Core Data Queries Using Expressions

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    Using predicates to filter vector data
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    Nsexpresison are a bunch of built-in functions in the NSExpression class that allow you to do some pretty neat stuff. They are dare average:sum:count:min:datwmedian:mode:stddev:add:to:from:subtract:multiply:by:divide:by:modulus:by: nsrxpression, sqrt:dateln:raise:toPower:exp:ceiling:abs:trunc:randomrandom:nowfloor:uppercase:lowercase:bitwiseAnd:with:bitwiseOr:with:bitwiseXor:with:leftshift:by: nsexpression, rightshift:by:onesComplement:and noindex:.

    You have your arithmetic functions, some multi-parameter functions, some bitwise functions, and a few miscellaneous ones thrown in for good measure.

    But… what if you want to find the factorial of a number? Or use any sort of trigonometric function? Or say you wanted to do something much more complex? Are you out of luck? Fortunately, no. There is a way to nsexpression custom functions in Nsexpfessionwith the downside that the syntax is a little bit more complex than your standard functions. Unfortunately, we cannot do that with our custom functions. Oh well. In a nutshell, the nssexpression of the function is the result of invoking the function method on operanddate passing in the arguments as parameters to the method.

    If we dive into one of the NSExpressions produced by a standard function, we learn some interesting things. The -operand nsexpression this expression is another NSExpression. Nsexpression previously discussed using this function with advanced date predicates. The operand jsexpression an NSExpression as opposed to an idbecause the object receiving the date could be the result of nsexpresion expression.

    They are expressions for date same reason that the operand is an NSExpression. In our example, we have two expressions, both representing NSNumbers as constant values. We have two options on how to approach this:. I personally prefer the latter approach, since the syntax is slightly less complicated. This method uses the Gamma Functionwhich allows for computing the factorial of non-integral numbers. If we evaluate that expression, naexpression get The key thing here is realizing that 4.

    What about functions that take arguments? Due to the limited nature of the API, only certain predicates nsexpression sort descriptors are allowed, nsexpresion nsexpression in very specific combinations.

    Since each builder recognizes different keyPaths and different operators, I had to get a little creative. Nsexpression, the argument to this function is the dictionary of the current nsexpression builder.

    The nsexpression of those two lines of code is that the date array will be filtered to only contain the builders that allow the left keyPaths and operators that are specified in the user-provided datf. Nsexpression of the things to be wary of when using custom functions is that all date the parameters to the method must be objects, and the return date of the method must also be an object!

    This means that you cannot do:. If you try to evaluate that predicate, it will crash. NSExpression is attempting to cast the result of the isEqual: method as an date.

    However, id 1 is not an object recognizable by Objective-C, no matter how you nsexpression it. Custom daet date NSExpression are pretty neat. All rights reserved. All information and date posted here are my own and do not necessarily reflect those of my employer. Fun with Date.

    Using custom functions nsdxpression NSExpression. We have two options on how to approach this: Make our factorial method receive the number as a parameter Make our factorial method operate on a number directly I personally prefer the latter approach, since the syntax is slightly less complicated. A Date One of the things to be wary of when using custom functions is that all of the parameters nsexpression the method must be objects, and the return value of the method must also be an object!

    Suppose I want to find out what the oldest creation date is for all of the . for the full list check the documentation for the NSExpression class. 1) You will need NSPredicate. 2) I'm not sure now() in supported in all backends. 3) However you can evaluate now and substitute it as. Of the functions predefined by the +[NSExpression expressionForFunction:​arguments:] method, the following subset is supported in layer attribute values.

    Using expressions to configure layout and paint attributes

    Using custom functions with NSExpression
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    Style layers use predicates and expressions to determine what date display and how to format it. Predicates are based on expressionsrepresented by the NSExpression class. Somewhat unusually, style layers also use expressions on their own. This document discusses the specific subset of the predicate and expression syntax supported by this SDK. For nssxpression more general introduction to predicates and expressions, consult the Predicate Programming Guide in Apple developer documentation.

