I'd created a Map function in C# to act, in many ways, as it's JavaScript equivalent to project object types. I've since renamed these methods to 'Select' to use as overloads so they feel more 'integrated'. This is a chain, so bear with me, but the affected functions look like this...
public static TResult Project<TInput, TResult>(this TInput input, Func<TInput, TResult> projectionMapping)
=> projectionMapping(input);
public static TResult Project<TInput, TAccumulatedValue, TIncrementingValue, TResult>(this TInput input, Func<TInput, TAccumulatedValue, TResult> projectionMapping,
Func<TAccumulatedValue, TIncrementingValue, TAccumulatedValue> accumulator, TAccumulatedValue initialAccumulatorValue, TIncrementingValue increment)
=> projectionMapping(input, accumulator(initialAccumulatorValue, increment));
public static IEnumerable<TResult> Select<TInput, TAccumulatedValue, TIncrementingValue, TResult>(this IEnumerable<TInput> input,
Func<TInput, TAccumulatedValue, TResult> projectionMapping, Func<TAccumulatedValue, TIncrementingValue, TAccumulatedValue> accumulator,
TAccumulatedValue initialAccumulatorValue, TIncrementingValue increment)
=> input.Select(item => item.Project(projectionMapping, accumulator, initialAccumulatorValue, increment));
// This doesn't work.
public static IEnumerable<TResult> Select<TInput, TResult>(this IEnumerable<TInput> input,
Func<TInput, int, TResult> projectionMapping, int initialAccumulatorValue = -1, int increment = 1)
{
return input.Select(projectionMapping, (acc, inc) => acc + inc,
initialAccumulatorValue, increment);
}
I am using the int version of the map method, with the accumulator written into it, as follows...
MyList.Add(new List<MyObject>(rowValues.Map((val, headerNumber)
=> new MyObject(headerNumber, val), 0, 10)));
The problem is, that the value of headerNumber never changes (It's always 10) - The accumulator runs once and then is running for each Mapping but it's not remembering it's accumulation between runs. I feel I'm missing something glaringly obvious here but I can't see the wood for the trees.
If I input (for example) an array like this...
rowValues = new string[] { "Item 1", "Item 2", "Item 3" };
I would expect a list of MyObject items that contain the following data...
10 | "Item 1"
20 | "Item 2"
30 | "Item 3"
The problem is that you always call the accumulator with initialAccumulatorValue.
In order to achieve the goal, you need to maintain the accumulated value, and the easiest correct way to do that is using C# iterator method:
public static IEnumerable<TResult> Map<TInput, TAccumulatedValue, TIncrementingValue, TResult>(this IEnumerable<TInput> input,
Func<TInput, TAccumulatedValue, TResult> projectionMapping, Func<TAccumulatedValue, TIncrementingValue, TAccumulatedValue> accumulator,
TAccumulatedValue initialAccumulatorValue, TIncrementingValue increment)
{
var accumulatedValue = initialAccumulatorValue;
foreach (var item in input)
yield return projectionMapping(item, accumulatedValue = accumulator(accumulatedValue, increment));
}
Please note that the naïve attempt to use a combination of closure and Select
var accumulatedValue = initialAccumulatorValue;
return input.Select(item => projectionMapping(item, accumulatedValue = accumulator(accumulatedValue, increment)));
simply doesn't work because the accumulatedValue will be shared by the multiple executions of the returned select query, hence they will produce incorrect result. The iterator method has no such issue because the code is actually executed anytime the GetEnumerator() method is called.
