I have dictionary indices and want to add several keys to it from another dictionary using LINQ.
var indices = new Dictionary<string, int>();
var source = new Dictionary<string, int> { { "1", 1 }, { "2", 2 } };
source.Select(name => indices[name.Key] = 0); // doesn't work
var res = indices.Count; // returns 0
Then I replace Select with Min and everything works as expected, LINQ creates new keys in my dictionary.
source.Min(name => indices[name.Key] = 0); // works!!!
var res = indices.Count; // returns 2
Question
All I want to do is to initialize dictionary without foreach. Why dictionary keys disappear when LINQ is executed? What iterator or aggregator I could use instead of Min to create keys for a dictionary declared outside of LINQ query?
Update #1
Decided to go with System.Interactive extension.
Update #2
I appreciate and upvote all answers, but need to clarify that, purpose of the question is not to copy a dictionary, but to execute some code in a LINQ query. To add more sense to it, I actually have hierarchical structure of classes with dictionaries and at some point they need to be synchronized, so I want to create flat, non-hierarchical dictionary, used for tracking, that includes all hierarchical keys.
class Account
{
Dictionary<string, User> Users;
}
class User
{
Dictionary<string, Activity> Activities;
}
class Activity
{
string Name;
DateTime Time;
}
Now I want to sync all actions by time, so I need a tracker that will help me to align all actions by time, and I don't want to create 3 loops for Account, User, and Activity. Because that would be considered a hierarchical hell of loops, the same as async or callback hell. With LINQ I don't have to create loop inside loop, inside loop, etc.
Accounts.ForEach(
account => account.Value.Users.ForEach(
user => user.Value.Activities.ForEach(
activity => indices[account.Key + user.Key + activity.Key] = 0));
Also, having loops where it can be replaced with LINQ can be considered as a code smell, not my opinion, but I totally agree, because having too many loops you will probably end up in duplicated code.
https://jasonneylon.wordpress.com/2010/02/23/refactoring-to-linq-part-1-death-to-the-foreach/
You can say that LINQ is used for querying, not for setting a variable, I would say I'm querying ... the KEYS.
Linq is not intended to be used to mutate the elements of a sequence. Rather, it is intended to be used to traverse, filter and project elements of a sequence. In this respect, it is intended to be used more in a "functional programming" style.
As you have discovered, Linq can be used in other than a functional programming style - but by using it in that way you are really misusing it.
Technically, the reason that source.Min() has the effect you were looking for is that it has to visit each of the elements of your sequence in order to determine the minimum element.
Because your selector for Min() has a side-effect (i.e. indices[name.Key] = 0) then a side-effect of finding the minimum value is to add each element's key to indices, but with a value of zero rather than the original value.
(I suspect you might have meant to put indices[name.Key] = name.Value...)
The reason that your use of Select() has no effect is that it has not been used to traverse the sequence - it uses "deferred execution".
You can force it to traverse the sequence by counting the elements, like so:
source.Select(name => indices[name.Key] = 0).Count();
However, that is also counter-intuitive and is a misuse of Linq.
The correct solution is to use foreach. This expresses your intent clearly and unambiguously.
An alternative approach is to write an AddRange() extension method for Dictionary like so:
public static class DictionaryExt
{
public static Dictionary<TKey, TValue> AddRange<TKey, TValue>(
this Dictionary<TKey, TValue> self,
IEnumerable<KeyValuePair<TKey, TValue>> items)
{
foreach (var item in items)
{
self[item.Key] = item.Value;
}
return self;
}
}
Then you can just call indices.AddRange(source); to achieve your aim.
Interestingly, the ImmutableDictionary type does already have an AddRange() method that you could use like so:
var indices = ImmutableDictionary.Create<string, int>();
var source = new Dictionary<string, int> { { "1", 1 }, { "2", 2 } };
indices = indices.AddRange(source);
Console.WriteLine(indices.Count);
But I wouldn't recommend you change over to using ImmutableDictionary just so you can use its AddRange().