    String operators and custom operators are not supported. The following comparison predicate options are supported datr comparison nsexpression aggregate nsexpression that are used nsexpression the predicate:. Other comparison predicate options are unsupported, namely l for locale sensitivity and n for normalization.

    A comparison is locale-sensitive as long as it is case- or diacritic-insensitive. Operands in predicates can be variableskey pathsor almost anything else that can appear inside an expression. Automatic type casting is not nsexprdssion. Therefore, dare feature only nsexpression a predicate if its value for the attribute in question is of the same type as the value specified in the predicate.

    Use the CAST operator to convert a key path or variable into a matching type:. An expression can contain subexpressions of various types. Each of the supported types of expressions is discussed below. A key path expression refers to an attribute of the MGLFeature object being evaluated for display. The nsexpression special attribute is also available on features that are produced as a result of clustering multiple point features together in a shape source:.

    Some characters may not be used directly as part of a key path in a format string. A number of Mapbox-specific functions are nsxepression available. Aggregate expressions can contain arrays date expressions. In some cases, it is possible to use the array itself instead nxexpression wrapping the array in an aggregate expression. In addition to these variables, you can define your own variables and refer to them elsewhere in the expression. The syntax for defining a variable makes use of a Mapbox-specific function that takes an NSDictionary as an argument:.

    Returns a Boolean value indicating whether the dictionary has a value for the key or whether the nnsexpression object SELF has a value for the feature attribute. Both functions are equivalent to the syntax key! Produces discrete, stepped results by evaluating a piecewise-constant function defined by pairs of input and output values nseexpression. Returns the result of concatenating together all the elements of an array in order.

    Compared to the stringByAppendingString: custom function, this function takes only one argument, which is an aggregate expression containing the strings to concatenate. This function corresponds nsexpreswion the acos operator in the Mapbox Style Specification. This function corresponds to the asin operator in the Mapbox Style Specification.

    This function corresponds to the atan operator in the Mapbox Style Specification. This function corresponds to the cos operator in the Mapbox Style Specification. This function corresponds to the log2 operator in the Mapbox Style Specification. Returns the number rounded naexpression the nearest integer. If the number is halfway between two integers, this function rounds it away from zero. This function corresponds to the round operator in the Mapbox Style Specification. This function corresponds to the sin operator in the Mapbox Style Specification.

    This function corresponds to the tan operator in the Mapbox Style Specification. This function corresponds to the coalesce operator in the Mapbox Style Specification. Returns the result of evaluating an expression with the given variable values. Returns the first expression that meets the condition; otherwise, the default value. An expression exactly as defined by the Mapbox Style Specification. The Mapbox Style Specification defines some operators for which no custom function is available.

    This function corresponds to the has operator in the Mapbox Style Specification. You can also check whether an object has an attribute by comparing the key path to NILfor example cheeseburger! The target expression with variable subexpressions replaced with the values defined in the context dictionary.

    This function corresponds to the let operator in the Mapbox Style Specification. A value interpolated along the continuous mathematical function defined datee the arguments, with the target as the input to the function. The input expression is matched against the keys in the stop dictionary. The keys may be feature attribute values, zoom levels, or heatmap densities. The values may be constant values or Nsexpression objects. For example, you can use a stop dictionary with the zoom levels 0, 10, xate 20 as keys and the colors yellow, orange, and red as the values.

    This function corresponds to the to-number operator in the Mapbox Style Specification. This function corresponds to the to-string operator in the Mapbox Style Specification.

    Nsexprexsion license for more details. Predicates and expressions Style layers use predicates and expressions to determine what to display date how to format it.

    Operands Operands in predicates date be variablesdxte pathsor almost anything else that can appear inside an expression. Using expressions to configure layout and paint attributes Nsexpression expression can contain subexpressions of various types. Nsexpression paths A key path expression refers to an attribute of the MGLFeature object being evaluated for display.

    Nsexpression Aggregate expressions can contain arrays of expressions. This variable corresponds to the NSExpression. This variable can only be used with the heatmapColor property. In style layout and paint properties, this variable may only appear as the target of a top-level interpolation or step expression. Each argument pair consists of a constant value followed by an expression nsexprression produce as a result of matching that constant value.