I started by changing your 3rd function so that it just takes an accumulator function that returns the next index in the sequence. This allows the function to have state which you need to calculate the increasing accumulator values.
public static IEnumerable<TResult> Project<TInput, TAccumulatorValue, TResult>(this IEnumerable<TInput> input,
Func<TInput, TAccumulatorValue, TResult> projectionMapping,
Func<TAccumulatorValue> accumulator)
{
return input.Select(item => projectionMapping(item, accumulator()));
}
Then your function that takes the range arguments that didn't work can be written like this, which solves your problem.
public static IEnumerable<TResult> Project<TInput, TResult>(this IEnumerable<TInput> input,
Func<TInput, int, TResult> projectionMapping, int initialAccumulatorValue = 0, int increment = 1)
{
int curValue = initialAccumulatorValue;
return input.Project(projectionMapping,
() => { var ret = curValue; curValue += increment; return ret; });
}
Alternatively
Thinking about this problem in a different way you can make it more generic. All you are really doing is combining two sequences together using projectionMapping to combine the elements. In this case the second sequence happens to contain the accumulator values. Then to use this you just use the standard Linq function Zip, passing in the accumulator sequence and the projectionMapping function.
To get a linear sequence we can use Enumerable.Range, but to get a non-linear range we need to write a Range generator like this
public static IEnumerable<int> Range(int start, int increment)
{
for (; ; )
{
yield return start;
start += increment;
}
}
Examples
Showing both solutions in action
var items = new[] { "a", "b", "c" };
// use Project, returns: a10, b20, c30
var results = items.Project((s, i) => s + i.ToString(), 10, 10).ToList();
// use Zip with custom range, returns: a10, b20, c30
var results2 = items.Zip(Range(10, 10), (s, i) => s + i.ToString()).ToList();
// use Zip with standard range, returns: a1, b2, c3
var results3 = items.Zip(Enumerable.Range(1, int.MaxValue), (s, i) => s + i.ToString()).ToList();
Assuming the following
public class MyObject
{
public int Row { get; }
public string Value { get; }
public MyObject(int row, string value)
{
Value = value;
Row = row;
}
}
With the input source being:
var rows = new[] { "Item 1", "Item 2", "Item 3", "Item 4" };
Solution
To achieve the specific result given to the input you've presented, you can simply use the Select that is provided in the framework as such:
var result = rows.Select((v, i) => new MyObject((i+1)*10, v)).ToList();
Granted, that doesn't look very nice, but does the trick.
Ivan's answer is neater, but I'll persist the above in case someone finds it useful.
Related
In JavaScript ES6, you are able to destructure arrays like this:
const [a,b,...rest] = someArray;
where a is the first element in the array, b is the second, and rest is an array with the remaining elements.
I know in C#7 that you can destructure tuples during assignment, but could not find anything related to destructuring arrays/enumerables like this:
var (a,b) = someTuple;
I have an IEnumerable where I need the first and second elements as variables, and I need the rest of the elements as another IEnumerable. I have a solution, but feel that destructuring will look cleaner.
It turns out not only tuples can be deconstructed but any type which has Deconstruct static (or extension) method with matching signature. Doing deconstruction correctly for IEnumerable is not trivial (see library suggested by David Arno in this answer), so let's see how it works with simple IList instead (implementation is irrelevant, this one is for example and of course can be better/different):
public static class Extensions {
public static void Deconstruct<T>(this IList<T> list, out T first, out IList<T> rest) {
first = list.Count > 0 ? list[0] : default(T); // or throw
rest = list.Skip(1).ToList();
}
public static void Deconstruct<T>(this IList<T> list, out T first, out T second, out IList<T> rest) {
first = list.Count > 0 ? list[0] : default(T); // or throw
second = list.Count > 1 ? list[1] : default(T); // or throw
rest = list.Skip(2).ToList();
}
}
Then (after adding relevant using statements if necessary) you can use exactly the syntax you want:
var list = new [] {1,2,3,4};
var (a,rest) = list;
var (b,c,rest2) = list;
Or you can chain deconstruction like this (because last returned value can itself be deconstructed):
var (a, (b, (c, rest))) = list;
With last version, you can deconstruct to any number of items using single Deconstruct method (that one which returns first item and the rest).
For real usage for IEnumerables, I'd suggest to not reimplement the wheel and use David Arno's library mentioned in this answer.
If you want a solution that is fully integrated with the C# language features, use Evk's answer, which hides some of the implementation detail. If you don't care about that, you can use either of the answers.
To my knowledge there is not. However, it is not very hard to make something similar.