Also note that ImmutableDictionary is, well, immutable - so you can't just do indices.AddRange(source);; you have to assign the result back as in indices = indices.AddRange(source); (like when you modify a string using ToUpper()).
You wrote:
All I want to do is to initialize dictionary without foreach
Do you want to replace the values in your indices dictionary with the values in source? Use Enumerable.ToDictionary
indices = (KeyValuePair<string, int>)source // regard the items in the dictionary as KeyValuePairs
.ToDictionary(pair => pair.Key, // the key is the key from original dictionary
pair => pair.Value); // the value is the value from the original
Or do you want to add the values from source to the already existing values in indices? If you don't want a foreach you'll have to take the current values from both dictionaries and Concat them to the values from source. Then use the ToDictionary to create a new Dictionary.
indices = (KeyValuePair<string, int>) indices
.Concat(KeyValuePair<string, int>) source)
.ToDictionary(... etc)
However this would be a waste of processing power.
Consider creating extension functions for Dictionary. See Extension Methods Demystified
public static Dictionary<TKey, TValue> Copy>Tkey, TValue>(
this Dictionary<TKey, TValue> source)
{
return source.ToDictionary(x => x.Key, x => x.Value);
}
public static void AddRange<TKey, TValue>(
this Dictionary<TKey, TValue> destination,
Dictionary<TKey, TValue> source)
{
foreach (var keyValuePair in source)
{
destination.Add(keyValuePair.Key, keyValuePair.Value);
// TODO: decide what to do if Key already in Destination
}
}
Usage:
// initialize:
var indices = source.Copy();
// add values:
indices.AddRange(otherDictionary);
How do I iterate over a Dictionary's keys while maintaining the index of the key.
What I've done is merge a foreach-loop with a local variable i which gets incremented by one for every round of the loop.
Here's my code that works:
public IterateOverMyDict()
{
int i=-1;
foreach (string key in myDict.Keys)
{
i++;
Console.Write(i.ToString() + " : " + key);
}
}
However, it seems really low tech to use a local variable i.
I was wondering if there's a way where I don't have to use the "extra" variable?
Not saying this is a bad way, but is there a better one?
There's no such concept as "the index of the key". You should always treat a Dictionary<TKey, TValue> as having an unpredictable order - where the order which you happen to get when iterating over it may change. (So in theory, you could add one new entry, and the entries could be in a completely different order next time you iterated over them. In theory this could even happen without you changing the data, but that's less likely in normal implementations.)
If you really want to get the numeric index which you happened to observe this time, you could use:
foreach (var x in dictionary.Select((Entry, Index) => new { Entry, Index }))
{
Console.WriteLine("{0}: {1} = {2}", x.Index, x.Entry.Key, x.Entry.Value);
}
... but be aware that that's a fairly misleading display, as it suggests an inherent ordering.
From the documentation:
For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair<TKey, TValue> structure representing a value and its key. The order in which the items are returned is undefined.
EDIT: If you don't like the Select call here, you could create your own extension method:
public struct IndexedValue<T>
{
private readonly T value;
private readonly int index;
public T Value { get { return value; } }
public int Index { get { return index; } }
public IndexedValue(T value, int index)
{
this.value = value;
this.index = index;
}
}
public static class Extensions
{
public static IEnumerable<IndexedValue<T>> WithIndex<T>
(this IEnumerable<T> source)
{
return source.Select((value, index) => new IndexedValue<T>(value, index));
}
}
Then your loop would be:
foreach (var x in dictionary.WithIndex())
{
Console.WriteLine("{0}: {1} = {2}", x.Index, x.Value.Key, x.Value.Value);
}
Technically, the key is the index in a Dictionary<TKey, TValue>. You're not guaranteed to get the items in any specific order, so there's really no numeric index to be applied.
Not really. Note that keys in a dictionary are not logically "ordered". They don't have an index. There is no first or last key, from the Dictionary's point of view. You can keep track on your own whether this is the first key returned by the enumerator, as you are doing, but the Dictionary has no concept of "give me the 5th key", so you couldn't use a for loop with an indexer as you could with a list or array.
