Develop Reference

Fast random access to a collection

I'm consuming a stream of semi-random tokens. For each token, I'm maintaining a lot of data (including some sub-collections).
The number of unique tokens is unbounded but in practice tends to be on the order of 100,000-300,000.
I started with a list and identified the appropriate token object to update using a Linq query.
public class Model {
public List<State> States { get; set; }
...
}
var match = model.States.Where(x => x.Condition == stateText).SingleOrDefault();
Over the first ~30k unique tokens, I was able to find and update ~1,100 tokens/sec.
Performance analysis shows that 85% of the total Cpu cycles are being spent on the Where(...).SingleOrDefault() (which makes sense, lists are inefficient way to search).
So, I switched the list over to a HashSet and profiled again, confident that HashSet would be able to random seek faster. This time, I was only processing ~900 tokens/sec. And a near-identical amount of time was spent on the Linq (89%).
So... First up, am I misusing the HashSet? (Is using Linq is forcing a conversion to IEnumerable and then an enumeration / something similar?)
If not, what's the best pattern to implement myself? I was under the impression that HashSet already does a Binary seek so I assume I'd need to build some sort of tree structure and have smaller sub-sets?
To answer some questions form comments... The condition is unique (if I get the same token twice, I want to update the same entry), the HashSet is the stock .Net implementation (System.Collections.Generic.HashSet<T>).
A wider view of the code is...
var state = new RollingList(model.StateDepth); // Tracks last n items and drops older ones. (Basically an array and an index that wraps around
var tokens = tokeniser.Tokenise(contents); // Iterator
foreach (var token in tokens) {
var stateText = StateToString(ref state);
var match = model.States.Where(x => x.Condition == stateText).FirstOrDefault();
// ... update the match as appropriate for the token
}

var match = model.States.Where(x => x.Condition == stateText).SingleOrDefault();
If you're doing that exact same thing with a hash set, that's no savings. Hash sets are optimized for quickly answering the question "is this member in the set?" not "is there a member that makes this predicate true in the set?" The latter is linear time whether it is a hash set or a list.
Possible data structures that meet your needs:
Make a dictionary mapping from text to state, and then do a search in the dictionary on the text key to get the resulting state. That's O(1) for searching and inserting in theory; in practice it depends on the quality of the hash.
Make a sorted dictionary mapping from text to state. Again, search on text. Sorted dictionaries keep the keys sorted in a balanced tree, so that's O(log n) for searching and inserting.

30k is not that much so if state is unique you can do something like this.
Dictionary access is much faster.
var statesDic = model.States.ToDictionary(x => x.Condition, x => x);
var match = statesDic.ConstainsKey(stateText) ? statesDic[stateText] : default(State);
Quoting MSDN:
The Dictionary generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary class is implemented as a hash table.
You can find more info about Dictionaries here.
Be also aware that Dictionaries use memory space to improve performance, you can do a quick test for 300k items and see what kind of space I'm talking about like this:
var memoryBeforeDic = GC.GetTotalMemory(true);
var dic = new Dictionary<string,object>(300000);
var memoryAfterDic = GC.GetTotalMemory(true);
Console.WriteLine("Memory: {0}", memoryAfterDic - memoryBeforeDic);

Find key with max value from SortedDictionary?

I have a SortedDictionary how do I find the key associated with the max value? Do I have to loop through every KeyValuePair?

If dict is your SortedDictionary<,> (or any IDictionary<,>) and you want all the keys that correspond to the max value, first check that dict is not null or empty (you need at least one element). Then maybe this works:
var max = dict.Values.Max();
var relevantKeys = dict.Where(pair => max.Equals(pair.Value))
.Select(pair => pair.Key);
Maybe it can be done more efficiently?