What about an extension method like this:
public static class EX
{
public static void Deconstruct<T>(this T[] items, out T t0)
{
t0 = items.Length > 0 ? items[0] : default(T);
}
public static void Deconstruct<T>(this T[] items, out T t0, out T t1)
{
t0 = items.Length > 0 ? items[0] : default(T);
t1 = items.Length > 1 ? items[1] : default(T);
}
}
And you can use it like so:
int[] items = { 1, 2 };
items.Deconstruct(out int t0);
The drawback is that you need an extension method per number of items to return. So if you have more than a few variables to return, this method might not be very useful.
Note that I left out checking the length, and related stuff, but you understand what needs to be done I guess.
What you are describing is generally known in functional languages as "cons", which often takes the form:
let head :: tail = someCollection
I did propose this be added to C#, but it didn't receive very favourable feedback. So I wrote my own, which you can use via the Succinc<T> nuget package.
It uses deconstruction to achieve that splitting of the head and tail of any IEnumerable<T>. Deconstructs can be nested, so you can use it to extract multiple elements in one go:
var (a, (b, rest)) = someArray;
This potentially could provide the functionality you are after.
To extend the solutions hinted by other contributors, I provide an answer that uses IEnumerable. It might not be optimized but it works quite well.
public static class IEnumerableExt
{
public static void Deconstruct<T>(this IEnumerable<T> seq, out T first, out IEnumerable<T> rest)
{
first = seq.FirstOrDefault();
rest = seq.Skip(1);
}
public static void Deconstruct<T>(this IEnumerable<T> seq, out T first, out T second, out IEnumerable<T> rest)
=> (first, (second, rest)) = seq;
public static void Deconstruct<T>(this IEnumerable<T> seq, out T first, out T second, out T third, out IEnumerable<T> rest)
=> (first, second, (third, rest)) = seq;
public static void Deconstruct<T>(this IEnumerable<T> seq, out T first, out T second, out T third, out T fourth, out IEnumerable<T> rest)
=> (first, second, third, (fourth, rest)) = seq;
public static void Deconstruct<T>(this IEnumerable<T> seq, out T first, out T second, out T third, out T fourth, out T fifth, out IEnumerable<T> rest)
=> (first, second, third, fourth, (fifth, rest)) = seq;
}
Then just use these deconstructors like this:
var list = new[] { 1, 2, 3, 4 };
var (a, b, rest1) = list;
var (c, d, e, f, rest2) = rest1;
Console.WriteLine($"{a} {b} {c} {d} {e} {f} {rest2.Any()}");
// Output: 1 2 3 4 0 0 False
Really quick: No.
C# does not support destructuring for Arrays yet.
Currently, I cannot find any information of this on the roadmap, either. Seems like there will be a lot of waiting involved until we get this syntactic sugar by default.
As #Nekeniehl added in the comments, it can be implemented though: gist.github.com/waf/280152ab42aa92a85b79d6dbc812e68a
In C# you will need to write your own, like this one I'm using:
public static class ArrayExtensions
{
public static void Deconstruct<T>(this T[] array, out T first, out T[] rest)
{
first = array.Length > 0 ? array[0] : default(T);
rest = array.Skip(1).ToArray();
}
public static void Deconstruct<T>(this T[] array, out T first, out T second, out T[] rest)
=> (first, (second, rest)) = array;
public static void Deconstruct<T>(this T[] array, out T first, out T second, out T third, out T[] rest)
=> (first, second, (third, rest)) = array;
public static void Deconstruct<T>(this T[] array, out T first, out T second, out T third, out T fourth, out T[] rest)
=> (first, second, third, (fourth, rest)) = array;
public static void Deconstruct<T>(this T[] array, out T first, out T second, out T third, out T fourth, out T fifth, out T[] rest)
=> (first, second, third, fourth, (fifth, rest)) = array;
// .. etc.
}
Then simply do:
var (first, second,_ , rest) = new[] { 1, 2, 3, 4 }
There is no special syntax for it in the language.
