I have the following array
ArrayList<Car> list = new ArrayList<>();
I want to iterate it and throw an exception if it contains a certain value
i.e.
if at least one
list.stream.filter(x -> x.color.equals("Black"));
then I want to stop iteration and throw an exception.
Is there a way?
You could use anyMatch for this:
boolean matched = list.stream().anyMatch(x -> x.color.equals("Black"));
if(matched) throw new SomeException();
Since it does not evaluate the rest of the pipeline if the condition is satisfied for one element when iterating through it, and that it returns false if the stream is empty, I think this is what you are looking for.
Of course, you could do it in a single statement but it may not improve readability depending on the situation:
if(list.stream().anyMatch(x -> x.color.equals("Black"))) {
throw new SomeException();
}
Easiest is:
list.forEach( x -> {
if(x.color.equals("Black")) throw new RuntimeException();
});
Related
I'm trying to create a loop to read, for example, 4200 users from 1000 to 1000 but I can't get it to cut when it reaches the end. I tried it with if, for and I couldn't do it.
I have programmed in JAVA but with Groovy I see that the structure is different.
urlUsers = urlUsers.concat("/1/1000");
List<UserConnectorObject> usersList = null;
while({
gesdenResponse = GesdenUtils.sendHttpRequest(urlUsers, "LOOKUP", null,
request.getMetaData()?.getLogin(), request.getMetaData()?.getPassword());
log.info("Users data in JSON: "+gesdenResponse.getOutput())
usersList = GesdenUtils.fromJSON(gesdenResponse.getOutput(), GesdenConstants.USER_IDENTITY_KEY);
usersList.size() == 10;
log.info("List size in JSON "+usersList.size());
}()) continue
Groovy has lots of loop structures, but it is crucial to separate the regular ones (lang built-ins) and the api functions which take closure as an argument
take closure - no plain way to escape
If you want to iterate from A to B users, you can use, for instance,
(10..20).each { userNo -> // Here you will have all 10 iterations
if ( userNo == 5) {
return
}
}
If something outrageous happens in the loop body and you cannot use return to escape, as loop boddy is a closure (separate function) and this resurn just exits this closure. Next iteration will happen just after.
use regular lang built-in loop structures - make use of break/continue
for (int userNo in 1..10) { // Here you will have only 5 iterations
if (userNo == 5) {
break
}
}
It looks like your closure always return falsy because there is no explicit return, and the last statement evaluated is the call to log.info(String) which returns void.
Use an explicit return or move/delete the log statement.
I was wondering why this is valid go code:
func FindUserInfo(id string) (Info, bool) {
it, present := all[id]
return it, present
}
but this isn't
func FindUserInfo(id string) (Info, bool) {
return all[id]
}
is there a way to avoid the temporary variables?
To elaborate on my comment, the Effective Go mentions that the multi-value assignment from accessing a map key is called the "comma ok" pattern.
Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that the empty string because it's not in the map at all? You can discriminate with a form of multiple assignment.
var seconds int
var ok bool
seconds, ok = timeZone[tz]
For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false.
Playground demonstrating this
We can see that this differs from calling a regular function where the compiler would tell you that something is wrong:
package main
import "fmt"
func multiValueReturn() (int, int) {
return 0, 0
}
func main() {
fmt.Println(multiValueReturn)
asgn1, _ := multiValueReturn()
asgn2 := multiValueReturn()
}
On the playground this will output
# command-line-arguments
/tmp/sandbox592492597/main.go:14: multiple-value multiValueReturn() in single-value context
This gives us a hint that it may be something the compiler is doing. Searching the source code for "commaOk" gives us a few places to look, including types.unpack
At the time of writing this it this the method's godoc reads:
// unpack takes a getter get and a number of operands n. If n == 1, unpack
// calls the incoming getter for the first operand. If that operand is
// invalid, unpack returns (nil, 0, false). Otherwise, if that operand is a
// function call, or a comma-ok expression and allowCommaOk is set, the result
// is a new getter and operand count providing access to the function results,
// or comma-ok values, respectively. The third result value reports if it
// is indeed the comma-ok case. In all other cases, the incoming getter and
// operand count are returned unchanged, and the third result value is false.
