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Assign a formula at runtime for accessing elements of List

The scenario is that I have say two different types of cases - case 1 and case 2. For case 1 and case 2 each I have a certain startIndex, endIndex and a formula for accessing the elements of a List.
Now for assigning values startIndex and endIndex I am preferring a normal switch case, however I am at loss for the formula for accessing elements. For case 1 it is say something like List[ a+i ] and for case 2 it is say List[a + (i-b)].
One way can be to have a for loop like this
for(int i=0;;i++)
{
if(case is 1)
then f=a+i
else if(case 2)
then f=a+(i-b)
}
I thought of using delegates. however, as per my knowledge they need to be made global. Actions do not return value. Func can be used but one expression/formula takes only one element (int) and the other takes 3. I need something in lines to this like that anonymous function can be assigned any of above mentioned formulae at runtime from the switch case (as the cases might and will increase in future).
Thank you.

I thought of using delegates. however, as per my knowledge they need
to be made global.
This is not true (actually, there are no truly global variables in C#, since each and every variable needs to be encapsulated inside an object). A public delegate type is indeed visible to all code after referencing the assembly containing this type's code, but a variable of such type can be private.
What I recommend in your situation is to have some sort of mapping from case numbers to delegates. A good idea is to use a Dictionary<TKey, TValue> if you have at most one delegate per case. This dictionary can be stored as a private variable inside the class where your method resides.
public class MyClass
{
private Dictionary<int, Delegate> _delegateMapping = new Dictionary<int, Delegate>;
}
There are a couple of ways you can add elements do the dictionary in the constructor: passing the already populated dictionary, passing an array of delegates, creating these delegates in the constructor itself. Either way, you'll end up with a dictionary of Delegate types, so you'll need to use a cast to be able to use them in your code properly.
for (int i = 1; i < _delegateMapping.Count; i++)
{
switch (i)
{
case 1:
var f = (Action<int>)_delegateMapping[1];
f(i);
break;
case 2:
var f = (Action<int, int>)_delegateMapping[2];
f(i, a);
break;
}
}
Of course I'm greatly improvising here.
It is important to note that if the type of delegate changes inside the dictionary, you will have to modify the cast accordingly inside the switch statement. Otherwise, if no implicit cast exists, you'll get a runtime exception.

Hi guys thank you so very much for your feedbacks. I finally found the solution with Func. This is what my code looks like. I had to manipulate the Func usage a little. I made almost all the vars which I have to use in the Func as global/local to the function where I write these Funcs. My apologies, if I was not able to explain my problem properly.
int i = -1;
Func<int,int> formula = null;
switch(f)
{
case 1:
{
formula = new Func<int,int>(index => { return i; });
}
break;
case 2:
{
formula = new Func<int, int>( index => { return s- (i * c); } );//here s and c are global variables.
}
break;
}
i = startIndex;
while(i < endIndex)
{
var Obj= List[formula.Invoke(i)];
//my code goes here
i++;
}
Let me know if my solution is correct w.r.t performance, logic, C# programming, etc.. :)
EDITED::
#usr and #Kapol I tried the way you suggested and tried to improvise the code like this.
private Dictionary<int, Func<int[], int>> indexFormulae;
private void assignDelegates()
{
indexFormulae = new Dictionary<int, Func<int[], int>>();
indexFormulae.Add(0, getFormula_1);
indexFormulae.Add(1, getFormula_2);
}
private void someFunction(int sp)
{
int i = 0;
Func<int[], int> formula = null;
indexFormulae.TryGetValue(formation,out formula);
i = startIndex;
while (i < endIndex)
{
int[] intValues = new int[] {i,sp,globalVar };
var Obj = List[formula.Invoke(intValues)];
//My code here
i++;
}
}
private int getFormula_1(params int[] intValues)
{
return intValues[0];
}
private int getIndex_Vertical(params int[] intValues)
{
return intValues[1] - (intValues[0] * intValues[2]);
}
So, that with this now I can use these two getFormula methods anywhere in this class rather than keeping them anonymous. and also I think I will stick to params because I might have N number of int params in future for other functions.

Closures and java anonymous inner classes

Would anyone be so kind to post the equivalent Java code for a closure like this one (obtained using C#) with anonymous inner classes?
public static Func<int, int> IncrementByN()
{
int n = 0; // n is local to the method
Func<int, int> increment = delegate(int x)
{
n++;
return x + n;
};
return increment;
}
static void Main(string[] args)
{
var v = IncrementByN();
Console.WriteLine(v(5)); // output 6
Console.WriteLine(v(6)); // output 8
}
Furthermore, can anyone explain how partial applications can be obtained if lexical closures are available and viceversa? For this second question, C# would be appreciated but it's your choice.
Thanks so much.