Dictionaries are not exactly lists, arrays, or vectors. They take those constructs a step further. The key can be the index:
Dictionary myDictionary<int, string> = new Dictionary<int, string>()
{
{0, "cat"},
{1, "dog"},
{2, "pig"},
{3, "horse"}
};
myDictionary[4] = "hat";
for int i = 0; i <5; i++){
Console.Writeline(myDictionary[i]);
}
At this point you are probably missing most of the benefits of a dictionary (which is similar to enumeration with the benefit of sorting quickly on key values), and using it like a list.
The Select((Entry, Index) => new { Entry, Index }) approach is probably best in the specific context of this question but, as an alternative, System.Linq.Enumerable now lets you convert a dictionary into a list. Something like this would work:
var x = dictionary.ToList();
for (int y=0; y<x.Count; y++) Console.WriteLine(y + " = " + x[y].Key);
There are pros & cons to both approaches depending on what you're trying to do.
I've seen a few different ways to iterate over a dictionary in C#. Is there a standard way?
foreach(KeyValuePair<string, string> entry in myDictionary)
{
// do something with entry.Value or entry.Key
}
If you are trying to use a generic Dictionary in C# like you would use an associative array in another language:
foreach(var item in myDictionary)
{
foo(item.Key);
bar(item.Value);
}
Or, if you only need to iterate over the collection of keys, use
foreach(var item in myDictionary.Keys)
{
foo(item);
}
And lastly, if you're only interested in the values:
foreach(var item in myDictionary.Values)
{
foo(item);
}
(Take note that the var keyword is an optional C# 3.0 and above feature, you could also use the exact type of your keys/values here)
In some cases you may need a counter that may be provided by for-loop implementation. For that, LINQ provides ElementAt which enables the following:
for (int index = 0; index < dictionary.Count; index++) {
var item = dictionary.ElementAt(index);
var itemKey = item.Key;
var itemValue = item.Value;
}
Depends on whether you're after the keys or the values...
From the MSDN Dictionary(TKey, TValue) Class description:
// When you use foreach to enumerate dictionary elements,
// the elements are retrieved as KeyValuePair objects.
Console.WriteLine();
foreach( KeyValuePair<string, string> kvp in openWith )
{
Console.WriteLine("Key = {0}, Value = {1}",
kvp.Key, kvp.Value);
}
// To get the values alone, use the Values property.
Dictionary<string, string>.ValueCollection valueColl =
openWith.Values;
// The elements of the ValueCollection are strongly typed
// with the type that was specified for dictionary values.
Console.WriteLine();
foreach( string s in valueColl )
{
Console.WriteLine("Value = {0}", s);
}
// To get the keys alone, use the Keys property.
Dictionary<string, string>.KeyCollection keyColl =
openWith.Keys;
// The elements of the KeyCollection are strongly typed
// with the type that was specified for dictionary keys.
Console.WriteLine();
foreach( string s in keyColl )
{
Console.WriteLine("Key = {0}", s);
}
Generally, asking for "the best way" without a specific context is like asking
what is the best color?
One the one hand, there are many colors and there's no best color. It depends on the need and often on taste, too.
On the other hand, there are many ways to iterate over a Dictionary in C# and there's no best way. It depends on the need and often on taste, too.
Most straightforward way
foreach (var kvp in items)
{
// key is kvp.Key
doStuff(kvp.Value)
}
If you need only the value (allows to call it item, more readable than kvp.Value).
foreach (var item in items.Values)
{
doStuff(item)
}
If you need a specific sort order
Generally, beginners are surprised about order of enumeration of a Dictionary.
LINQ provides a concise syntax that allows to specify order (and many other things), e.g.:
foreach (var kvp in items.OrderBy(kvp => kvp.Key))
{
// key is kvp.Key
doStuff(kvp.Value)
}
Again you might only need the value. LINQ also provides a concise solution to:
iterate directly on the value (allows to call it item, more readable than kvp.Value)
but sorted by the keys
Here it is:
foreach (var item in items.OrderBy(kvp => kvp.Key).Select(kvp => kvp.Value))
{
doStuff(item)
}
There are many more real-world use case you can do from these examples.