Use Enumerable.OrderByDescending() and then access the Key property of what First() returns like so:
var dict = new SortedDictionary<string, string>
{
{"key1", "value3"},
{"key2", "value1"},
{"key3", "value2"},
};
var max = dict.OrderByDescending(d => d.Value).First();
var key = max.Key;

Getting the key associated with the max value, means you are not actually using the default ordering of the SortedDictionary. This is because the SortedDictionar orders by Key, rather than Value. So to do what you want, you'd do it the old fashioned LINQ way:
sortedDict.OrderByDescending(kvp => kvp.Value).First().Key

You can use the MaxBy method in MoreLinq to efficiently run this query.
var result = dictionary.MaxBy(pair => pair.Value).Key;
This will only need to iterate the data once, as opposed to sorting the values and taking the first result (which will be O(n * log(n))).
Since only the keys, rather then the values, are sorted, there is no way of performing this query without at least looping through every keypair once.
Another option would be to have two SortedDictionaries. One would be the one that you already have, and the other would be a reverse dictionary. For each value in your current dictionary you could add it as a key to the second dictionary, and the value of the second dictionary would be the key in the first (if it's a one to many relationship rather than a one to one the value of the reverse lookup will need to be a list of items). While it will be programmatically "expensive" (more in memory than in time, but still some of both) to create this second dictionary, once you do you would be able to efficiently query based on values rather than keys.

To get all keys that hold a maximum of the value you are
interested in, some dataprocessing has to be done.
Actually this is quite comfortable in C#.
It can be accomplished by doing some combination of Linq
// first of all, group your dictionary by the value you want to have
var groups = dict.GroupBy(d => d.Value);
// then, order those groups by the value
var orderedGroups = groups.OrderBy(g => g.Key);
// after that, you have all the KeyValuePairs that hold the MaxValue here:
var maxKeys = orderedGroups.Last().ToList();
Have fun with it!

adding multiple bitmap values for one key in dictionary with C#

I had a dictionary to store pattern images for OCR purposes. I grabbed these bitmaps from dictionary and compared to ones that I cropped from image, if they matched => grabbed the key (OCR part is done).
The problem arises here. One Key should be represented by several different bitmaps (i.e. values). How do you add multiple bitmaps to the dictionary, to represent the same key?
that's how I used dictionary:
Dictionary<string, Bitmap> lookup = new Dictionary<string, Bitmap>();
lookup.Add("A", new Bitmap(#"C:\08\letters\1\a1.bmp", true));
lookup.Add("A", new Bitmap(#"C:\08\letters\1\a2.bmp", true)); // Error will be here, because key A already exists for one Bitmap value.
lookup.Add("a", new Bitmap(#"C:\08\letters\1\aa1.bmp", true));
lookup.Add("B", new Bitmap(#"C:\08\letters\1\b1.bmp", true));
Now, to grab images and value I did following:
var target = lookup.ToList();
bitmap b1 = target[j].Value; //grab value
//if value = cropped bitmap => proceed
string key = target[j].Key; //grab key
How will this process change according to your solution?
P.s. I have heard of "System.Linq.Lookup(Of TKey, TElement)", but never used it before. Will this "lookup" help me solve my problem or is it a completely different tool? google doesn't know much about it either, so an example would be welcomed
please note, that I load dictionary only once, at program start, so it doesn't matter how fast adding is.
Lookup, on the other side is what bothers me the most. I have 120 elements in two of my dictionaries, and according to this article http://www.dotnetperls.com/dictionary-time - Lookup in the List is much more slower, than in dictionary..
anyway I'll be doing some measures to test out how List solution that was suggested below - compares to Dictionary solution which I have right now and tell the results later, probably this evening.

Lookup. It is basically a dictionary of Key to list of Values, instead of key to value.
lookup.Add("a", "123"); // creates 'a' key and adds '123' to it
lookup.Add("a", "456"); // adds '456' to existing 'a' key
lookup.Add("b", "000"); // creates 'b' key and adds '000' to it

