The past few days, I've been desperately trying to fix an issue with my list count in C++.
This is my function that handles Count & Add new plr in list.
/*Iterator for players*/
list<Player*>::iterator itc;
list<Player*>PlayerCount;
int FindPosition(list<Player*>List, Player* plr)
{
int pos = 1;
for (itc = List.begin(); itc != List.end(); itc++, pos++)
{
if ((*itc) = plr)
break;
}
return pos;
}
What i'm trying to do is implement each time someone Queue in my list.
My output is currently showing this :
Player One Queue. Player Two Queue
it keep saying : Player in queue [1]
like there was no implementation for next row
How it Should be.
Player One : Player in queue [1]
Player Two : Player in Queue [2]
Thanks for helping !
As I mentioned in my comment above, the problem was the if ((*itc) = plr) was assigning plr to *itc instead of performing a comparison if ((*itc) == plr).
One friendly suggestion,
In FindPosition function, list<Player*>List is being passed by value which means that you are creating a copy of the whole list every time that you are calling the function. If that is the intended behavior then you can keep it as is otherwise you could rewrite it as follows:
// pass List by value
// int FindPosition(list<Player*>List, Player* plr)
// pass List by reference
int FindPosition(list<Player*>& List, Player* plr)
{
int pos = 1;
for (itc = List.begin(); itc != List.end(); itc++, pos++)
{
// changed from assigment (*itc) = plr
// to comparison (*itc) == plr
if ((*itc) == plr)
break;
}
return pos;
}
I need a thread safe data structure with three operations: remove, getRandom, reset.
I have only two ideas by now.
First: Seq in syncronized var.
val all: Array[String] = ... //all possible.
var current: Array[String] = Array.empty[String]
def getRandom(): = {
val currentAvailable = current
currentAvailable(Random.nextInt(currentAvailable.length))
}
def remove(s: String) = {
this.syncronized {
current = current diff Seq(s)
}
}
def reset(s: String) = {
this.syncronized {
current = all
}
}
Second:
Maintain some Map[String,Boolean], there bool is true when element currently is present. The main problem is to make a fast getRandom method (not something like O(n) in worst case).
Is there a better way(s) to implement this?
Scala's Trie is a lock free data structure that supports snapshots (aka your currentAvailable) and fast removals
Since I'm not a Scala expert so this answer is general as an example I used Java coding.
in short the answer is YES.
if you use a map such as :
Map<Integer,String> map=new HashMap<Integer,String>(); //is used to get random in constant time
Map<String,Integer> map1=new HashMap<String,Integer>(); //is used to remove in constant time
to store date,
the main idea is to keep the key( in this case the integer) synchronized to be {1 ... size of map}
for example to fill this structure, you need something like this:
int counter=0; //this is a global variable
for(/* all your string (s) in all */ ){
map.put(counter++, s);
}
//then , if you want the removal to be in constant time you need to fill the second map
for(Entry e : map.EntrySet(){
map1.put(e.getValue(),e.getKey());
}
The above code is the initialization. everytime you want to set things you need to do that
then you can achieve a random value with O(1) complexity
String getRandom(){
int i; /*random number between 0 to counter*/
return map.get(i);
}
Now to remove things you use map1 to achive it in constant time O(1);
void remove(String s){
if(!map1.containsKey(s))
return; //s doesn't exists
String val=map.get(counter); //value of the last
map.remove(counter) //removing the last element
int thisCounter= map1.get(s); //pointer to this
map1.remove(s); // remove from map1
map.remove(counter); //remove from map
map1.put(thisCounter,val); //the val of the last element with the current pointer
counter--; //reducing the counter by one
}
obviously the main issue here is to keep the synchronization ensured. but by carefully analyzing the code you should be able to do that.
Why is the output of the given 200 and not 20000 as I had expected ????
Please help me on this !!!
class Program
{
static void Main(string[] args)
{
mukul x = new mukul();
x.b= 200;
Console.WriteLine(Convert.ToString(x.calculate));
Console.ReadLine()
}
}
class mukul
{
public int b;
public int calculate
{
get { return b; }
set { b = value * 100; }
}
}
You're setting x.b directly - you're not using the calculate setter, so it's not multiplying by 100.