You could leverage the tuple syntax, though, to arrive at this
class Program
{
static void Main(string[] args)
{
int[] ints = new[] { 1, 2, 3 };
var (first, second, rest) = ints.Destruct2();
}
}
public static class Extensions
{
public static (T first, T[] rest) Desctruct1<T>(this T[] items)
{
return (items[0], items.Skip(1).ToArray());
}
public static (T first, T second, T[] rest) Destruct2<T>(this T[] items)
{
return (items[0], items[1], items.Skip(2).ToArray());
}
}
(which should be extended with error handling for obvious error scenarios before being used in production code).
You need to be slightly careful if you want to handle infinite streams, as from a while(true) yield return block for example. In that case, you can't practically check the length of the stream to make sure you have enough items to populate the requested tuple.
If your source is actually infinite a combination of the above approaches would work -- rather than counting the length of the IEnumerable<T>, just check that it has any content at all, and then implement the multi-parameter overloads in terms of the single-parameter overload:
public static void Deconstruct<T>(this IEnumerable<T> list, out T head, out IEnumerable<T> tail)
{
head = list.First(); // throws InvalidOperationException for empty list
tail = list.Skip(1);
}
public static void Deconstruct<T>(this IEnumerable<T> list, out T head, out T next, out IEnumerable<T> tail)
{
head = list.First();
(next, tail) = list.Skip(1);
}
The critical question is what you want to happen when the stream runs out. The code above will throw an InvalidOperationException. Returning default<T> might not be what you want instead. In a functional context you'd typically be doing a cons, and splitting the stream into a single head an stream tail - and then checking for empty streams outside of your cons implementation (so outside of the Deconstruct method).
I tried to make it more shorter and performance effective. So I avoided the calling of IEnumerable<T>.ToList() method, which would be expensive if we had a large list.
public static void Deconstruct<T>(this IEnumerable<T> list, out T first, out IEnumerable<T> rest) {
first = list.FirstOrDefault();
rest = list.Skip(1);
}
public static void Deconstruct<T>(this IEnumerable<T> list, out T first, out T second, out IEnumerable<T> rest) {
first = list.FirstOrDefault();
(second, rest) = list.Skip(1);
}
public static void Deconstruct<T>(this IEnumerable<T> list, out T first, out T second, out T third, out IEnumerable<T> rest) {
first = list.FirstOrDefault();
(second, third, rest) = list.Skip(1);
}
We have it in C# 11 now as part of list patterns.
Usage examples:
List<int> list = new() { 1, 2, 3 };
// Here array must contain exactly 3 elements to match
// taking first element, disregarding last 2 ("_" disregards a single element)
if (list is [var firstElm, _, _])
{
// Do stuff with firstElm
}
// Do stuff with firstElm here too
Or
public void SomeFunc(int[] arr)
{
// guard
if (arr is not [var a, var b, var c]) return;
// act
a = b - c;
}
Or
Queue<int[]> queue = new ();
queue.Enqueue(new int[] { 1, 2, 3 });
// Here matching an array with length >= 2
// and taking first 2 elements (".." matches and disregards 0 or more elements)
// Arrays with less then 2 elements won't match
while (queue.TryDequeue(out var arr) && arr is [var a, var b, ..])
{
// Do stuff with a & b
}
See language reference on topic for full info.
As what such code is lowered to.
Or more pattern examples like
[var a, .., var y, _] for first and (second to last) elements
[_, >= 0, .., >=0, var z] to check & assign simultaneously
[.., [.., var zz]] to match last element of last subarray
PS Atm using stuff mentioned in first 2 examples is plenty for me.
Would be nice to have optional elements, but this already saves a lot of boilerplate.
Examples: Suppose the predicate is i == 0.
Then
[1] -> [(1)]
[0] -> []
[1, 0] -> [(1)]
[0, 1] -> [(1)]
[0, 0] -> []
[1, 1, 0] -> [(1, 1)]
[1, 0, 1] -> [(1), (1)]
[1, 1, 0, 0, 1, 0, 1, 1, 1] -> [(1, 1), (1), (1, 1, 1)]
Basically, returning contiguous subsegments where the predicate is false.
