corner cases.

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• SQL Server: Writing CASE expressions properly when NULLs are involved

  - by Mladen Prajdic

  We’ve all written a CASE expression (yes, it’s an expression and not a statement) or two every now and then. But did you know there are actually 2 formats you can write the CASE expression in? This actually bit me when I was trying to add some new functionality to an old stored procedure. In some rare cases the stored procedure just didn’t work correctly. After a quick look it turned out to be a CASE expression problem when dealing with NULLS. In the first format we make simple “equals to” comparisons to a value: SELECT CASE <value> WHEN <equals this value> THEN <return this> WHEN <equals this value> THEN <return this> -- ... more WHEN's here ELSE <return this> END Second format is much more flexible since it allows for complex conditions. USE THIS ONE! SELECT CASE WHEN <value> <compared to> <value> THEN <return this> WHEN <value> <compared to> <value> THEN <return this> -- ... more WHEN's here ELSE <return this> END Now that we know both formats and you know which to use (the second one if that hasn’t been clear enough) here’s an example how the first format WILL make your evaluation logic WRONG. Run the following code for different values of @i. Just comment out any 2 out of 3 “SELECT @i =” statements. DECLARE @i INTSELECT  @i = -1 -- first resultSELECT  @i = 55 -- second resultSELECT  @i = NULL -- third resultSELECT @i AS OriginalValue, -- first CASE format. DON'T USE THIS! CASE @i WHEN -1 THEN '-1' WHEN NULL THEN 'We have a NULL!' ELSE 'We landed in ELSE' END AS DontUseThisCaseFormatValue, -- second CASE format. USE THIS! CASE WHEN @i = -1 THEN '-1' WHEN @i IS NULL THEN 'We have a NULL!' ELSE 'We landed in ELSE' END AS UseThisCaseFormatValue When the value of @i is –1 everything works as expected, since both formats go into the –1 WHEN branch. When the value of @i is 55 everything again works as expected, since both formats go into the ELSE branch. When the value of @i is NULL the problems become evident. The first format doesn’t go into the WHEN NULL branch because it makes an equality comparison between two NULLs. Because a NULL is an unknown value: NULL = NULL is false. That is why the first format goes into the ELSE Branch but the second format correctly handles the proper IS NULL comparison.   Please use the second more explicit format. Your future self will be very grateful to you when he doesn’t have to discover these kinds of bugs.

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• CFilterEdit: Use Regular Expressions to Filter Your Input

  The definitive approach to filtering input text. Includes configurable error display.

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• Finding the html tag value with Python [on hold]

  - by MrWho

  Consider a html page, which contains a line like below: file: 'http://google.com/video.mp4' I want to search for google.com/video.mp4 in that file and save it in a variable.I want to code it with python. Shortly, I want to elicit a link from a html page, so I need to get the link by using regular expressions or the other techniques in which I'm asking about. PS: What should I exactly try to clarify?it's really annoying that the administrators don't even say what is exactly unclear about the question, they've just learned to close or on hold the topics!

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• What are some examples of MemberBinding linq expressions?

  - by michielvoo

  There are three possibilities, but I can't find examples: System.Linq.Expressions.MemberAssignment System.Linq.Expressions.MemberListBinding System.Linq.Expressions.MemberMemberBinding I want to write some unit tests to see if I can handle them, but I don't know how to write them except for the first one, which seems to be new Foo { Property = "value" } where Property = "value" is an expression of type MemberAssignment. See also this MSDN article.

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• Freely-available, well-debugged regular expressions

  - by fsb

  I was reading ICU documentation and came across this fine advice: For common tasks like this there are libraries of freely available regular expressions that have been well debugged. It's worth making a quick search before writing a new expression. To which libraries of well-debugged regular expressions do you commonly refer? I'm not much taken with http://regexlib.com where the expressions don't seem all that well debugged. It appears to have no QA process besides user comments and ratings.

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• iPhone App Development - mathematical sysmbols

  - by mcglynnconsultancy

  I am quite the beginner - but I have a lot of experience with respect to Electrical Engineering and formulas - over 30 years worth! Trying to construct an app for the iPhone. Loaded the SDK, bought myself a Mac, got a couple of "Chapter" pages written by John at Alpha Aviation - who recomended I try this site. How do I - well for example I enetered 400/1.732 - it came out like it was a web address with an underline! How can I get mathematical symbols like - sqare root, to the power of, subscript, superscript. I'd like to insert a few "calculator" pages but don't know how to. Appreciate I have only been doing this for the last week or so - so I am still quite rusty compared to what I wrote many years ago. Can anyone help? cheers

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• syntax to express mathematical formula concisely in your language of choice

  - by aaa

  hello. I am developing functional domain specific embedded language within C++ to translate formulas into working code as concisely and accurately as possible. Right now my language looks something like this: // implies two nested loops j=0:N, i=0,j (range(i) < j < N)[T(i,j) = (T(i,j) - T(j,i))/e(i+j)]; // implies summation over above expression sum(range(i) < j < N))[(T(i,j) - T(j,i))/e(i+j)]; I am looking for possible syntax improvements/extensions or just different ideas about expressing mathematical formulas as clearly and precisely as possible. Can you give me some syntax examples relating to my question which can be accomplished in your language of choice which consider useful. In particular, if you have some ideas about how to translate the above code segments, I would be happy to hear them Thank you

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• Question about the mathematical properties of hashes

  - by levand

  Take a commonly used binary hash function - for example, SHA-256. As the name implies, it outputs a 256 bit value. Let A be the set of all possible 256 bit binary values. A is extremely large, but finite. Let B be the set of all possible binary values. B is infinite. Let C be the set of values obtained by running SHA-256 on every member of B. Obviously this can't be done in practice, but I'm guessing we can still do mathematical analysis of it. My Question: By necessity, C ? A. But does C = A?

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• Lazy sequence or recur for mathematical power function?

  - by StackedCrooked

  As an exercise I implemented the mathematical power function. Once using recur: (defn power [a n] (let [multiply (fn [x factor i] (if (zero? i) x (recur (* x factor) factor (dec i))))] (multiply a a (dec n)))) And once with lazy-seq: (defn power [a n] (letfn [(multiply [a factor] (lazy-seq (cons a (multiply (* a factor) factor))))] (nth (multiply a a) (dec n)))) Which implementation do you think is superior? I truly have no idea.. (I'd use recur because it's easier to understand.) I read that lazy-seq is fast because is uses internal caching. But I don't see any opportunities for caching in my sample. Am I overlooking something?

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