You can not add item into a dictionary with the same key. I think you are using the wrong data structure. I thing you might have to look into using a list<> instead. Like this:
var lookup=new List<KeyValuePair<string,Bitmap>>();
lookup.Add(new KeyValuePair<string,Bitmap>("A", new Bitmap(#"C:\08\letters\1\a1.bmp", true)));
lookup.Add(new KeyValuePair<string,Bitmap>("A", new Bitmap(#"C:\08\letters\1\a2.bmp", true)));
lookup.Add(new KeyValuePair<string,Bitmap>("a", new Bitmap(#"C:\08\letters\1\aa1.bmp", true)));
lookup.Add(new KeyValuePair<string,Bitmap>("B", new Bitmap(#"C:\08\letters\1\b1.bmp", true)));
The you can do this instead. Without doing ToList():
bitmap b1 = target[j].Value; //grab value
string key = target[j].Key; //grab key
Edit
But if you are doing a ToList() on a Dictionary then you are missing the point of having a Dictionary in the first place. Because then you are accessing the Dictionary anyway the list way. I can also see a problem doing a ToList() on a Dictionary because the sorting in a Dictionary is not as you insert them, it is by hash. That means that you can not be sure that Index 1 is index 1. You also have to take in considerations that the operation add on a Dictionary is not as effective as the add on a List. The god things with a Dictionary is that the lookup is fast. But that are you not using with your current solution.
So there is two ways what i can see. The one above or make sure that the keys are unique and get it by the lookup in the Dictionary. Like this:
Dictionary<string, Bitmap> lookup = new Dictionary<string, Bitmap>();
lookup.Add("A", new Bitmap(#"C:\08\letters\1\a1.bmp", true));
lookup.Add("B", new Bitmap(#"C:\08\letters\1\a2.bmp", true));
lookup.Add("C", new Bitmap(#"C:\08\letters\1\aa1.bmp", true));
lookup.Add("D", new Bitmap(#"C:\08\letters\1\b1.bmp", true));
Then you can get the Bitmap like this:
Bitmap bm;
if(lookup.TryGetValue("A",out bm))
{
//Do something
}
Or if you know that the key is present in the Dictionary then you can do like this:
Bitmap bm;
bm= lookup["A"];

First, it's different to have 'multiple bmp-s' to 'represent the key' - or have 'one key' associated (mapped) into multiple 'values' - which is what Yorye suggested rightly.
So, if you want more values attached to a single key - then you can use something like Dictionary<TKey, IList<TValue>> - where TKey and TValue are types you need.
But that doesnt' solve indexing and querying of the data.
That assumes that your 'key' is just 'A' in your case - which is not clear what it is.
So in that case you're using the 'dictionary' for something that it shouldn't be used. Dictionary is a hashing structure (basically indexes all its entries into buckets etc.) which servers the purpose of speeding up the querying process, locating the 'right' value.
As I see in your case the 'key' is/are the 'set of bitmaps' which sort of present a 'signature' of the OCR-ed image, if I"m right? I'm not much into the OCR but I'm guessing here.
That complicates things a bit, you'd need to create a 'composite' key of a sort.
The 'key' and not the 'value' (or value list) would be the bitmaps (providing they can be made to be comparable and equal or non-equal, also there's a problem of how you compare multiple values to multiple values etc.).
If that's the case usually (but for simpler cases than yours), you'd create a custom class and make that class have a GetHashCode() Equals override (or IEqualityComparer) etc. so that it can be used as a key in dictionaries. And then you use that as a key.
Again, in your case I think that's a bit of a stretch (in a sense that it's not easy to implement).
Basically you need to think about 'querying' the data, not much as storing. What are the real 'keys' for your system. If it's a bitmap, that's always the same (or if not how you compare to signature bmps) then you might save some bmp hashcode instead and use that as a key - and compare that, instead of bmps.
i.e. you need to think about things like that - and then the solution will usually be obvious, what you need to use.
I would not recommend a list, as that's a poor-men's choice - unless you might have only a couple so it's easy to go through it by hand, somehow I dont' think that's the case in your case.
If you need some way of 'indexing' by some key or keys - then it's usually the dictionary (or dictionary is involved in some way or part) - but you can have many 'dictionaries' - or combinations. Also you can have 'many types of keys' and values etc.
You'd need to give us some data for that.
hope this helps
EDIT: And lastly - getting the right 'hash-code' is also not a simple thing to do - as with your custom structure, comnparing, that's something you need to work out yourself - so that boils down to what's your key - and what represents the 'key' (as in which property, value best describes it and makes it unique, hard thing to do for an image/bmp?), the distribution of hash values etc.