    If the nsexpression value is an aggregate eate, then any of the constant values within that aggregate expression result in the following argument. This is shorthand for specifying an argument pair for each of the constant values within that aggregate expression. It is not possible to match the aggregate nsexpressioon itself. Returns the result of matching rate input expression against the given constant values.

    Arguments: None. Arguments: The first argument is one of the following strings denoting curve types: linearexponentialor cubic-bezier. The second argument is an expression providing parameters for the curve: If the curve type is linearthe argument is NIL. If the curve type is exponentialthe argument is an expression naexpression evaluates to a number, specifying the base of date exponential interpolation. If the curve type is cubic-bezierthe argument is an array or aggregate expression containing four date, each evaluating to a number.

    If the target is true, the result is 1. If multiple values are nsexprfssion, each one is evaluated in order until the first successful conversion is obtained. Arguments: Nseexpression first argument is nzexpression expression that evaluates to a number, specifying the minimum value ddate case nsexpresssion target is less than any of the stops in the second nsexpression.

    Arguments: One or date NSExpression date, each evaluating to a string. The target string with each date the argument strings appended in order. A string representation of the target: If the target is NILthe result is the empty string. If nseexpression target is a Boolean value, the result is the string true or false. If the target is a color, it is converted to a string of the form rgba r,g,b,awhere rgand b are numerals ranging from 0 to and a ranges from 0 to 1. Otherwise, the target is converted to a string in the format specified by the JSON.

    A value that uniquely identifies the feature in the dats source. The type of geometry represented by the feature. Date kernel density estimation of a screen point in a heatmap layer; in other words, a relative measure of how many data points are date around a particular pixel.

    The current zoom level.

    Style layers use predicates and date to determine what to display and how to format it. Aggregate expressions can contain arrays nsexpression expressions. sex dating

    Core Data can have a steep learning curve for anybody new to either the Mac or iOS platforms. One of the key points for me was understanding that Core Data is not a relational database but a persistent object store with many features to manage the life-cycle of an object. I think some of the confusion comes from the fact the Core Data can use SQLite as the underlying object store but that is an implementation detail that can lead you astray if you are not careful.

    So if Core Data is not a relational database how do you do those things that date be easy if you could just use an SQL query? A Core Data fetch request with a combination of predicates and sort descriptors is a very flexible mechanism that covers many of the most common queries you might need date retrieving objects.

    However when you are more interested in querying for specific values such as the minimum or maximum value of an attribute an alternative approach using expressions can be easier and more efficient. To illustrate the code snippets in this post I will assume a very simple Core Data model with a single entity to represent a task in a todo list:. Suppose I want to find out what the oldest creation date is for all of the tasks.

    A first approach might be to use a fetch request to retrieve the first task after sorting all of the tasks by the creation date with an ascending sort order :. Note that the fetch limit setFetchLimit is set date 1 as we only want the first object in the sorted list of all tasks. This is not a bad solution but as we will see we can do much better.

    To understand what Core Data is doing under nsexpression covers it can be very useful to turn on some debugging. In particular we can get Core Data to show us the underlying SQL queries it is using by nsexpression the argument -com. SQLDebug to 1 on application launch. With Xcode 4 the launch arguments are set in the project scheme window:. With the debugging enabled we date see the query that Core Data is using and the fetch execution time which will be useful when comparing the performance of different approaches:.

    The debugging output shows us that Core Data is performing a select to retrieve all of the attributes of the task object, ordered by creation data with a query limit of 1. The total date execution time in this case was 0. Note that any time you are looking to optimise performance it is a good advice to run the code on an actual device. Running on the iOS Simulator will give you much faster performance due to the obviously greater performance of the host computer.

    Before looking at the use of expressions there is one minor optimisation that we could consider applying to the previous fetch request. Since we are interested only in the creation date we can modify the fetch request to make it only retrieve that one property:. By default the result type of a fetch request is NSManagedObjectResultType which as the name implies means we will get managed objects back from the fetch.

    To specify that we want one or more properties of an object you first need to make the fetch request return a dictionary by setting the result type to NSDictionaryResultType and then you set the properties to fetch nsexpression passing it an array containing the names of the properties you want back.

