I'm browsing sample asp.net code in an MVC project and need a better understanding of the => operator. Plugging => into search engines is non-helpful.
thx
The => syntax creates lambda expressions, which are small functions.
For example, the line
Func<int, int> myFunc = i => 2 * i;
declares a variable of type Func<int, int> (a delegate that takes one integer and returns another one), and assigns it to a lambda expressions that takes a parameter called i (the compiler automatically figures out that i is an int) and returns 2 * i.
Yo can search it as lambda. See here http://msdn.microsoft.com/en-us/library/bb397687.aspx
The => operator, as has been mentioned, represents a lambda expression. This is short-hand for an anonymous delegate. Here is practical example:
If you want to filter all Person objects in a collection to return only Male people, the Where() extension method requires a Func delegate you could create a named delegate like this:
Func<Person, bool> isMale = delegate(Person peep) { return peep.Gender == "male"; };
var men = from p in peeps.Where(isMale)
select p;
Or you could use an anonymous delegate like this:
var women = from p in peeps.Where(delegate(Person peep) { return peep.Gender != "male"; })
select p;
The lambda allows you to declare the anonymous delegate using a short-hand, like this:
var women = from p in peeps.Where(x => x.Gender != "male")
select p;
Notice the correspondence between delegate(Person peep) and x, and between 'return peep.Gender != "male"and 'x.Gender != "male".
Related
According to official Dart documentation, it is mentioned that:
Chaining method calls
a..b = true..c = 5;
Cascade used for chaining access to methods and other members.
Equivalent:
a.b = true; a.c = 5;
Can someone explain the meaning of the above lines?
Consider user as an instace of following class:
class User {
int age;
String firstName;
String lastName;
}
Now you can update data in user as following:
user.age = 5;
user.firstName = 'John';
user.lastName = 'Doe';
but if you use chaining access as described in the documentation.
Then it will look something like this:
user..age = 5
..firstName = 'John'
..lastName = 'Doe';
Note: sequence of assignment of the properties does not matter, but it might be of importance while calling functions or methods like this.
Just a simple & random example:
painter..computeLayout()
..initializePaint()
..render();
above is same as:
painter.computeLayout();
painter.initializePaint();
painter.render();
in the above example all of the methods were called on painter object/instance but the sequence here must be followed as without paint or layout the render will not work.
Cascades
To perform a sequence of operations on the same object, use cascades (..). We’ve all seen an expression like this:
myObject.someMethod()
It invokes someMethod() on myObject, and the result of the expression is the return value of someMethod().
Here’s the same expression with a cascade:
myObject..someMethod()
Although it still invokes someMethod() on myObject, the result of the expression isn’t the return value — it’s a reference to myObject! Using cascades, you can chain together operations that would otherwise require separate statements. For example, consider this code:
var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));
With cascades, the code becomes much shorter, and you don’t need the button variable:
querySelector('#confirm')
..text = 'Confirm'
..classes.add('important')
..onClick.listen((e) => window.alert('Confirmed!'));
I would like to obtain an object from a List based on a specific search criteria of its member variable
this is the code I am using
class foo
{
foo(this._a);
int _a;
}
List<foo> lst = new List<foo>();
main()
{
foo f = new foo(12);
lst.add(f);
List<foo> result = lst.where( (foo m) {
return m._a == 12;
});
print(result[0]._a);
}
I am getting the error and not sure how to resolve this
Uncaught exception:
TypeError: Instance of 'WhereIterable<foo>': type 'WhereIterable<foo>' is not a subtype of type 'List<foo>'
I am trying to search for an object whose member variable a == 12. Any suggestions on what I might be doing wrong ?
The Iterable.where method returns an iterable of all the members which satisfy your test, not just one, and it's a lazily computed iterable, not a list. You can use lst.where(test).toList() to create a list, but that's overkill if you only need the first element.
You can use lst.firstWhere(test) instead to only return the first element, or you can use lst.where(test).first to do effectively the same thing.
In either case, the code will throw if there is no element matched by the test.
To avoid throwing, you can use var result = lst.firstWhere(test, orElse: () => null) so you get null if there is no such element.