I thought this would work
internal static IEnumerable<IEnumerable<T>> PartitionBy<T>(this IEnumerable<T> source, Func<T, bool> condition)
{
IEnumerator<T> mover = source.GetEnumerator();
for (; mover.MoveNext() ; )
{
var chunk = mover.MoveUntil(condition);
if (chunk.Any())
{
yield return chunk;
}
}
}
private static IEnumerable<T> MoveUntil<T>(this IEnumerator<T> mover, Func<T, bool> condition)
{
bool hitCondition = false;
do
{
if (condition(mover.Current))
{
hitCondition = true;
}
else
{
yield return mover.Current;
}
}
while (!hitCondition && mover.MoveNext());
}
but I was seeing that for example with [1, 1, 0] it will return [(1), (1)]. I don't completely understand why. I can make it work if I change
var chunk = mover.MoveUntil(condition);
to have mover.MoveUntil(condition).ToList(); but if possible I'd like to not have to hold any of the subsegments in memory.
It is possible to stream the results using LINQ calls. The below implementation:
Does not create temporary Lists to reduce memory consumption, I think it would be O(1) for the memory as only one sub-segment is dealt with at a time.
There will be no double enumeration and the predicate will be called exactly once per record.
It would be O(n) for the runtime because like this answer suggests, the GroupBy operation should be O(n) and the other LINQ calls are single-pass operations so should also be O(n).
public static IEnumerable<IEnumerable<T>> PartitionBy<T>(this IEnumerable<T> a, Func<T, bool> predicate)
{
int groupNumber = 0;
Func<bool, int?> getGroupNumber = skip =>
{
if (skip)
{
// prepare next group, we don't care if we increment more than once
// we only want to split groups
groupNumber++;
// null, to be able to filter out group separators
return null;
}
return groupNumber;
};
return a
.Select(x => new { Value = x, GroupNumber = getGroupNumber(predicate(x))} )
.Where(x => x.GroupNumber != null)
.GroupBy(x => x.GroupNumber)
.Select(g => g.Select(x => x.Value));
}
Firstly, I think you wanted O(n) as memory complexity, as your output length is linearly proportional to the input. As a big fan of functional programming, I have chosen to use a fold (that corresponds to the LINQ function Aggregate in C#).
Basically, we start with an empty collection of collection and a flag that indicates whether the next iteration has to create a new sub-collection (we only know that when the predicate matches, i.e. in the previous iteration). I use a tuple that contains those two elements as an accumulator. I extracted the logic of the Aggregate in a separated function for clarity purpose.
static IEnumerable<IEnumerable<T>> Partition<T>(this IEnumerable<T> a, Func<T, bool> predicate)
{
// The accumulator is a tuple defined as: (collection, mustCreateNewList)
return a.Aggregate((new List<List<T>>(), true), (acc, e) => ForEachElement(acc, e, predicate)).Item1;
}
static (List<List<T>>, bool) ForEachElement<T>((List<List<T>>, bool) acc, T e, Func<T, bool> predicate)
{
var (collection, mustCreateNewList) = acc;
// The predicate matches, continue to iterate!
if (predicate(e)) return (collection, true);
// The previous iteration requests to create a new list
if(mustCreateNewList) collection.Add(new List<T>());
// Add the current element to the last list
collection[collection.Count - 1].Add(e);
return (collection, false);
}
The initial collection is walked through once (O(n)) and the length of the output has the length of the input in the worst case (O(n)).
Example of call:
var array = new int[] { 1, 1, 0, 0, 1, 0, 1, 1, 1 };
var result = array.Partition(i => i == 0);
I'm curious when execution occurs, especially when updating data and calling a second time. Is it whenever the query variable is being used, such as in the foreach statement? Or, is it when I update the list, such as nums[1] = 99?
int[] nums = { 1, -2, 3, 0, -4, 5 };
var posNums = from n in nums
where n > 0
select n;
foreach (int i in posNums)
Console.Write("" + i + " ");
Console.WriteLine();
nums[1] = 99;
foreach (int i in posNums)
Console.Write("" + i + " ");
Console.WriteLine();
Linq defers analysis until the sequence is iterated over, either by a Foreach statement or getting the iterator. Note that under the hood, .ToArray() and .ToList calls perform such an iteration. You can see this by using the method-call version and pressing F9 to breakpoint the passed in lambda.
var posNums = nums
.Where(n => n > 0);
Note that because Linq functions create Enumerators, they will also re-evaluate all your query functions each time you iterate the sequence, so it is often advantageous to copy the collection to memory using .ToArray() if you want to perform multiple (or nested!) iterations over the results of the query. If you want to perform multiple iterations over the changing data source then you wan to reuse the same Linq result.