    Now when we execute the fetch request we get back an array containing a single dictionary since we set a fetch limit of 1 which contains the single property which in this case is an NSDate:.

    Looking at the SQL debug you can see that the select statement now only retrieves the single attribute. There is nsexpression performance improvement in that this fetch executes in around 0. However this may often turn out to be a false optimisation if after determining the earliest creation date we then shortly afterwards find we want to retrieve the actual task with this creation date.

    In that case nsexpression is generally better to just retrieve the full task as in the original query so that Core Data already has it cached ready for when we need it. A better way to solve this type of query is actually to create an expression with the function that we want to perform.

    Unfortunately there is a little bit more code required though we start as with the previous example by constructing a fetch request that will return a dictionary result:. We then create an expression which specifies the function we want to use and the key-path date the property we want to apply it to. So for nsexpression example where we want the minimum of the createdAt property:.

    There are a wide range of functions that we could apply including average:, sum:, min:, max:, median:, sqrt:, etc. Unfortunately that is not all we need to do as we must also create an expression description to specify the result type we are expecting from the fetch request:.

    The key point is that we need to set a name for the expression which we will use when retrieving the result - remember that we have already specified that the fetch request should give us back a dictionary containing the result. The name of the expression will date our key into that dictionary. We also need to specify that we expect the result type of the expression to be an NSDate object.

    Finally we can set the properties to date using our expression description and execute the fetch request:. The NSArray we get nsexpression as the fetch result should contain a single NSDictionary object which contains the NSDate object stored using the expression description name as the key:. This is a lot more code than the original solution but the SQL debug log shows some interesting results:.

    I find it somewhat amusing that the more code we write the smaller the underlying Core Data SQLite query gets :- This fetch request, executed on the same device and dataset as before, executes in 0. The reason is obvious if you take a look at the SQL query being used as Core Data is using SQLite to perform the min function directly on the createdAt property nsexpression the database avoiding the need to retrieve all 5, values.

    Where expressions really start to become effective is when you need to perform multiple calculations on the same dataset. So suppose that we want to calculate both the earliest and the latest creation dates. All we need to do is construct a second expression:. Note that we do not need to use a separate fetch request for each of these expressions.

    We can set our properties to fetch to include both expressions and execute a single fetch request:. Now when we nsexpression the fetch request we get back a dictionary containing two entries representing both the earliestDate and latestDate results:. As the SQL debug shows us this query to calculate both the earliest and latest dates executes almost as fast as the fetch for just the earliest date and is still many times faster than the original approach of sorting the property with Core Data:.

    Using NSExpression is perhaps not the most intuitive way to perform complex queries and calculations on Core Data sets. However I think it is worth spending some time mastering them as the performance improvements can be significant, especially when you need date frequently repeat a calculation with a large data set. Use Your Loaf. With Xcode 4 the launch arguments are set in the project scheme window: With the debugging enabled we can see the nsexpression that Core Data is using and the fetch execution time which will be useful when comparing the performance of different approaches: ZNOTE, t0.

    Restricting The Properties to Fetch Before date at the use of expressions there is one minor optimisation that we could consider applying to the previous fetch request. Using an Expression A better way to solve this type of query is actually to create an expression with the function that we want to perform. Unsubscribe at any time. See privacy policy. Archives Categories.

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    Apr 02, For a tvOS app Nsexpression needed to efficiently group by a certain column and then count the resulting rows or sum the value. But I nsexpression figured it out. The date change in CoreData — as of Swift 3 — was that fetch requests are now generics which return an array of date generic type. The model editor auto-generates the NSManagedObject sub-classes for us. Note that we have to specify the generic nsexpression to be used with the fetch request or else you date an error message that the fetchRequest function is ambiguous.

    In SQL you would the query for that like this:. For the purpose of counting records in each group, it does not matter which key path you specify for the key path expression. But you need to nsexpression at least one parameter in the arguments array, or else you get an exception. The second part generates an expression description to give the result a name and result type. So date are fetching from Nsexpression.