Another alternative is
foo result;
int index = lst.indexWhere(test);
if (index >= 0) result = lst[index];
The answer is simple. Iterable.where returns an Iterable, not a List. AFAIK this is because _WhereIterable does its computations lazily.
If you really need to return a List, call lst.where(...).toList().
Otherwise, you can set result to be an Iterable<foo>, instead of a List<foo>.
or you can go crazy and do this:
bool checkIfProductNotFound(Map<String, Object> trendingProduct) {
bool isNotFound = this
._MyProductList
.where((element) => element["id"] == trendingProduct["id"])
.toList()
.isEmpty;
return isNotFound ;
}
so I have a form with several fields which are criteria for searching in a database.
I want to formulate a query using LINQ like so:
var Coll = (from obj in table where value1 = criteria1 && value2 = criteria2...)
and so on.
My problem is, I don't want to write it using If statements to check if every field has been filled in, nor do I want to make separate methods for the various search cases (criteria 1 and criteria 5 input; criteria 2 and criteria 3 input ... etc.)
So my question is: How can I achieve this without writing an excessive amount of code? If I just write in the query with comparison, will it screw up the return values if the user inputs only SOME values?
Thanks for your help.
Yes, it will screw up.
I would go with the ifs, I don't see what's wrong with them:
var query = table;
if(criteria1 != null)
query = query.Where(x => x.Value1 == criteria1);
if(criteria2 != null)
query = query.Where(x => x.Value2 == criteria2);
If you have a lot of criteria you could use expressions, a dictionary and a loop to cut down on the repetitive code.
In an ASP.NET MVC app, chances are your user input is coming from a form which is being POSTed to your server. In that case, you can make use of strongly-typed views, using a viewmodel with [Required] on the criteria that MUST be provided. Then you wrap your method in if (ModelState.IsValid) { ... } and you've excluded all the cases where the user hasn't given you something they need.
Beyond that, if you can collect your criteria into a list, you can filter it. So, you could do something like this:
filterBy = userValues.Where(v => v != null);
var Coll = (from obj in table where filterBy.Contains(value1) select obj);
You can make this more complex by having a Dictionary (or Lookup for non-unique keys) that contains a user-entered value along with some label (an enum, perhaps) that tells you which field they're filtering by, and then you can group them by that label to separate out the filters for each field, and then filter as above. You could even have a custom SearchFilter object that contains other info, so you can have filters with AND, NOT and OR conditions...
Failing that, you can remember that until you trigger evaluation of an IQueryable, it doesn't hit the database, so you can just do this:
var Coll = (from obj in table where value1 == requiredCriteria select obj);
if(criteria1 != null)
{
query = query.Where(x => x.Value1 == criteria1);
}
//etc...
if(criteria5 != null)
{
query = query.Where(x => x.Value5 == criteria5);
}
return query.ToList();
That first line applies any criteria that MUST be there; if there aren't any mandatory ones then it could just be var Coll = table;.
That will add any criteria that are provided will be applied, any that aren't will be ignored, you catch all the possible combinations, and only one query is made at the end when you .ToList() it.
As I understand of your question you want to centralize multiple if for the sake of readability; if I were right the following would be one of some possible solutions
Func<object, object, bool> CheckValueWithAnd = (x, y) => x == null ? true : x==y;
var query = from obj in table
where CheckValue(obj.value1, criteria1) &&
CheckValue(obj.value2, criteria2) &&
...
select obj;
It ls flexible because in different situations or scenarios you can change the function in the way that fulfill your expectation and you do not need to have multiple if.
If you want to use OR operand in your expression you need to have second function
Func<object, object, bool> CheckValueWithOr = (x, y) => x == null ? false : x==y;
I'm designing an interface where the user can join a publicaiton to a keyword, and when they do, I want to suggest other keywords that commonly occur in tandem with the selected keyword. The trick is getting the frequency of correlation alongside the properties of the suggested keywords.
The Keyword type (EF) has these fields:
int Id
string Text
string UrlString
...and a many-to-many relation to a Publications entity-set.
I'm almost there. With :
var overlappedKeywords =
selectedKeyword.Publications.SelectMany(p => p.Keywords).ToList();
Here I get something very useful: a flattened list of keywords, each duplicated in the list however many times it appears in tandem with selectedKeyword.