If you're curious, you can also view the source code the .NET framework uses for various Linq statements at the Reference Source
The posNums query will be executed each time you iterate over the result in the foreach's.
A simple way to see the this in action is to introduce a side-effect in the query. The compiler converts your query expression to:
var posNums = nums.Where(n => n > 0);
We can modify your code with a bit more console output and see exactly where things are getting executed:
int[] nums = { 1, -2, 3, 0, -4, 5 };
Console.WriteLine("Before query creation");
var posNums = nums.Where(n => { Console.WriteLine(" Evaluating " + n); return n > 0; });
Console.WriteLine("Before foreach 1");
foreach (int i in posNums)
Console.WriteLine(" Writing " + i);
Console.WriteLine("Before array modification");
nums[1] = 99;
Console.WriteLine("Before foreach 2");
foreach (int i in posNums)
Console.WriteLine(" Writing " + i);
Output is:
Before query creation
Before foreach 1
Evaluating 1
Writing 1
Evaluating -2
Evaluating 3
Writing 3
Evaluating 0
Evaluating -4
Evaluating 5
Writing 5
Before array modification
Before foreach 2
Evaluating 1
Writing 1
Evaluating 99
Writing 99
Evaluating 3
Writing 3
Evaluating 0
Evaluating -4
Evaluating 5
Writing 5
The easiest way of seeing exactly what's going on is to actually build something equivalent to what Where would return and step through it. Here is an implementation that is functionally equivalent to Where (at least in as much as where the source sequence is iterated and the result).
I've omitted some of the performance optimizations to keep attention on what's important, and written out some operations "the long way" for clarity:
public static IEnumerable<T> Where<T>(
this IEnumerable<T> source,
Func<T, bool> predicate)
{
return new WhereEnumerable<T>(source, predicate);
}
public class WhereEnumerable<T> : IEnumerable<T>
{
private IEnumerable<T> source;
private Func<T, bool> predicate;
public WhereEnumerable(IEnumerable<T> source, Func<T, bool> predicate)
{
this.source = source;
this.predicate = predicate;
}
public IEnumerator<T> GetEnumerator()
{
return new WhereEnumerator<T>(source.GetEnumerator(), predicate);
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
public class WhereEnumerator<T> : IEnumerator<T>
{
private IEnumerator<T> source;
private Func<T, bool> predicate;
public WhereEnumerator(IEnumerator<T> source, Func<T, bool> predicate)
{
this.source = source;
this.predicate = predicate;
}
public T Current { get; private set; }
public void Dispose()
{
source.Dispose();
}
object IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
while (source.MoveNext())
if (predicate(source.Current))
{
Current = source.Current;
return true;
}
return false;
}
public void Reset()
{
throw new NotImplementedException();
}
}
It's also worth having, for reference, what a foreach loop is equivalent to:
foreach (int i in posNums)
Console.Write("" + i + " ");
is equivalent to:
using(IEnumerator<int> iterator = posNums.GetEnumerator())
while(iterator.MoveNext())
{
int i = iterator.Current;
Console.Write("" + i + " ");
}
So now you can walk through and see when the sequence's values are actually pulled. (I'd encourage you to walk through this code with a debugger, using this Where in place of LINQ's Where in your own code, to see what's going on here.)
Calling Where on a sequences doesn't affect the sequence at all, and the sequence changing doesn't affect the result of Where at all. It is when MoveNext is actually called that the enumerable begins to pull the values from the underlying enumerable, and MoveNext is called when you have a foreach loop (among other possibilities).