If you don't need a specific order, just stick to the "most straightforward way" (see above)!
C# 7.0 introduced Deconstructors and if you are using .NET Core 2.0+ Application, the struct KeyValuePair<> already include a Deconstruct() for you. So you can do:
var dic = new Dictionary<int, string>() { { 1, "One" }, { 2, "Two" }, { 3, "Three" } };
foreach (var (key, value) in dic) {
Console.WriteLine($"Item [{key}] = {value}");
}
//Or
foreach (var (_, value) in dic) {
Console.WriteLine($"Item [NO_ID] = {value}");
}
//Or
foreach ((int key, string value) in dic) {
Console.WriteLine($"Item [{key}] = {value}");
}
I would say foreach is the standard way, though it obviously depends on what you're looking for
foreach(var kvp in my_dictionary) {
...
}
Is that what you're looking for?
You can also try this on big dictionaries for multithreaded processing.
dictionary
.AsParallel()
.ForAll(pair =>
{
// Process pair.Key and pair.Value here
});
I appreciate this question has already had a lot of responses but I wanted to throw in a little research.
Iterating over a dictionary can be rather slow when compared with iterating over something like an array. In my tests an iteration over an array took 0.015003 seconds whereas an iteration over a dictionary (with the same number of elements) took 0.0365073 seconds that's 2.4 times as long! Although I have seen much bigger differences. For comparison a List was somewhere in between at 0.00215043 seconds.
However, that is like comparing apples and oranges. My point is that iterating over dictionaries is slow.
Dictionaries are optimised for lookups, so with that in mind I've created two methods. One simply does a foreach, the other iterates the keys then looks up.
public static string Normal(Dictionary<string, string> dictionary)
{
string value;
int count = 0;
foreach (var kvp in dictionary)
{
value = kvp.Value;
count++;
}
return "Normal";
}
This one loads the keys and iterates over them instead (I did also try pulling the keys into a string[] but the difference was negligible.
public static string Keys(Dictionary<string, string> dictionary)
{
string value;
int count = 0;
foreach (var key in dictionary.Keys)
{
value = dictionary[key];
count++;
}
return "Keys";
}
With this example the normal foreach test took 0.0310062 and the keys version took 0.2205441. Loading all the keys and iterating over all the lookups is clearly a LOT slower!
For a final test I've performed my iteration ten times to see if there are any benefits to using the keys here (by this point I was just curious):
Here's the RunTest method if that helps you visualise what's going on.
private static string RunTest<T>(T dictionary, Func<T, string> function)
{
DateTime start = DateTime.Now;
string name = null;
for (int i = 0; i < 10; i++)
{
name = function(dictionary);
}
DateTime end = DateTime.Now;
var duration = end.Subtract(start);
return string.Format("{0} took {1} seconds", name, duration.TotalSeconds);
}
Here the normal foreach run took 0.2820564 seconds (around ten times longer than a single iteration took - as you'd expect). The iteration over the keys took 2.2249449 seconds.
Edited To Add:
Reading some of the other answers made me question what would happen if I used Dictionary instead of Dictionary. In this example the array took 0.0120024 seconds, the list 0.0185037 seconds and the dictionary 0.0465093 seconds. It's reasonable to expect that the data type makes a difference on how much slower the dictionary is.
What are my Conclusions?
Avoid iterating over a dictionary if you can, they are substantially slower than iterating over an array with the same data in it.
If you do choose to iterate over a dictionary don't try to be too clever, although slower you could do a lot worse than using the standard foreach method.
As already pointed out on this answer, KeyValuePair<TKey, TValue> implements a Deconstruct method starting on .NET Core 2.0, .NET Standard 2.1 and .NET Framework 5.0 (preview).
With this, it's possible to iterate through a dictionary in a KeyValuePair agnostic way:
var dictionary = new Dictionary<int, string>();
// ...
foreach (var (key, value) in dictionary)
{
// ...
