When I add to array of Actions with a for loop a delegate the I is updated across the array. How to prevent that from happening?
I have tried assigning the "I" to an integer before adding that works.
Action[] actions = new Action[100];
for (int i = 0;i< actions.Length; i++)
{
actions[i] = () => Console.WriteLine("Hello"+ i);
}
"I" in each Action in Action[] is 100;
Why is that?
because they are all assigned to the same local variable "int i"
and after the loop end "i" is 100
Action[] actions = new Action[100];
for (int i = 0;i< actions.Length; i++)
{
int a = i;
actions[i] = () => Console.WriteLine("Hello"+ a);
}
after declare int a = i , you have respective a for each actions
Here and Here are good explanations on similar question. Here is also good explanation on C# closures by Jon Skeet.
In for loop there is only one single variable i used. That's why later on when you are executing actions, they all reference to same value i=100. If action need to use actual value of the current i you have to capture a copy of it and store copy to action.
for (int i = 0;i< actions.Length; i++)
{
int copy = i;
actions[i] = () => Console.WriteLine("Hello"+ copy);
}
Related
Hey I'm in a situation where I have a for loop that does some stuff and I want to make a line of code either call a function passing in an array indexed by the for loops index, or run a single (not array) variable for every call of that function, I know I could do that by putting an if statement inside the for loop but i'd be repeating the same if statement over and over getting the same result. So is there a way good way I can run the if statement before the for loop and the result of that if statement run the same for loop but that one call passes in the array or the variable?
Code Example
for (int i = 0; i < CurrentVerticalList.Count; i++)
{
GuiGeneral CGroup = CurrentVerticalList[i];
CGroup.ResizeUsingStandard(ForcedResize[i]); //I want the condition before the for
//loop to have ForcedResize[i] here if
//true and another variable here of the
//same type but not an array if false.
for (int j = 0; j < 2; j++)
{
GlobalListIndex[j]++;
}
CGroup.MoveElementTo(CCoord, false);
CCoord.y += CGroup.ElementRect.WidthHeight.y;
}
Here you go, moving your condition check out of your for loop:
Func<int, double> GetResizeFromForcedResize = (index => ForcedResize[index]);
Func<int, double> GetResizeFromVariable = (index => fixVariable);
var GetResizeValue = condition? GetResizeFromForcedResize : GetResizeFromVariable;
for (int i = 0; i < CurrentVerticalList.Count; i++)
{
GuiGeneral CGroup = CurrentVerticalList[i];
CGroup.ResizeUsingStandard(GetResizeValue(i));
for (int j = 0; j < 2; j++)
{
GlobalListIndex[j]++;
}
CGroup.MoveElementTo(CCoord, false);
CCoord.y += CGroup.ElementRect.WidthHeight.y;
}
Edit: Wanted to let you know that the other answers here are still doing a check at every iteration, but not this one.
Eh, hard to understand the question to me but i recon what you want is something along these lines, could've helped with more types supplied, but you could make the intend achievable using a local function:
ForcedResizeArrayType other = new object(); //TODO: Define return type
bool condition = ResolveCondition(); //TODO: Define condition to be true or false
ForcedResizeArrayType GetOneOr(int i, bool condition,
ForcedResizeArrayType[] forcedResizeArray)
{
return condition ? forcedResizeArray[i] : other;
}
for (int i = 0; i < CurrentVerticalList.Count; i++)
{
CGroup.ResizeUsingStandard(GetOneOr(i, condition, ForcedResize));
}
It varies from week to week if i love or hate those local functions, but they have uses
I have the following method. When the Writeline Methods are calling, missing results arise. The result of
the second Writeline must true otherwise, it is false, could you please advise me?