The remaining Challenge:
So I want to get a count of the number of times each keyword appears in this list, and project the distinct keyword entities onto a new type, called KeywordCounts, having the same fields as Keyword but with one extra field: int PublicationsCount, into which I will populate the count of each Keyword within overlappedKeywords. How can I do this??
So far I've tried 2 approaches:
var keywordCounts = overlappingKeywords
.Select(oc => new KeywordCount
{
KeywordId = oc.Id,
Text = oc.Text,
UrlString = oc.UrlString,
PublicationsCount = overlappingKeywords.Count(ok2 => ok2.Id == oc.Id)
})
.Distinct();
...PublicationsCount is getting populated correctly, but Distinct isn't working here. (must I create an EqualityComarer for this? Why doesn't the default EqualityComarer work?)
var keywordCounts = overlappingKeywords
.GroupBy(o => o.Id)
.Select(c => new KeywordCount
{
Id = ???
Text = ???
UrlString = ???
PublicationsCount = ???
})
I'm not very clear on GroupBy. I don't seem to have any access to 'o' in the Select, and c isn't comping up with any properties of Keyword
UPDATE
My first approach would work with a simple EqualityComparer passed into .Distinct() :
class KeywordEqualityComparer : IEqualityComparer<KeywordCount>
{
public bool Equals(KeywordCount k1, KeywordCount k2)
{
return k1.KeywordId== k2.KeywordId;
}
public int GetHashCode(KeywordCount k)
{
return k.KeywordId.GetHashCode();
}
}
...but Slauma's answer is preferable (and accepted) because it does not require this. I'm still stumped as to what the default EqualityComparer would be for an EF entity instance -- wouldn't it just compare based on primary ids, as I did above here?
You second try is the better approach. I think the complete code would be:
var keywordCounts = overlappingKeywords
.GroupBy(o => o.Id)
.Select(c => new KeywordCount
{
Id = c.Key,
Text = c.Select(x => x.Text).FirstOrDefault(),
UrlString = c.Select(x => x.UrlString).FirstOrDefault(),
PublicationsCount = c.Count()
})
.ToList();
This is LINQ to Objects, I guess, because there doesn't seem to be a EF context involved but an object overlappingKeywords, so the grouping happens in memory, not in the database.
I have a Blogs table related to BlogComments table with a FK.
I need to get, through Linq, all the BlogComments items that match a certain flag
If i do:
db.Blogs.Where(b => b.BlogComments.Where(bc=>bc.Where(bc.Flag1==true));
I get "Cannot implicity convert type IEnumerable to bool"
Which is the best way to solve this problem?
Because this expression:
b.BlogComments.Where(...)
returns an IEnumerable (of BlogComments), but you are then passing it into this method:
db.Blogs.Where(...)
which expects a function that returns a bool, not an IEnumerable.
You probably need something like this:
var blogId = 5;
db.BlogComments.Where(bc => bc.BlogId == blogId && bc.Flag1 == true)
If you need to select comments from multiple blogs, then you could try using Contains:
var blogIds = new [] {1,2,3,4,5};
db.BlogComments.Where(bc => blogIds.Contains(bc.BlogId) && bc.Flag1 == true)
If you want to place criteria on the set of blogs, as well as the comments, then you could do this in one query using a join:
var query = from b in db.Blogs
join c in db.BlogComments on c.Blog equals b
where b.SomeField == "some value"
&& c.Flag1 == true
select c;
You could write it in LINQ form.
var blogs = from b in db.Blogs
join c in db.BlogComments
on b.BlogId equals c.BlogId
where c.Flag1
select b;
If you have a composite key you can write
on new { A = b.BlogKey1, B = b.BlogKey2 }
equals new { A = c.CommentKey1, B = c.CommentKey2 }
If it were me, I would just have another DbSet in your DbContext.
DbSet<BlogComment> BlogComments
and just search through there without going through Blogs.
db.BlogComments.Where(bc => bc.Flag1 == true);
If anyone knows if there's anything wrong in doing so, then I'm all ears :)