}
There are plenty of options. My personal favorite is by KeyValuePair
Dictionary<string, object> myDictionary = new Dictionary<string, object>();
// Populate your dictionary here
foreach (KeyValuePair<string,object> kvp in myDictionary)
{
// Do some interesting things
}
You can also use the Keys and Values Collections
With .NET Framework 4.7 one can use decomposition
var fruits = new Dictionary<string, int>();
...
foreach (var (fruit, number) in fruits)
{
Console.WriteLine(fruit + ": " + number);
}
To make this code work on lower C# versions, add System.ValueTuple NuGet package and write somewhere
public static class MyExtensions
{
public static void Deconstruct<T1, T2>(this KeyValuePair<T1, T2> tuple,
out T1 key, out T2 value)
{
key = tuple.Key;
value = tuple.Value;
}
}
As of C# 7, you can deconstruct objects into variables. I believe this to be the best way to iterate over a dictionary.
Example:
Create an extension method on KeyValuePair<TKey, TVal> that deconstructs it:
public static void Deconstruct<TKey, TVal>(this KeyValuePair<TKey, TVal> pair, out TKey key, out TVal value)
{
key = pair.Key;
value = pair.Value;
}
Iterate over any Dictionary<TKey, TVal> in the following manner
// Dictionary can be of any types, just using 'int' and 'string' as examples.
Dictionary<int, string> dict = new Dictionary<int, string>();
// Deconstructor gets called here.
foreach (var (key, value) in dict)
{
Console.WriteLine($"{key} : {value}");
}
foreach is fastest and if you only iterate over ___.Values, it is also faster
Using C# 7, add this extension method to any project of your solution:
public static class IDictionaryExtensions
{
public static IEnumerable<(TKey, TValue)> Tuples<TKey, TValue>(
this IDictionary<TKey, TValue> dict)
{
foreach (KeyValuePair<TKey, TValue> kvp in dict)
yield return (kvp.Key, kvp.Value);
}
}
And use this simple syntax
foreach (var(id, value) in dict.Tuples())
{
// your code using 'id' and 'value'
}
Or this one, if you prefer
foreach ((string id, object value) in dict.Tuples())
{
// your code using 'id' and 'value'
}
In place of the traditional
foreach (KeyValuePair<string, object> kvp in dict)
{
string id = kvp.Key;
object value = kvp.Value;
// your code using 'id' and 'value'
}
The extension method transforms the KeyValuePair of your IDictionary<TKey, TValue> into a strongly typed tuple, allowing you to use this new comfortable syntax.
It converts -just- the required dictionary entries to tuples, so it does NOT converts the whole dictionary to tuples, so there are no performance concerns related to that.
There is a only minor cost calling the extension method for creating a tuple in comparison with using the KeyValuePair directly, which should NOT be an issue if you are assigning the KeyValuePair's properties Key and Value to new loop variables anyway.
In practice, this new syntax suits very well for most cases, except for low-level ultra-high performance scenarios, where you still have the option to simply not use it on that specific spot.
Check this out: MSDN Blog - New features in C# 7
Simplest form to iterate a dictionary:
foreach(var item in myDictionary)
{
Console.WriteLine(item.Key);
Console.WriteLine(item.Value);
}
I found this method in the documentation for the DictionaryBase class on MSDN:
foreach (DictionaryEntry de in myDictionary)
{
//Do some stuff with de.Value or de.Key
}
This was the only one I was able to get functioning correctly in a class that inherited from the DictionaryBase.