List containing Dictionary Containing List

Question
I need a data structure like the above in C# but I am having problems accessing the contents, I have seen that I can do this in C++ how is the same done in c#?
List<Dictionary<string, List<int>>> data =
new List<Dictionary<string, List<int>>>(5);
when i do Data[0] i get an error stating index out of bounds..
Can anyone point me as to how I can do this. or what I am doing wrong ?
Addendum:
->What im trying to do
I am basically creating a data structure which keeps a plan of the month.
So a List of the 5 weeks each containing->
a Dictionary of activities, where the value for each activity is a ->
list of 7 integers representing the hours spent each day of the week on that activity.
I have got that I need to add blank entries.
Additional Question is like this.
->Query
Right now I will have to Add 5 blank dictionaries to the list.
and a list of 7 integers(initialized to zero) in each Key of the dictionary representing each days time consumed.
Is there a way to initialize 5 blank dictionaries in the list and same inside, without manually creating new dictionaries and adding them.
With c++ i believe that you can do this and even enter the value to which you want to initialize each entry to be, so..
A statement like
Vector<int> *vec = new Vector<int>(5,0);
Initialized the vec with 5 entries each being 0. Can I do the same in c# ?
Thanks for the help.

This has nothing to do with the list being a complex type. You'll get the same thing with this:
List<string> list = new List<string>(5);
string x = list[0]; // This will fail too
The List<T>(int) constructor creates a list with the given capacity (i.e. the array backing it will have an initial size of 5, assuming it doesn't decide that's just too small to be useful). It still has a size of 0.
You still need to add elements to the list before you can access them.
EDIT: There's no constructor to "fill" a list with a given set of values. With LINQ you could write:
List<string> list = Enumerable.Repeat("foo", 5).ToList();
... but you wouldn't want to do that for your dictionary one, as otherwise you'll end up with 5 references to the same dictionary. You could use:
var list = Enumerable.Range(0, 5)
.Select(x => new Dictionary<string, List<int>>())
.ToList();
Note that each dictionary here is empty though.
Personally I'd probably just create the list and fill it with a for loop though...

You have initialized the List with a default size of 5, but the list is still empty, thus when you attempt to access index 0 in the data object, it is out of bounds, because the current amount of items in the list is 0, thus there is no objects to access.
if you add a new item to data, with data.Add(...) you can access the object at the first index with data[0]

What is the most efficient way to do look-up table in C#

What is the most efficient way to do look-up table in C#
I have a look-up table. Sort of like
0 "Thing 1"
1 "Thing 2"
2 "Reserved"
3 "Reserved"
4 "Reserved"
5 "Not a Thing"
So if someone wants "Thing 1" or "Thing 2" they pass in 0 or 1. But they may pass in something else also.
I have 256 of these type of things and maybe 200 of them are reserved.
So what is the most efficient want to set this up?
A string Array or dictionary variable that gets all of the values. And then take the integer and return the value at that place.
One problem I have with this solution is all of the "Reserved" values. I don't want to create those redundant "reserved" values. Or else I can have an if statement against all of the various places that are "reserved" but they might now be just 2-3, might be 2-3, 40-55 and all different places in the byte. This if statement would get unruly quick
My other option that I was thinking was a switch statement. And I would have all of the 50ish known values and would fall through through and default for the reserved values.
I am wondering if this is a lot more processing than creating a string array or dictionary and just returning the appropriate value.
Something else? Is there another way to consider?