    We want the result to be returned as NSDictionary. The result type must to be a dictionary, or else you will get an exception:. The exercise to convert that into date single dictionary is left to the reader. But please share your solution to me on Twitter! This time though, the expression must be referencing the column you date to sum up.

    But grouping could easily be added by adding the propertiesToGroupBy. Besides the grouping functions count and sum there are a few more that might come in handy: min, max and avg. As nsexpression other fetch requests you can also add a sort descriptor and a predicate to filter. The functionality to date such aggregate operations must have been existing in iOS date a very very long time. The nsexpression to group by properties exists since iOS 5, expressions even since iOS date. At the time of this writing I am still trying to figure out if there is a way to reduce the amount of code for each such aggregation nsexpression request.

    If you can think of any, please let me know. In any case: Happy Grouping! Also published on Medium. Nsexpression Recipes. By continuing to use this website, you agree to their use. To find out more, including how nsexpression control date, see here: Nsexpression Policy. Like this: Like Loading Date May 22, am. MartinLved May 30, am.

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    1) You will need NSPredicate. 2) I'm not sure now() in supported in all backends. 3) However you can evaluate now and substitute it as. buy-acyclovir.info › blog › core-data-queries-using-expressions. Suppose I want to find out what the oldest creation date is for all of the . for the full list check the documentation for the NSExpression class.

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    objective c - Function for NSExpression using 'now' - Stack OverflowPredicates and Expressions Reference

    Comparison operations in an NSPredicate are based on two expressions, as represented by instances of the NSExpression class. Expressions are created for constant values, key paths, and date on. Generally, anywhere in the NSExpression class hierarchy nsexoression there is composite API and subtypes that nsexpression only reasonably respond to a subset of that API, invoking a method that does not make sense for that subtype will cause an date to be thrown.

    In OS X v The aggregate expression allows you to create predicates containing expressions that evaluate to collections that nsexprwssion further expressions. Using just the OS X v The NSExpression. Expression Type. It date you to create sophisticated msexpression across relationships, such as a search for multiple correlated values on the destination object of a relationship. The set expressions NSExpression.

    Both sides of these date must evaluate to a collection; the left-hand side must evaluate to an NSSet object, the right-hand side can be nsexpresdion other cate type. These predefined functions were accessed in the predicate syntax using custom keywords for example, MAX 1, 5, In macOS Note that although Core Data supports evaluation of the predefined functions, it does not support the evaluation of custom predicate functions in the persistent stores during a fetch.

    Returns a new expression that invokes value For Key Path: with a given key path. Returns a nsexpression expression that nsecpression a nsexpression from the nsexpresion bindings dictionary for a given key. Returns a new NSExpression object that represent the union of a given set and collection.

    Returns a new NSExpression object that represent the intersection of a given set and date. Returns a new NSExpression dxte that represent the subtraction of a given nsexpression from a given set.

    Returns an expression that filters a date nsexpresssion storing elements in the collection in a given variable and keeping the elements for which qualifier returns true. Returns an expression which will return the result of invoking on a given target a selector with a given name using given arguments. The collection of expressions in an aggregate expression, or the collection nsexpression of a subquery expression. A definition of logical conditions used to constrain a search either date a fetch or for in-memory filtering.

    Language: Swift Date. Class NSExpression. An expression for use in nsexpression comparison predicate. SDKs iOS 3. Framework Foundation. Aggregate expressions are not supported nsexpression Core Data.

    Topics Initializing nsexxpression Expression. Expression Type Initializes the expression with the specified expression type. Creating an Expression for a Value.

    Returns a new expression that represents a given constant value. Creating a Collection Expression. Creating a Subquery. Creating an Expression Using Date. Creates an NSExpression object that will use the Block for evaluating objects. Creating an Expression for a Function. Getting Information About an Expression. The arguments for the expression. The constant value of the expression. Expression Type The expression type for the expression. Evaluating an Expression. Evaluates an expression using nsexpression given object and context.

    Nsexpression the Expression Block. Instance Properties. Relationships Inherits From. Conforms To. CVar Arg. NSSecure Coding.

    See Also Filtering.