public static void IsGreater()
{
var biggerThanNumber = new Predicate<int>[10];
for (int index = 0; index < biggerThanNumber.Length; ++index)
{
biggerThanNumber[index] = value => value > index;
}
Console.WriteLine(biggerThanNumber[5](20)); //True
Console.WriteLine(biggerThanNumber[5](6)); //False
}
Change the method to copy loop variable into a local reference to avoid lambda closure issue in a loop. In your method value of index is always 10 hence second the console is returning false. For more info check this post
public static void IsGreater()
{
var biggerThanNumber = new Predicate<int>[10];
for (int index = 0; index < biggerThanNumber.Length; ++index)
{
int localCopy = index;
biggerThanNumber[index] = value => value > localCopy;
}
Console.WriteLine(biggerThanNumber[5](20)); //True
Console.WriteLine(biggerThanNumber[5](6)); //True
}
So I have this homework assignment that requires me to assign output to labels after I compare two arrays. My problem is that after I compare the two arrays, the output I assign is wrong. I'm supposed to out 'Y' if at a specific index of the two arrays are equal and 'N' if they're not equal but every time I run the code, it outputs 'Y' to all the labels no matter what. How can I fix what is being outputted after the comparison?
private void evaluateStudentAnswers()
{
/* Use a "for" loop to cycle through the answerKey[] and studentAnswers[] arrays, and compare the answers
* in the two arrays at each index. If they match, then increment the global variable "correctAnswers"
* and assign the value 'Y' to the corresponding index in the correctOrIncorrect[] array. if they
* don't match, then increment the global variable "incorrectAnswers" and assign the value 'N' to the
* corresponding indes in the correctOrIncorrec[] array. These two variables will be used to calculate
* the grade percentage.
*/
for (int i = 0; i < studentAnswers.Length; i++)
{
for(int j = 0; j < answerKey.Length; j++)
{
// I think the indexes below are being checked if they're the same and I need to make sure not just the
//indexes are the same but the values as well
if (studentAnswers[i] == answerKey[j])
{
correctAnswers++;
for(int k = 0; k < correctOrIncorrect.Length; k++)
{
correctOrIncorrect[k] = 'Y';
}
}
else
{
incorrectAnswers++;
for (int k = 0; k < correctOrIncorrect.Length; k++)
{
correctOrIncorrect[k] = 'N';
}
}
}
}
}
I think your code can be simplified quite a lot. assuming there's a 1-1 mapping between studentAnswers and answerKey.
for (int i = 0; i < studentAnswers.Length; i++)
{
var studentAnswer = studentAnswers[i];
var answer = answerKey[i];
if (studentAnswer == answer)
{
++correctAnswers;
correctOrIncorrect[i] = 'Y';
}
else
{
++incorrectAnswers;
correctOrIncorrect[i] = 'N'
}
}
All of the arrays are the same size. So when we loop over each answer the student provided, we know we can find the corresponding correct answer in answerKey. Also, the tracking of correct answers also follows the same pattern, for each studentAnswer, we want to record the correctness in correctOrIncorrect, which corresponds to the particular answer the student provided. As such, we only need to perform a single loop, since the i refers to the appropriate index in all the arrays as we're processing.
If studentAnswers.Length == answerKey.Length == correctOrIncorrect.Length
Then
for (int i = 0; i < studentAnswers.Length; i++)
{
if(studentAnswers[i] == answerKey[j])
{
correctAnswers++;
correctOrIncorrect[k] = 'Y';
}
else
{
incorrectAnswers++;
correctOrIncorrect[k] = 'N';
}
}
Since it is an assignment I wont give an answer :) but since you are stuck I encourage you use the guidance below.