Something else that we can see here is that each time we call foreach we call GetEnumerator again, which gets a brand new enumerator form Where, which gets a brand new enumerator from the underlying source sequence. This means that each time you call foreach you're iterating the underlying sequence again, from the start.
I'm running throuth Microsoft's 101 LINQ Samples, and I'm stumped on how this query knows how to assign the correct int value to the correct int field:
public void Linq12()
{
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
var numsInPlace = numbers.Select((num, index) => new { Num = num, InPlace = (num == index) });
Console.WriteLine("Number: In-place?");
foreach (var n in numsInPlace)
{
Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
}
}
I saw in SO #336758 that there have been errors in the examples before, but it is much more likely that I am just missing something.
Could someone explain this and how the compiler knows how to interpret this data correctly?
EDIT:
OK, I think my confusion comes from the LINQ extension that enables the Select feature to work. The Func and two int parameters IEnumerable<TResult> IEnumerable<int>.Select(Func<int,int,TResult> selector) are most likely the key to my lack of understanding.
I'm not really sure what you are asking of but the Select iterates over the list starting at index 0. If the value of the element at the current index is equal to the index it will set the InPlace property in the anonymous object to true. My guess is that the code above prints true for 3, 6 and 7, right?
It would also make it easier to explain if you write what you don't understand.
Jon Skeet has written a series of blog post where he implement linq, read about Select here: Reimplementation of Select
UPDATE: I noticed in one of your comment to one of the other comments and it seems like it is the lambda and not linq itself that is confusing you. If you read Skeet's blog post you see that Select has two overloads:
public static IEnumerable<TResult> Select<TSource, TResult>(
this IEnumerable<TSource> source,
Func<TSource, TResult> selector)
public static IEnumerable<TResult> Select<TSource, TResult>(
this IEnumerable<TSource> source,
Func<TSource, int, TResult> selector)
The Select with index matches the second overload. As you can see it is an extension of IEnumerable<TSource> which in your case is the list of ints and therefor you are calling the Select on an IEnumerable<int> and the signature of Select becomes: Select<int, TResult>(this IEnumerable<int> source, Func<int, int, TResult> selector). As you can see I changed TSource against int, since that is the generic type of your IEnumerable<int>. I still have TResult since you are using anonymous type. So that might explain some parts?
Looks correct to me.
First you have an anonymous type with Num and InPlace being created. Then the LINQ Select is just iterating over the elements with the element and the index of that element. If you were to rewrite it without linq and anonymous classes, it would look like this:
class NumsInPlace
{
public int Num { get; set; }
public bool InPlace { get; set; }
}
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
List<NumsInPlace> numsInPlace = new List<int>();
for (int index = 0; i < numbers.length; i++)
{
int num = numers[index];
numsInPlace.Add(new NumsInPlace() { Num = num, InPlace = (index == num) });
}
Console.WriteLine("Number: In-place?");
foreach (var n in numsInPlace)
{
Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
}
The MSDN on Enumerable.Select has the details, but the projection function (num, index) always has the item first, then the index second (if supplied).
how does the compiler know that LINQ Select is using the index value as an index to the array?
The compiler doesn't know about index values. The implementation of that overload of Select knows about index values.
//all the compiler sees is a method that accepts 2 int parameters and returns a bool.
Func<int, int, bool> twoIntFunc = (x, y) => (x == y);
//The compiler sees there's an overload of Enumerable.Select which accepts such a method.
IEnumerable<bool> query = numbers.Select(twoIntFunc);
Enumerable.Select's implementation does the rest of the work by calling that method with the appropriate parameters.
The first argument to selector represents the element to process. The second argument to selector represents the zero-based index of that element in the source sequence.
So - Select will call your method with (5, 0) first, then Select calls it with (4, 1).
The lambda expression (num, index) => new { Num = num, InPlace = (num == index) } is executed once for each element in the input sequence and passed the item and it's index as the arguments.
Update
Lambda expressions can be implicitly typed, that is, from the required type, the compiler can figure out (or imply) what types you intend the arguments to be.