There is no closure yet in Java. Lambda expressions are coming in java 8. However, the only issue with what you're trying to translate is that it has state, which not something that lamba expressions will support i don't think. Keep in mind, it's really just a shorthand so that you can easily implement single method interfaces. You can however still simulate this I believe:
final AtomicInteger n = new AtomicInteger(0);
IncrementByN v = (int x) -> x + n.incrementAndGet();
System.out.println(v.increment(5));
System.out.println(v.increment(6));
I have not tested this code though, it's just meant as an example of what might possibly work in java 8.
Think of the collections api. Let's say they have this interface:
public interface CollectionMapper<S,T> {
public T map(S source);
}
And a method on java.util.Collection:
public interface Collection<K> {
public <T> Collection<T> map(CollectionMapper<K,T> mapper);
}
Now, let's see that without closures:
Collection<Long> mapped = coll.map(new CollectionMapper<Foo,Long>() {
public Long map(Foo foo) {
return foo.getLong();
}
}
Why not just write this:
Collection<Long> mapped = ...;
for (Foo foo : coll) {
mapped.add(foo.getLong());
}
Much more concise right?
Now introduce lambdas:
Collection<Long> mapped = coll.map( (Foo foo) -> foo.getLong() );
See how much nicer the syntax is? And you can chain it too (we'll assume there's an interface to do filtering which which returns boolean values to determine whether to filter out a value or not):
Collection<Long> mappedAndFiltered =
coll.map( (Foo foo) -> foo.getLong() )
.filter( (Long val) -> val.longValue() < 1000L );

This code is equivalent I believe (at least it produces the desired output):
public class Test {
static interface IncrementByN {
int increment(int x);
}
public static void main(String[] args) throws InterruptedException {
IncrementByN v = new IncrementByN() { //anonymous class
int n = 0;
#Override
public int increment(int x) {
n++;
return x + n;
}
};
System.out.println(v.increment(5)); // output 6
System.out.println(v.increment(6)); // output 8
}
}

Assuming we have a generic function interface:
public interface Func<A, B> {
B call A();
}
Then we can write it like this:
public class IncrementByN {
public static Func<Integer, Integer> IncrementByN()
{
final int n_outer = 0; // n is local to the method
Func<Integer, Integer> increment = new Func<Integer, Integer>() {
int n = n_outer; // capture it into a non-final instance variable
// we can really just write int n = 0; here
public Integer call(Integer x) {
n++;
return x + n;
}
};
return increment;
}
public static void main(String[] args) {
Func<Integer, Integer> v = IncrementByN();
System.out.println(v.call(5)); // output 6
System.out.println(v.call(6)); // output 8
}
}
Some notes:
In your program, you capture the variable n by reference from the enclosing scope, and can modify that variable from the closure. In Java, you can only capture final variables (thus capture is only by value).
What I did here is capture the final variable from the outside, and then assign it into a non-final instance variable inside the anonymous class. This allows "passing info" into the closure and at the same time having it be assignable inside the closure. However, this information flow only works "one way" -- changes to n inside the closure is not reflected in the enclosing scope. This is appropriate for this example because that local variable in the method is not used again after being captured by the closure.
If, instead, you want to be able to pass information "both ways", i.e. have the closure also be able to change things in the enclosing scope, and vice versa, you will need to instead capture a mutable data structure, like an array, and then make changes to elements inside that. That is uglier, and is rarer to need to do.

Can an Action/delegate change it's arguments value?

I ran into what was to me an unexpected result when testing a simple ForEach extension method.
ForEach method
public static void ForEach<T>(this IEnumerable<T> list, Action<T> action)
{
if (action == null) throw new ArgumentNullException("action");
foreach (T element in list)
{
action(element);
}
}
Test method
[TestMethod]
public void BasicForEachTest()
{
int[] numbers = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
numbers.ForEach(num =>
{
num = 0;
});
Assert.AreEqual(0, numbers.Sum());
}
Why would numbers.Sum() be equal to 55 and not 0?

num is the copy of the value of the current element you are iterating over. So you are just changing the copy.
What you do is basically this:
foreach(int num in numbers)
{
num = 0;
}
Surely you do not expect this to change the content of the array?
Edit: What you want is this:
for (int i in numbers.Length)
{
numbers[i] = 0;
}
In your specific case you could maintain an index in your ForEach extension method and pass that as second argument to the action and then use it like this:
numbers.ForEachWithIndex((num, index) => numbers[index] = 0);
However in general: Creating Linq style extension methods which modify the collection they are applied to are bad style (IMO). If you write an extension method which cannot be applied to an IEnumerable<T> you should really think hard about it if you really need it (especially when you write with the intention of modifying the collection). You have not much to gain but much to loose (like unexpected side effects). I'm sure there are exceptions but I stick to that rule and it has served me well.