Sometimes if you only needs the values to be enumerated, use the dictionary's value collection:
foreach(var value in dictionary.Values)
{
// do something with entry.Value only
}
Reported by this post which states it is the fastest method:
http://alexpinsker.blogspot.hk/2010/02/c-fastest-way-to-iterate-over.html
I know this is a very old question, but I created some extension methods that might be useful:
public static void ForEach<T, U>(this Dictionary<T, U> d, Action<KeyValuePair<T, U>> a)
{
foreach (KeyValuePair<T, U> p in d) { a(p); }
}
public static void ForEach<T, U>(this Dictionary<T, U>.KeyCollection k, Action<T> a)
{
foreach (T t in k) { a(t); }
}
public static void ForEach<T, U>(this Dictionary<T, U>.ValueCollection v, Action<U> a)
{
foreach (U u in v) { a(u); }
}
This way I can write code like this:
myDictionary.ForEach(pair => Console.Write($"key: {pair.Key}, value: {pair.Value}"));
myDictionary.Keys.ForEach(key => Console.Write(key););
myDictionary.Values.ForEach(value => Console.Write(value););
If you want to use a for loop, you can do as below:
var keyList=new List<string>(dictionary.Keys);
for (int i = 0; i < keyList.Count; i++)
{
var key= keyList[i];
var value = dictionary[key];
}
I will take the advantage of .NET 4.0+ and provide an updated answer to the originally accepted one:
foreach(var entry in MyDic)
{
// do something with entry.Value or entry.Key
}
If say, you want to iterate over the values collection by default, I believe you can implement IEnumerable<>, Where T is the type of the values object in the dictionary, and "this" is a Dictionary.
public new IEnumerator<T> GetEnumerator()
{
return this.Values.GetEnumerator();
}
The standard way to iterate over a Dictionary, according to official documentation on MSDN is:
foreach (DictionaryEntry entry in myDictionary)
{
//Read entry.Key and entry.Value here
}
I wrote an extension to loop over a dictionary.
public static class DictionaryExtension
{
public static void ForEach<T1, T2>(this Dictionary<T1, T2> dictionary, Action<T1, T2> action) {
foreach(KeyValuePair<T1, T2> keyValue in dictionary) {
action(keyValue.Key, keyValue.Value);
}
}
}
Then you can call
myDictionary.ForEach((x,y) => Console.WriteLine(x + " - " + y));
Dictionary< TKey, TValue > It is a generic collection class in c# and it stores the data in the key value format.Key must be unique and it can not be null whereas value can be duplicate and null.As each item in the dictionary is treated as KeyValuePair< TKey, TValue > structure representing a key and its value. and hence we should take the element type KeyValuePair< TKey, TValue> during the iteration of element.Below is the example.
Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1,"One");
dict.Add(2,"Two");
dict.Add(3,"Three");
foreach (KeyValuePair<int, string> item in dict)
{
Console.WriteLine("Key: {0}, Value: {1}", item.Key, item.Value);
}
The best answer is of course: Think, if you could use a more appropriate data structure than a dictionary if you plan to iterate over it- as Vikas Gupta mentioned already in the (beginning of the) discussion under the question. But that discussion as this whole thread still lacks surprisingly good alternatives. One is:
SortedList<string, string> x = new SortedList<string, string>();
x.Add("key1", "value1");
x.Add("key2", "value2");
x["key3"] = "value3";
foreach( KeyValuePair<string, string> kvPair in x )
Console.WriteLine($"{kvPair.Key}, {kvPair.Value}");
Why it could be argued a code smell of iterating over a dictionary (e.g. by foreach(KeyValuePair<,>) ?
A basic principle of Clean Coding:
"Express intent!"
Robert C. Martin writes in "Clean Code": "Choosing names that reveal intent". Obviously naming alone is too weak. "Express (reveal) intent with every coding decision" expresses it better.
A related principle is "Principle of least surprise" (=Principle of Least Astonishment).
Why this is related to iterating over a dictionary? Choosing a dictionary expresses the intent of choosing a data structure which was made for primarily finding data by key. Nowadays there are so much alternatives in .NET, if you want to iterate through key/value pairs that you could choose something else.
Moreover: If you iterate over something, you have to reveal something about how the items are (to be) ordered and expected to be ordered!
Although the known implementations of Dictionary sort the key collection in the order of the items added-
AFAIK, Dictionary has no assured specification about ordering (has it?).
But what are the alternatives?