//
// In other words, if there's exactly one operand that - after type-checking
// by calling get - stands for multiple operands, the resulting getter provides
// access to those operands instead.
//
// If the returned getter is called at most once for a given operand index i
// (including i == 0), that operand is guaranteed to cause only one call of
// the incoming getter with that i.
//
The key bits of this being that this method appears to determine whether or not something is actually a "comma ok" case.
Digging into that method tells us that it will check to see if the mode of the operands is indexing a map or if the mode is set to commaok (where this is defined does give us many hints on when it's used, but searching the source for assignments to commaok we can see it's used when getting a value from a channel and type assertions). Remember the bolded bit for later!
if x0.mode == mapindex || x0.mode == commaok {
// comma-ok value
if allowCommaOk {
a := [2]Type{x0.typ, Typ[UntypedBool]}
return func(x *operand, i int) {
x.mode = value
x.expr = x0.expr
x.typ = a[i]
}, 2, true
}
x0.mode = value
}
allowCommaOk is a parameter to the function. Checking out where unpack is called in that file we can see that all callers pass false as an argument. Searching the rest of the repository leads us to assignments.go in the Checker.initVars() method.
l := len(lhs)
get, r, commaOk := unpack(func(x *operand, i int) { check.expr(x, rhs[i]) }, len(rhs), l == 2 && !returnPos.IsValid())
Since it seems that we can only use the "comma ok" pattern to get two return values when doing a multi-value assignment this seems like the right place to look! In the above code the length of the left hand side is checked, and when unpack is called the allowCommaOk parameter is the result of l == 2 && !returnPos.IsValid(). The !returnPos.IsValid() is somewhat confusing here as that would mean that the position has no file or line information associated with it, but we'll just ignore that.
Further down in that method we've got:
var x operand
if commaOk {
var a [2]Type
for i := range a {
get(&x, i)
a[i] = check.initVar(lhs[i], &x, returnPos.IsValid())
}
check.recordCommaOkTypes(rhs[0], a)
return
}
So what does all of this tell us?
Since the unpack method takes an allowCommaOk parameter that's hardcoded to false everywhere except in assignment.go's Checker.initVars() method, we can probably assume that you will only ever get two values when doing an assignment and have two variables on the left-hand side.
The unpack method will determine whether or not you actually do get an ok value in return by checking if you are indexing a slice, grabbing a value from a channel, or doing a type assertion
Since you can only get the ok value when doing an assignment it looks like in your specific case you will always need to use variables
You may save a couple of key strokes by using named returns:
func FindUserInfo(id string) (i Info, ok bool) {
i, ok = all[id]
return
}
But apart from that, I don't think what you want is possible.
Simply put: the reason why your second example isn't valid Go code is because the language specification says so. ;)
Indexing a map only yields a secondary value in an assignment to two variables. Return statement is not an assignment.
An index expression on a map a of type map[K]V used in an assignment or initialization of the special form
v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]
yields an additional untyped boolean value. The value of ok is true if the key x is present in the map, and false otherwise.
Furthermore, indexing a map is not a "single call to a multi-valued function", which is one of the three ways to return values from a function (the second one, the other two not being relevant here):
There are three ways to return values from a function with a result type:
The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and assignable to the corresponding element of the function's result type.
The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply.
The expression list may be empty if the function's result type specifies names for its result parameters. The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables.
As for your actual question: the only way to avoid temporary variables would be using non-temporary variables, but usually that would be quite unwise - and probably not much of an optimization even when safe.
So, why doesn't the language specification allow this kind of special use of map indexing (or type assertion or channel receive, both of which can also utilize the "comma ok" idiom) in return statements? That's a good question. My guess: to keep the language specification simple.
I'm no Go expert but I believe you are getting compile time error when you are trying to return the array i.e. return all[id]. The reason could be because the functions return type is specially mentioned as (Info, bool) and when you are doing return all[id] it can't map the return type of all[id] to (Info, bool).