Because num is a copy.
It's as if you were doing this:
int i = numbers[0];
i = 0;
You wouldn't expect that to change numbers[0], would you?

Because int is a value type and is passed to your extension method as a value parameter. Thus a copy of numbers is passed to your ForEach method. The values stored in the numbers array that is initialized in the BasicForEachTest method are never modified.
Check this article by Jon Skeet to read more on value types and value parameters.

I am not claiming that the code in this answer is useful, but (it works and) I think it illustrates what you need in order to make your approach work. The argument must be marked ref. The BCL does not have a delegate type with ref, so just write your own (not inside any class):
public delegate void MyActionRef<T>(ref T arg);
With that, your method becomes:
public static void ForEach2<T>(this T[] list, MyActionRef<T> actionRef)
{
if (actionRef == null)
throw new ArgumentNullException("actionRef");
for (int idx = 0; idx < list.Length; idx++)
{
actionRef(ref list[idx]);
}
}
Now, remember to use the ref keyword in your test method:
numbers.ForEach2((ref int num) =>
{
num = 0;
});
This works because it is OK to pass an array entry ByRef (ref).
If you want to extend IList<> instead, you have to do:
public static void ForEach3<T>(this IList<T> list, MyActionRef<T> actionRef)
{
if (actionRef == null)
throw new ArgumentNullException("actionRef");
for (int idx = 0; idx < list.Count; idx++)
{
var temp = list[idx];
actionRef(ref temp);
list[idx] = temp;
}
}
Hope this helps your understanding.
Note: I had to use for loops. In C#, in foreach (var x in Yyyy) { /* ... */ }, it is not allowed to assign to x (which includes passing x ByRef (with ref or out)) inside the loop body.

Is there an "upto" method in C#?

Here's a bit of code which prints out the squares of the numbers from 0 to 9:
for (int i = 0; i < 10; i++)
Console.WriteLine(i*i);
Doing something from 0 to N by 1 via a for loop is a very common idiom.
Here's an UpTo method which expresses this:
class MathUtil
{
public static void UpTo(int n, Action<int> proc)
{
for (int i = 0; i < n; i++)
proc(i);
}
}
The squares example above is now:
MathUtil.UpTo(10, (i) => Console.WriteLine(i * i));
My question is, does the standard C# library come with something like the above UpTo?
Ideally, I'd like a way to have 'UpTo' be a method on all integer objects. So I could do:
var n = 10;
n.UpTo(...);
Is this possible in C#?

Turn it into an extension method (note the this before the n parameter, which defines the type this method operates on):
static class MathUtil
{
public static void UpTo(this int n, Action<int> proc)
{
for (int i = 0; i < n; i++)
proc(i);
}
}
Usage:
10.UpTo((i) => Console.WriteLine(i * i));
Note: The above method call isn't particularly intuitive though. Remember code is written once and read many times.
Maybe allowing something like below might be slightly better, but to be honest i'd still just write a foreach loop.
0.UpTo(10 /*or 9 maybe*/, (i) => Console.WriteLine(i * i));
If you wanted this, then you could write an extension method like this:
public static void UpTo(this int start, int end, Action<int> proc)
{
for (int i = start; i < end; i++)
proc(i);
}
Change < to <= if you want an inclusive upper bound.

Take a look at LINQ TakeWhile or for your specific case of integers, use Enumerable.Range
Enumerable.Range(1, 10).Select(i => ...);
Arguably you shouldn't be putting an Action on the end there, see comments on ForEach here.