TLDR:
SortedList: If your collection is not getting too large, a simple solution would be to use SortedList<,> which gives you also full indexing of key/value pairs.
Microsoft has a long article about mentioning and explaining fitting collections:
Keyed collection
To mention the most important: KeyedCollection<,> and SortedDictionary<,> .
SortedDictionary<,> is a bit faster than SortedList for only inserting if it gets large, but lacks indexing and is needed only if O(log n) for inserting is preferenced over other operations. If you really need O(1) for inserting and accept slower iterating in exchange, you have to stay with simple Dictionary<,>.
Obviously there is no data structure which is the fastest for every possible operation..
Additionally there is ImmutableSortedDictionary<,>.
And if one data structure is not exactly what you need, then derivate from Dictionary<,> or even from the new ConcurrentDictionary<,> and add explicit iteration/sorting functions!
var dictionary = new Dictionary<string, int>
{
{ "Key", 12 }
};
var aggregateObjectCollection = dictionary.Select(
entry => new AggregateObject(entry.Key, entry.Value));
Just wanted to add my 2 cent, as the most answers relate to foreach-loop.
Please, take a look at the following code:
Dictionary<String, Double> myProductPrices = new Dictionary<String, Double>();
//Add some entries to the dictionary
myProductPrices.ToList().ForEach(kvP =>
{
kvP.Value *= 1.15;
Console.Writeline(String.Format("Product '{0}' has a new price: {1} $", kvp.Key, kvP.Value));
});
Altought this adds a additional call of '.ToList()', there might be a slight performance-improvement (as pointed out here foreach vs someList.Foreach(){}),
espacially when working with large Dictionaries and running in parallel is no option / won't have an effect at all.
Also, please note that you wont be able to assign values to the 'Value' property inside a foreach-loop. On the other hand, you will be able to manipulate the 'Key' as well, possibly getting you into trouble at runtime.
When you just want to "read" Keys and Values, you might also use IEnumerable.Select().
var newProductPrices = myProductPrices.Select(kvp => new { Name = kvp.Key, Price = kvp.Value * 1.15 } );
in addition to the highest ranking posts where there is a discussion between using
foreach(KeyValuePair<string, string> entry in myDictionary)
{
// do something with entry.Value or entry.Key
}
or
foreach(var entry in myDictionary)
{
// do something with entry.Value or entry.Key
}
most complete is the following because you can see the dictionary type from the initialization, kvp is KeyValuePair
var myDictionary = new Dictionary<string, string>(x);//fill dictionary with x
foreach(var kvp in myDictionary)//iterate over dictionary
{
// do something with kvp.Value or kvp.Key
}
Lets say I have a Dictionary object:
Dictionary myDictionary<int, SomeObject> = new Dictionary<string, SomeObject>();
Now I want to iterate through the dictionary in reverse order. I can't use a simple for loop because I don't know the keys of the dictionary. A foreach is easy:
foreach (SomeObject object in myDictionary.Values)
{
// Do stuff to object
}
But how can I perform this in reverse?
A dictionary or any other form of hashtable has no ordering. So what you are trying to do is pointless :)
I'd use a SortedList instead of a dictionary. You can still access it by Key, but you can access it by index as well.
SortedList sCol = new SortedList();
sCol.Add("bee", "Some extended string matching bee");
sCol.Add("ay", "value matching ay");
sCol.Add("cee", "Just a standard cee");
// Go through it backwards.
for (int i = sCol.Count - 1; i >=0 ; i--)
Console.WriteLine("sCol[" + i.ToString() + "] = " + sCol.GetByIndex(i));
// Reference By Key
foreach (string i in sCol.Keys)
Console.WriteLine("sCol[" + i + "] = " + sCol[i]);
// Enumerate all values
foreach (string i in sCol.Values)
Console.WriteLine(i);
It's worth noting that a sorted list stores key/value pairs sorted by key only.