"Retrieving a value by using its key is very fast, close to O(1), because the Dictionary(TKey, TValue) class is implemented as a hash table."
var things = new Dictionary<int, string>();
things[0]="Thing 1";
things[1]="Thing 2";
things[4711]="Carmen Sandiego";

The absolute fastest way to do lookups of integer values in C# is with an array. This will be preferable to using a dictionary, maybe, if you are trying to do tens of thousands of lookups at a time. For most purposes, this is overkill; it's more likely that you need to optimize developer time than processor time.
If the reserved keys are not simply all keys that aren't in the lookup table (i.e. if a lookup for a key can return the found value, a not-found status, or a reserved status), you'll need to save the reserved keys somewhere. Saving them as dictionary entries with magic values (e.g. the key of any dictionary entry whose value is null is reserved) is OK unless you write code that iterates over the dictionary's entries without filtering them.
A way to solve that problem is to use a separate HashSet<int> to store the reserved keys, and maybe bake the whole thing into a class, e.g.:
public class LookupTable
{
public readonly Dictionary<int, string> Table { get; }
public readonly HashSet<int> ReservedKeys { get; }
public LookupTable()
{
Table = new Dictionary<int, string>();
ReservedKeys = new HashSet<int>();
}
public string Lookup(int key)
{
return (ReservedKeys.Contains(key))
? null
: Table[key];
}
}
You'll note that this still has the magic-value issue - Lookup returns null if the key is reserved, and throws an exception if it's not in the table - but at least now you can iterate over Table.Values without filtering magic values.

Checkout the HybridDictionary. It automatically adjusts it's underlying storage mechanism based on size to get the greatest efficiency.
http://msdn.microsoft.com/en-us/library/system.collections.specialized.hybriddictionary.aspx

If you have lots of reserved (currently unused) values or if the range of the integer values can get very big, then I would use a generic dictionary (Dictionary):
var myDictionary = new Dictionary<int, string>();
myDictionary.Add(0, "Value 1");
myDictionary.Add(200, "Another value");
// and so on
Otherwise, if you have a fixed number of values and only few of the are currently unused, then I'd use a string array (string[200]) and set/leave the reserved entries to null.
var myArray = new string[200];
myArray[0] = "Value 1";
myArray[2] = "Another value";
//myArray[1] is null

The in-built Dictionary object (preferably a generic dictionary) would be ideal for this, and is specifically designed for fast/efficient retrieval of the values relating to the keys.
From the linked MSDN article:
Retrieving a value by using its key is
very fast, close to O(1), because the
Dictionary<(Of <(TKey, TValue>)>)
class is implemented as a hash table.
As far as your "reserved" keys go, I wouldn't worry about that at all if we're only talking about a few hundred keys/values. It's only when you reach tens, maybe hundreds of thousands of "reserved" keys/values that you'll want to implement something more efficient.
In those cases, probably the most efficient storage container then would be an implementation of a Sparse Matrix.

I'm not quite sure I understand your problem correctly. You have a collection of strings. Each string is associated to an index. The consumer requests gives an index and you return the corresponding string, unless the index is reserved. Right?
Can't you simple set reserved items as null in the array.
If not, using a dictionary that doesn't contain the reserved items seems a reasonable solution.
Anyway, you'll probably get better answers if you clarify your problem.

I would use a Dictionary to do the lookups. This is the most efficient way to do look ups by far. Using a string will run somewhere in the region of O(n) to find the object.
It might be useful to have a 2nd Dictionary to all you to do a reverse lookup if its needed

Load all your values into
var dic = new Dictionary<int, string>();
And use this for retrieval:
string GetDescription(int val)
{
if(0 <= val && val < 256)
if(!dic.Contains(val))
return "Reserved";
return dic[val];
throw new ApplicationException("Value must be between 0 and 255");
}

Your question seems to imply that the query key is an integer. Since you have at most 256 items, then the query key is in the range 0..255, right? If so, just have a string array of 256 strings, and use the key as an index into the array.
If your query key is a string value, then it's more like a real lookup table. Using a Dictionary object is simple, but if you're after raw speed for a set of as few as 50 or so actual answers, a do-it-yourself approach such as binary search, or a trie, could be quicker. If you use binary search, since the number of items is so small, you could unroll it.
How often does the list of items change? If it only changes very seldom, you can get even better speed by generating code to do the search, which you can then compile and execute to do each query.
On the other hand, I assume you've proven that this lookup is your bottleneck, either by profiling or taking stackshots. If less than 10% of time-when-slow is spent in this query, then it is not your bottleneck so you may as well do the thing that is easiest to code.