If you changed this line:
x.b = 200;
to this:
x.calculate = 200;
then it would act as you expect.
There are several points to make though:
If you indent your code properly, it will make it easier to read
If your b field were private, you couldn't have set it directly. Fields should almost always be private.
You should follow normal .NET naming conventions (PascalCase for properties and types, for example)
Your property is very odd. It's very unusual for code like this:
x.calculate = x.calculate;
to actually make a difference. I would rethink your design if I were you.
If you're calling Console.WriteLine, there's already an overload to handle an int value, and even if there weren't the value could be boxed to object instead. So your code would be written more simply as:
Console.WriteLine(x.calculate);
In this " x.b= 200;" you calling only "b" variable.not "calculate" method.So every time you get the 200 as output with out performing calculate method.
For better understanding
Take two breakpoints at the below two instructions
get { return b; }
set { b = value * 100; }
Then perform stepinto debug(press F11) for the two instructons like "x.b=200" and "x.claculate=200".Then observe the difference between "x.b=200" and "x.claculate=200"
In x.b=200
"set{b=value*100;}"method cant execute.That means b=value*100 not executed.So every time you get "200" as output.
In x.calculate=200
"set{b=value*100;}"method is executed.That means b assigned with value*100.So you get "20000" as output.
Finally you have to call method("calculate") not variable("b").
Hope you got the answer.Have a happy Programming........
I am really confused about this concept:
/* Example with primitive data type */
public class Example1 {
public static void main (String[] args){
int a = 1;
System.out.println("a is " + a);
myMethod( a );
System.out.println("a is " + a);
}
public static void myMethod(int b){
b = 3;
System.out.println("b is " + b);
}
}
OUTPUT:
a is 1
b is 3
a is 1
Why does "a" not change?How does this primitive variable CHANGE like for a FOR LOOP or a WHILE LOOP when int i is initialed to zero? Like this:
int i = 1;
while (i < = 3) {
System.out.println(i);
i *= 2;
}
OUTPUT:
1
2
Please let me know in detail, as I am really confused.i is a primitive type, why does it get updated, and why does not int a in the first program?
myMethod() is void, if it returned an int and you assigned a=myMethod(a) then it would change
int a = 1;
System.out.println("a is " + a);
a= myMethod(a); //where myMethod is changed to return b instead of void
System.out.println("a is " + a);
a is 1
b is 3
a is 3
"Why does "a" not change?"
Because primitive a inside of your myMethod is not the same a that you had in your void main. Treat it as completely another variable and that its value got copied into myMethod. This primitive`s lifecycle ends in the end of this method execution.
If you have C++ background then maybe this explanation might help:
When you pass primitive type arguments into method - you are passing
variables that are being copied. You passed value, not instance.
When you pass objects as arguments
into method - you are passing references to that object, but to be more precise: in java, a copy of reference value is being passed. It is like passing a copy of the address of the object to the method. If you modify this object inside this method, the modifications will be visible outside the method. If you =null or =new Obj it, it will affect the object only inside your method.
Does C# 4.0 allow optional out or ref arguments?
No.
A workaround is to overload with another method that doesn't have out / ref parameters, and which just calls your current method.
public bool SomeMethod(out string input)
{
...
}
// new overload
public bool SomeMethod()
{
string temp;
return SomeMethod(out temp);
}
If you have C# 7.0, you can simplify:
// new overload
public bool SomeMethod()
{
return SomeMethod(out _); // declare out as an inline discard variable
}
(Thanks #Oskar / #Reiner for pointing this out.)
As already mentioned, this is simply not allowed and I think it makes a very good sense.
However, to add some more details, here is a quote from the C# 4.0 Specification, section 21.1:
Formal parameters of constructors, methods, indexers and delegate types can be declared optional:
fixed-parameter:
attributesopt parameter-modifieropt type identifier default-argumentopt
default-argument:
= expression
A fixed-parameter with a default-argument is an optional parameter, whereas a fixed-parameter without a default-argument is a required parameter.
A required parameter cannot appear after an optional parameter in a formal-parameter-list.