Try this:
public static class IntExtensions
{
public static void UpTo(this int n, Action<int> proc)
{
for (int i = 0; i < n; i++)
proc(i);
}
}
With this you could write
10.UpTo(i => Console.WriteLine(i * i));
The function I wrote is called an extension method.
At design time you notice is not a native function because it has a different icon.
Estension methods are static methods or functions included in a static class and type they work on is the first param on which you must use this keyword.
In IntExtensions class you could write all methods you please; grouping them inside the same static class makes you easy manage them.

wanna do it in one line ? here it goes:
Enumerable.Range(0, 9).Select(i => i * i).ToList().ForEach(j=>Console.WriteLine("%d",j));

Try Enumerable.Range, possibly in combination with Take or TakeWhile:
IEnumerable<int> values = Enumerable.Range(0, 20)
.Take(10); // Not necessary in this example
foreach(var value in values)
{
Console.WriteLine(value);
}
// or ...
foreach(var i in Enumerable.Range(0, 10))
{
Console.WriteLine(i * i);
}
There is a ForEach on List<T> that you could use to get closer syntax to what you want, but I consider it bad form. It takes a pure query/filter/transform syntax, that works in an effectively immutable fashion, and introduces side-effects.
For your future amusement you might want to check out extension methods, IEnumerable<T>, and yield return. A lot of generator-type functionality and interesting syntax becomes possible when you use those three things in combination. Although I would argue that this particular example isn't the best place to use them because the resulting syntax becomes a mess.

Make your method like this in a static class "Extensions" for example:
public static void UpTo(this int n, Action<int> proc)
{
for (var i = 0; i < n; i++)
proc(i);
}
And the usage:
var n = 10;
n.UpTo(i => Console.WriteLine(i * i));
Hope this helps! :)

List, not lose the reference

Back from interview. I share with you and a good and precise answer is welcome.
The purpose, you have a static method, this method receive an IList<int> you have
to get back the values you can divise by 3 and make the code.
Constraint :
The original list (in the main) has a reference on the stack and the values on the heap,
the result must be return (it's a void method) in the same space (on the heap) than the original list. The solution show here is not correct because in the method a new pointer
on the stack + heap are created in the method domain. Solution ?
Bonus : how change the code to receive not only int but float, double, ....
static void Main(string[] args)
{
IList<int> list = new List<int>() { 9, 3, 10, 6, 14, 16, 20};
CanBeDivedByThree(list);
}
static void CanBeDivedByThree(IList<int> list)
{
list = (from p in list
where p % 3 == 0
orderby p descending
select p).ToList<int>();
}

That's meaningless as the internal storage to an IList is not under your control. Adding (or possibly removing) items might re-allocate the internal data structures.
It is especially meaningless as the list in your sample contains value types which are copied anyway when you access them.
Last but not least it's basically the whole point of using a managed language that you don't have to worry about memory (al)locations. Such things are implementation details of the platform.
To take up on your bonus question: There is no simple way to achieve that. One could think that using generics with a type constraint would solve the problem here (something like static void CanBeDivedByThree<T>(IList<T> list) where T : struct), but the problem is that C# does not (yet?) have support for generic arithmetic. C# doesn't have a modulo operator that can take a generic parameter of type 'T' and 'int'.

list.RemoveAll(n => n % 3 == 0);
or
for (int i = list.Count - 1; i >= 0; --i)
{
if (list[i] % 3 != 0)
list.RemoveAt(i);
}
The first approach works only for List<T>.
One could make it a template method, but remainder operation doesn't make much sense on floats.

Unfortunately only List but not IList does implement RemoveAll. So I first implement it as an extension method.
public static int RemoveAll<T>(this IList<T> list, Predicate<T> match)
{
if (match == null)
throw new ArgumentNullException("match");
int destIndex=0;
int srcIndex;
for(srcIndex=0;srcIndex<list.Count;srcIndex++)
{
if(!match(list[srcIndex]))
{
//if(srcIndex!=destIndex)//Small optimization, can be left out
list[destIndex]=list[srcIndex];
destIndex++;
}
}
for(int removeIndex=list.Count-1;removeIndex>=destIndex;removeIndex--)
{
list.RemoveAt(removeIndex);
}
return srcIndex-destIndex;
}
Then you can use:
list.RemoveAll(n => n % 3 != 0);
You can then use overloads for other types. Unfortunately you can't (easily) make it generic since generics don't work with operator overloading.

Others have covered the list part - this is just for the bonus bit.
You can't do this in a statically typed way using C# generics, but if you're using C# 4 you can do it with dynamic typing. For example:
using System;
using System.Collections.Generic;
class Test
{
static void Main()
{
ShowDivisibleBy3(new List<int> { 1, 3, 6, 7, 9 });
ShowDivisibleBy3(new List<decimal> { 1.5m, 3.3m, 6.0m, 7m, 9.00m });
}
static void ShowDivisibleBy3<T>(IEnumerable<T> source)
{
foreach (dynamic item in source)
{
if (item % 3 == 0)
{
Console.WriteLine(item);
}
}
}
}