However the solution mentioned above, the variables being returned i and ok are the same that are mentioned in the return type of the function (i Info, ok bool) and hence the compiler knows what it's returning as opposed to just doing (i Info, ok bool).
By default, maps in golang return a single value when accessing a key
https://blog.golang.org/go-maps-in-action
Hence, return all[id] won't compile for a function that expects 2 return values.
At the beginning of a method I want to check if the method is called with these exact parameters before, and if so, return the result that was returned back then.
At first, with one parameter, I used a Dictionary, but now I need to check 3 parameters (a String, an Object and a boolean).
I tried making a custom Object like so:
var cacheKey:Object = { identifier:identifier, type:type, someBoolean:someBoolean };
//if key already exists, return it (not working)
if (resultCache[cacheKey]) return resultCache[cacheKey];
//else: create result ...
//and save it in the cache
resultCache[cacheKey] = result;
But this doesn't work, because the seccond time the function is called, the new cacheKey is not the same object as the first, even though it's properties are the same.
So my question is: is there a datatype that will check the properties of the object used as key for a matching key?
And what else is my best option? Create a cache for the keys as well? :/
Note there are two aspects to the technical solution: equality comparison and indexing.
The Cliff Notes version:
It's easy to do custom equality comparison
In order to perform indexing, you need to know more than whether one object is equal to another -- you need to know which is object is "bigger" than the other.
If all of your properties are primitives you should squash them into a single string and use an Object to keep track of them (NOT a Dictionary).
If you need to compare some of the individual properties for reference equality you're going to have a write a function to determine which set of properties is bigger than the other, and then make your own collection class that uses the output of the comparison function to implement its own a binary search tree based indexing.
If the number of unique sets of arguments is in the several hundreds or less AND you do need reference comparison for your Object argument, just use an Array and the some method to do a naive comparison to all cached keys. Only you know how expensive your actual method is, so it's up to you to decide what lookup cost (which depends on the number of unique arguments provided to the function) is acceptable.
Equality comparison
To address equality comparison it is easy enough to write some code to compare objects for the values of their properties, rather than for reference equality. The following function enforces strict set comparison, so that both objects must contain exactly the same properties (no additional properties on either object allowed) with the same values:
public static propsEqual(obj1:Object, obj2:Object):Boolean {
for(key1:* in obj1) {
if(obj2[key1] === undefined)
return false;
if(obj2[key1] != obj2[key1])
return false;
}
for(key2:* in obj2)
if(obj1[key2] === undefined)
return false;
return true;
}
You could speed it up by eliminating the second for loop with the tradeoff that {A:1, B:2} will be deemed equal to {A:1, B:2, C:'An extra property'}.
Indexing
The problem with this in your case is that you lose the indexing that a Dictionary provides for reference equality or that an Object provides for string keys. You would have to compare each new set of function arguments to the entire list of previously seen arguments, such as using Array.some. I use the field currentArgs and the method to avoid generating a new closure every time.
private var cachedArgs:Array = [];
private var currentArgs:Object;
function yourMethod(stringArg:String, objArg:Object, boolArg:Boolean):* {
currentArgs = { stringArg:stringArg, objArg:objArg, boolArg:boolArg };
var iveSeenThisBefore:Boolean = cachedArgs.some(compareToCurrent);
if(!iveSeenThisBefore)
cachedArgs.push(currentArgs);
}
function compareToCurrent(obj:Object):Boolean {
return someUtil.propsEqual(obj, currentArgs);
}
This means comparison will be O(n) time, where n is the ever increasing number of unique sets of function arguments.