A ref or out parameter cannot have a default-argument.
No, but another great alternative is having the method use a generic template class for optional parameters as follows:
public class OptionalOut<Type>
{
public Type Result { get; set; }
}
Then you can use it as follows:
public string foo(string value, OptionalOut<int> outResult = null)
{
// .. do something
if (outResult != null) {
outResult.Result = 100;
}
return value;
}
public void bar ()
{
string str = "bar";
string result;
OptionalOut<int> optional = new OptionalOut<int> ();
// example: call without the optional out parameter
result = foo (str);
Console.WriteLine ("Output was {0} with no optional value used", result);
// example: call it with optional parameter
result = foo (str, optional);
Console.WriteLine ("Output was {0} with optional value of {1}", result, optional.Result);
// example: call it with named optional parameter
foo (str, outResult: optional);
Console.WriteLine ("Output was {0} with optional value of {1}", result, optional.Result);
}
There actually is a way to do this that is allowed by C#. This gets back to C++, and rather violates the nice Object-Oriented structure of C#.
USE THIS METHOD WITH CAUTION!
Here's the way you declare and write your function with an optional parameter:
unsafe public void OptionalOutParameter(int* pOutParam = null)
{
int lInteger = 5;
// If the parameter is NULL, the caller doesn't care about this value.
if (pOutParam != null)
{
// If it isn't null, the caller has provided the address of an integer.
*pOutParam = lInteger; // Dereference the pointer and assign the return value.
}
}
Then call the function like this:
unsafe { OptionalOutParameter(); } // does nothing
int MyInteger = 0;
unsafe { OptionalOutParameter(&MyInteger); } // pass in the address of MyInteger.
In order to get this to compile, you will need to enable unsafe code in the project options. This is a really hacky solution that usually shouldn't be used, but if you for some strange, arcane, mysterious, management-inspired decision, REALLY need an optional out parameter in C#, then this will allow you to do just that.
ICYMI: Included on the new features for C# 7.0 enumerated here, "discards" is now allowed as out parameters in the form of a _, to let you ignore out parameters you don’t care about:
p.GetCoordinates(out var x, out _); // I only care about x
P.S. if you're also confused with the part "out var x", read the new feature about "Out Variables" on the link as well.
No, but you can use a delegate (e.g. Action) as an alternative.
Inspired in part by Robin R's answer when facing a situation where I thought I wanted an optional out parameter, I instead used an Action delegate. I've borrowed his example code to modify for use of Action<int> in order to show the differences and similarities:
public string foo(string value, Action<int> outResult = null)
{
// .. do something
outResult?.Invoke(100);
return value;
}
public void bar ()
{
string str = "bar";
string result;
int optional = 0;
// example: call without the optional out parameter
result = foo (str);
Console.WriteLine ("Output was {0} with no optional value used", result);
// example: call it with optional parameter
result = foo (str, x => optional = x);
Console.WriteLine ("Output was {0} with optional value of {1}", result, optional);
// example: call it with named optional parameter
foo (str, outResult: x => optional = x);
Console.WriteLine ("Output was {0} with optional value of {1}", result, optional);
}
This has the advantage that the optional variable appears in the source as a normal int (the compiler wraps it in a closure class, rather than us wrapping it explicitly in a user-defined class).
The variable needs explicit initialisation because the compiler cannot assume that the Action will be called before the function call exits.
It's not suitable for all use cases, but worked well for my real use case (a function that provides data for a unit test, and where a new unit test needed access to some internal state not present in the return value).
Use an overloaded method without the out parameter to call the one with the out parameter for C# 6.0 and lower. I'm not sure why a C# 7.0 for .NET Core is even the correct answer for this thread when it was specifically asked if C# 4.0 can have an optional out parameter. The answer is NO!