If you have .NET 3.5 you can use the .Reverse() extension method on IEnumerables. For example:
foreach (object o in myDictionary.Values.Reverse())
{
// Do stuff to object
}
Actually, in C# 2.0 you can create your own iterator that traverses a container in reverse. Then, you can use that iterator in your foreach statement. But your iterator would have to have a way of navigating the container in the first place. If it's a simple array, it could go backwards like this:
static IEnumerable<T> CreateReverseIterator<T>(IList<T> list)
{
int count = list.Count;
for (int i = count - 1; i >= 0; --i)
{
yield return list[i];
}
}
But of course you can't do that with a Dictionary as it doesn't implement IList or provides an indexer. Saying that a Dictionary does not have order is not true: of course it has order. That order can even be useful if you know what it is.
For a solution to your problem: I'd say copy the elements to an array, and use the above method to traverse it in reverse. Like this:
static void Main(string[] args)
{
Dictionary<int, string> dict = new Dictionary<int, string>();
dict[1] = "value1";
dict[2] = "value2";
dict[3] = "value3";
foreach (KeyValuePair<int, string> item in dict)
{
Console.WriteLine("Key : {0}, Value: {1}", new object[] { item.Key, item.Value });
}
string[] values = new string[dict.Values.Count];
dict.Values.CopyTo(values, 0);
foreach (string value in CreateReverseIterator(values))
{
Console.WriteLine("Value: {0}", value);
}
}
Copying your values to an array may seem like a bad idea, but depending on the type of value it's not really that bad. You might just be copying references!
I agree with #leppie, but think you deserve an answer to the question in general. It could be that you meant for the question to be in general, but accidentally picked a bad data structure. The order of the values in a dictionary should be considered implementation-specific; according to the documentation it is always the same order as the keys, but this order is unspecified as well.
Anyway, there's not a straightforward way to make foreach work in reverse. It's syntactic sugar to using the class's enumerator, and enumerators can only travel in one direction. Technically the answer could be "reverse the collection, then enumerate", but I think this is a case where you'll just have to use a "backwards" for loop:
for (int i = myCollection.Length - 1; i >= 0; i--)
{
// do something
}
If you don't have .NET 3.5 and therefore the Reverse extension method you can implement your own. I'd guess it probably generates an intermediate list (when necessary) and iterates it in reverse, something like the following:
public static IEnumerable<T> Reverse<T>(IEnumerable<T> items)
{
IList<T> list = items as IList<T>;
if (list == null) list = new List<T>(items);
for (int i = list.Count - 1; i >= 0; i-- )
{
yield return list[i];
}
}
That would be a Dictionary<int, SomeObject> myDictionary, and you would do it by:
foreach(SomeObject _object in myDictionary.Values.Reverse())
{
}
The only way I can come up with in .NET 2.0 is to first copy all the values to a List, reverse the list and then run the foreach on that list:
Dictionary<int, object> d;
List<object> tmplist;
foreach (object o in d.Values) tmplist.Add(s);
tmplist.Reverse();
foreach (object o in tmplist) {
//Do stuff
}
Literal answer:
Dictionary<int, SomeObject> myDictionary = new Dictionary<int, SomeObject>();
foreach (var pair in myDictionary.OrderByDescending(i => i.Key))
{
//Observe pair.Key
//Do stuff to pair.Value
}
If the ordering is most important, you could you a Stack and create a simple struct to store your int, Object pair.
If you want a dictionary type collection but you need to maintain the insertion order you can look into the KeyedCollection
here
It is a merger between a dictionary and a list. That way you can access elements in the collection via the key or the insertion index.
The only gotcha is if your element being stored in the collection has to have an int key. If you could change that to a string or another type (Guid Mabye). Since collection1 will be searching for the key of 1 rather than the index of 1.
A standard for loop would be best. You don't have to worry about the processing overhead of reversing the collection.
You can use the LINQ to Objects Enumerable.Reverse() function in .NET 2.0 using LinqBridge.
foreach (Sample in Samples)
try the following:
Int32 nEndingSample = Samples.Count - 1;
for (i = nEndingSample; i >= 0; i--)
{
x = Samples[i].x;
y = Samples[i].y;
}