If all the arguments to your function are primitive, see the very similar question In AS3, where do you draw the line between Dictionary and ArrayCollection?. The title doesn't sound very similar but the solution in the accepted answer (yes I wrote it) addresses the exact same techinical issue -- using multiple primitive values as a single compound key. The basic gist in your case would be:
private var cachedArgs:Object = {};
function yourMethod(stringArg:String, objArg:Object, boolArg:Boolean):* {
var argKey:String = stringArg + objArg.toString() + (boolArg ? 'T' : 'F');
if(cachedArgs[argKey] === undefined)
cachedArgs[argKey] = _yourMethod(stringArg, objArg, boolArg);
return cachedArgs[argKey];
}
private function _yourMethod(stringArg:String, objArg:Object, boolArg:Boolean):* {
// Do stuff
return something;
}
If you really need to determine which reference is "bigger" than another (as the Dictionary does internally) you're going to have to wade into some ugly stuff, since Adobe has not yet provided any API to retrieve the "value" / "address" of a reference. The best thing I've found so far is this interesting hack: How can I get an instance's "memory location" in ActionScript?. Without doing a bunch of performance tests I don't know if using this hack to compare references will kill the advantages gained by binary search tree indexnig. Naturally it would depend on the number of keys.
I have a many-to-one relationship that users can edit via checkboxes. PK of Foo is ID, and fid contains the id from the checkbox.
I'm checking to see if an element exists with:
Foo ent;
try
{
ent = ctx.Foo.First(f => f.ID == fid);
}
catch (System.InvalidOperationException ioe)
{
ent = new Foo();
}
It seems to me that I should be able to do this without throwing an exception. What would be the best way to do this?
The InvalidOperationException you get has the message:
Sequence contains no matching element
The behaviour of First is that it throws this exception if the element is not found.
You can use FirstOrDefault instead of First and check for null. The null-coalescing operator (??) can be used to make this check.
Foo ent = ctx.Foo.FirstOrDefault(f => f.ID == fid) ?? new Foo();
Note also that there is a similar pair of functions, Single and SingleOrDefault that throw an exception if more than one matching element is found. In your specific case, assuming that the identities should be unique, it might be more appropriate to use SingleOrDefault.
I need to translate the following Code to an Expression and I will explain why:
results = results.Where(answer => answer.Question.Wording.Contains(term));
results is IQueryable<ISurveyAnswer>
Question is ISurveyQuestion
Wording is String
The problem is, Question is not always the name of the LINQ to SQL property.
This will give me the PropertyInfo for the actual ISurveyQuestion property
private static PropertyInfo FindNaturalProperty<TMemberType>(Type search)
{
IDictionary<string,PropertyInfo> properties = new Dictionary<string,PropertyInfo>();
search.GetProperties().Each(prop =>
{
if (null != prop.PropertyType.GetInterface(typeof(TMemberType).Name))
properties.Add(prop.Name, prop);
});
if (properties.Count < 1) throw new ArgumentException(String.Format("{0} has no properties of type {1}", search.Name, typeof(TMemberType).Name));
if (properties.Count == 1) return properties.Values.First();
search.GetInterfaces().Each(inter =>
{
inter.GetProperties().Each(prop =>
{
if (null != prop.PropertyType.GetInterface(typeof(TMemberType).Name))
properties.Remove(prop.Name);
});
});
if (properties.Count < 1) throw new ArgumentException(String.Format("{0} has no properties of type {1} that are not members of an interface", search.Name, typeof(TMemberType).Name));
if (properties.Count > 1) throw new AmbiguousMatchException(String.Format("{0} has more than one property that are of type {1} and are not members of an interface", search.Name, typeof(TMemberType).Name));
return properties.Values.First();
}
Once I have the PropertyInfo how do I translate that to an Expression Tree?
EDIT:
What I basically need is:
results = results.Where(answer => answer.GetQuestionProperty().GetValue(answer).Wording.Contains(term));
But that doesn't work so I need to build the Expression Tree myself, for linq-to-sql.
Reading the question I think what you're after is Dynamic Linq - which is a helper library to let you build Linq queries dynamically (!) using strings as opposed to at design time. That means that if you can get your property name you should be able create your query on the fly.
ScottGu has an article here
What your trying to do is create a dynamic query and you want the action tables / properties your query against to be dynamic as well. I am not sure if this is easily possible based on how you want to use it.
Check out ScottGu's blog post:
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
and
Check out Rick Strahl's blog post:
http://www.west-wind.com/Weblog/posts/143814.aspx
http://www.linqpad.net/
linqpad will convert it for you.