For simple types you can do this using unsafe code, though it's not idiomatic nor recommended. Like so:
// unsafe since remainder can point anywhere
// and we can do arbitrary pointer manipulation
public unsafe int Divide( int x, int y, int* remainder = null ) {
if( null != remainder ) *remainder = x % y;
return x / y;
}
That said, there's no theoretical reason C# couldn't eventually allow something like the above with safe code, such as this below:
// safe because remainder must point to a valid int or to nothing
// and we cannot do arbitrary pointer manipulation
public int Divide( int x, int y, out? int remainder = null ) {
if( null != remainder ) *remainder = x % y;
return x / y;
}
Things could get interesting though:
// remainder is an optional output parameter
// (to a nullable reference type)
public int Divide( int x, int y, out? object? remainder = null ) {
if( null != remainder ) *remainder = 0 != y ? x % y : null;
return x / y;
}
The direct question has been answered in other well-upvoted answers, but sometimes it pays to consider other approaches based on what you're trying to achieve.
If you're wanting an optional parameter to allow the caller to possibly request extra data from your method on which to base some decision, an alternative design is to move that decision logic into your method and allow the caller to optionally pass a value for that decision criteria in. For example, here is a method which determines the compass point of a vector, in which we might want to pass back the magnitude of the vector so that the caller can potentially decide if some minimum threshold should be reached before the compass-point judgement is far enough away from the origin and therefore unequivocally valid:
public enum Quadrant {
North,
East,
South,
West
}
// INVALID CODE WITH MADE-UP USAGE PATTERN OF "OPTIONAL" OUT PARAMETER
public Quadrant GetJoystickQuadrant([optional] out magnitude)
{
Vector2 pos = GetJoystickPositionXY();
float azimuth = Mathf.Atan2(pos.y, pos.x) * 180.0f / Mathf.PI;
Quadrant q;
if (azimuth > -45.0f && azimuth <= 45.0f) q = Quadrant.East;
else if (azimuth > 45.0f && azimuth <= 135.0f) q = Quadrant.North;
else if (azimuth > -135.0f && azimuth <= -45.0f) q = Quadrant.South;
else q = Quadrant.West;
if ([optonal.isPresent(magnitude)]) magnitude = pos.Length();
return q;
}
In this case we could move that "minimum magnitude" logic into the method and end-up with a much cleaner implementation, especially because calculating the magnitude involves a square-root so is computationally inefficient if all we want to do is a comparison of magnitudes, since we can do that with squared values:
public enum Quadrant {
None, // Too close to origin to judge.
North,
East,
South,
West
}
public Quadrant GetJoystickQuadrant(float minimumMagnitude = 0.33f)
{
Vector2 pos = GetJoystickPosition();
if (minimumMagnitude > 0.0f && pos.LengthSquared() < minimumMagnitude * minimumMagnitude)
{
return Quadrant.None;
}
float azimuth = Mathf.Atan2(pos.y, pos.x) * 180.0f / Mathf.PI;
if (azimuth > -45.0f && azimuth <= 45.0f) return Quadrant.East;
else if (azimuth > 45.0f && azimuth <= 135.0f) return Quadrant.North;
else if (azimuth > -135.0f && azimuth <= -45.0f) return Quadrant.South;
return Quadrant.West;
}
Of course, that might not always be viable. Since other answers mention C# 7.0, if instead what you're really doing is returning two values and allowing the caller to optionally ignore one, idiomatic C# would be to return a tuple of the two values, and use C# 7.0's Tuples with positional initializers and the _ "discard" parameter:
public (Quadrant, float) GetJoystickQuadrantAndMagnitude()
{
Vector2 pos = GetJoystickPositionXY();
float azimuth = Mathf.Atan2(pos.y, pos.x) * 180.0f / Mathf.PI;
Quadrant q;
if (azimuth > -45.0f && azimuth <= 45.0f) q = Quadrant.East;
else if (azimuth > 45.0f && azimuth <= 135.0f) q = Quadrant.North;
else if (azimuth > -135.0f && azimuth <= -45.0f) q = Quadrant.South;
else q = Quadrant.West;
return (q, pos.Length());
}
(Quadrant q, _) = GetJoystickQuadrantAndMagnitude();
if (q == Quadrant.South)
{
// Do something.
}
What about like this?
public bool OptionalOutParamMethod([Optional] ref string pOutParam)
{
return true;
}
You still have to pass a value to the parameter from C# but it is an optional ref param.
void foo(ref int? n)
{
return null;
}