These two are unnecessary in your code
inner for-loop on correctOrIncorrect[]
variable "k", you can use "i" instead for correctOrIncorrect value assignment
Since the arrays have to have the same size/order, you only need to loop through them once. Also I find ternary assignments more clear than if blocks:
Func<bool, int> toInt = (b) => b ? 1 : 0;
for (int i = 0; i < studentAnswers.Length; i++)
{
var studentAnswer = studentAnswers[i];
var answer = answerKey[i];
var isCorrect = studentAnswer == answer;
correctOrIncorrect[i] = isCorrect ? 'Y' : 'N';
correctAnswers = isCorrect ? 1 : 0; // toInt(isCorrect)
incorrectAnswers = !isCorrect ? 1 : 0; // toInt(!isCorrect)
}
}
Or in LINQ (just because it's worth learning, but probably not appropriate for homework):
correctOrIncorrect = answerKey.Zip(studentAnswer, (a,b) => a == b ? "Y" : "N").ToArray();
incorrectAnswers = correctOrIncorrect.Count(x => x == "Y");
...
I often get strange resulst when passing int variables to tasks such as in this example:
List<List<object>> ListToProcess = new List<List<object>>();
// place some lists in list to process
foreach (var temp in Foo)
ListToProcess.Add(temp);
foreach (var tempArray in ListToProcess)
{
// initialize each list in ListToProcess
}
int numberOfChunks = ListToProcess.Count;
Task[] tasks = new Task[numberOfChunks];
for (int counter = 0; counter < numberOfChunks; counter++)
{
tasks[counter] = Task.Factory.StartNew(() =>
{
// counter is always = 5 why? <---------------------------
var t = ListToProcess[counter];
});
}
How can I solve this problem?
This is known as a closure. You are not using the value of the variable, you are using the variable itself. When the code executes, it uses the value at the time of execution, not the value when the Task was defined.
To fix this issue, I believe you would do something like this:
for (int counter = 0; counter < numberOfChunks; counter++)
{
int cur = counter;
tasks[counter] = Task.Factory.StartNew(() =>
{
// counter is always = 5 why? <---------------------------
var t = ListToProcess[cur];
});
}
There is no guarantee as to when the 'counter' variable in the Action block of StartNew will be accessed. What is likely to happen is that all 5 values are looped through, and the tasks are created, then the tasks are scheduled to run.
When they do run, the following is executed:
var t = ListToProcess[counter];
But at this stage count is already equal to 5.
Perhaps you should look at parallel collections?
ListToProcess.AsParallel().ForAll(list => dosomething(list));
There are many other options around this area.
for (int counter = 0; counter < numberOfChunks; counter++)
{
var referenceVariable = new{val=counter};
tasks[counter] = Task.Factory.StartNew(() =>
{
var t = ListToProcess[referenceVariable.val];
});
}
Since variables are captured, you can solve this by redeclaring a new variable in each loop.
for (int counter = 0; counter < numberOfChunks; counter++)
{
int localCounter = counter;
tasks[localCounter] = Task.Factory.StartNew(() =>
{
// counter is always = 5 why? <---------------------------
var t = ListToProcess[localCounter];
});
}
Hi all i am trying to write and anonymous delegate. as the integer variable is shared among the delegate i need it to be the local instance of every delegate such that rs[0] always gets nics[0], rs[1] always gets nics[1] and so on... how will i achieve this.
for (int i = 0; i < nics.Count; i++)
{
rs[i] = new RollingSeries(monitor, new RollingSeries.NextValueDelegate(delegate()
{
return GetNetworkUtilization(nics[i]);
}));
}
Abdul khaliq
Make a local copy of i:
for (int i = 0; i < nics.Count; i++)
{
int j = i;
rs[i] = new RollingSeries(monitor, new RollingSeries.NextValueDelegate(delegate()
{
return GetNetworkUtilization(nics[j]);
}));
}
The Beauty of Closures
Use a local to get a different value per iteration
for (int i = 0; i < nics.Count; i++)
{
int localI = i;
rs[i] = new RollingSeries(monitor, new RollingSeries.NextValueDelegate(delegate()
{
return GetNetworkUtilization(nics[localI]);
}));
}
Put int j = i inside your loop and refer to j within the lambda expression.
If you are curious about why this happens, here is an MSDN blog entry containing a detailed technical explanation: Closing over the loop variable considered harmful