(num, index) => new { Num = num, InPlace = (num == index) }
is equivalent to
someAnonymousType MyMethod(int num, int index)
{
return new
{
Num = num,
InPlace = (num == index)
};
}
obviously you can't write the latter because you can't type the name of an anonymous type, but the compiler can ;)
The compiler knows this because the overload of Select that you're using accepts a Func<TSource, Int32, TResult>, this is a Func that takes, two arguments of type TSource (the type of your IEnumberable<T>, in this case int) and an Int32 (which represents the index) and returns an object of TResult, being whatever you choose to return from your function, in this case, an anonymous type.
The lambda can be cast to the required type and therefore it just works.
The second argument in the select is the index, which increments as the compiler traverses the array of numbers. The compiler will see
num index num = index
5 0 false
4 1 false
1 2 false
3 3 true
9 4 false
8 5 false
6 6 true
7 7 true
2 8 false
0 9 false
It provides true for 3,6 and 7. You need to remember it starts with the index at 0.
I was wondering if someone could explain what Func<int, string> is and how it is used with some clear examples.
Are you familiar with delegates in general? I have a page about delegates and events which may help if not, although it's more geared towards explaining the differences between the two.
Func<T, TResult> is just a generic delegate - work out what it means in any particular situation by replacing the type parameters (T and TResult) with the corresponding type arguments (int and string) in the declaration. I've also renamed it to avoid confusion:
string ExpandedFunc(int x)
In other words, Func<int, string> is a delegate which represents a function taking an int argument and returning a string.
Func<T, TResult> is often used in LINQ, both for projections and predicates (in the latter case, TResult is always bool). For example, you could use a Func<int, string> to project a sequence of integers into a sequence of strings. Lambda expressions are usually used in LINQ to create the relevant delegates:
Func<int, string> projection = x => "Value=" + x;
int[] values = { 3, 7, 10 };
var strings = values.Select(projection);
foreach (string s in strings)
{
Console.WriteLine(s);
}
Result:
Value=3
Value=7
Value=10
A Func<int, string> eats ints and returns strings. So, what eats ints and returns strings? How about this ...
public string IntAsString( int i )
{
return i.ToString();
}
There, I just made up a function that eats ints and returns strings. How would I use it?
var lst = new List<int>() { 1, 2, 3, 4, 5 };
string str = String.Empty;
foreach( int i in lst )
{
str += IntAsString(i);
}
// str will be "12345"
Not very sexy, I know, but that's the simple idea that a lot of tricks are based upon. Now, let's use a Func instead.
Func<int, string> fnc = IntAsString;
foreach (int i in lst)
{
str += fnc(i);
}
// str will be "1234512345" assuming we have same str as before
Instead of calling IntAsString on each member, I created a reference to it called fnc (these references to methods are called delegates) and used that instead. (Remember fnc eats ints and returns strings).
This example is not very sexy, but a ton of the clever stuff you will see is based on the simple idea of functions, delegates and extension methods.
One of the best primers on this stuff I've seen is here. He's got a lot more real examples. :)
It is a delegate that takes one int as a parameter and returns a value of type string.
Here is an example of its usage:
using System;
class Program
{
static void Main()
{
Func<Int32, String> func = bar;
// now I have a delegate which
// I can invoke or pass to other
// methods.
func(1);
}
static String bar(Int32 value)
{
return value.ToString();
}
}
Func<int, string> accepts an int value parameter and returns a string value. Here's an example where an additional supporting method is unnecessary.
Func<int, string> GetDogMessage = dogAge =>
{
if (dogAge < 3) return "You have a puppy!";
if (dogAge < 7) return "Strong adult dog!";
return "Age is catching up with the dog!";
};
string youngDogMessage = GetDogMessage(2);
NOTE: The last object type in Func (i.e. "string" in this example) is the functions return type (i.e. not limited to primitives, but any object). Therefore, Func<int, bool, float> accepts int and bool value parameters, and returns a float value.
Func<int, bool, float> WorthlessFunc = (intValue, boolValue) =>
{
if(intValue > 100 && boolValue) return 100;
return 1;
};
float willReturn1 = WorthlessFunc(21, false);
float willReturn100 = WorthlessFunc(1000, true);
HTH