Incrementing a numerical value in a dictionary

Question

I'm using the code below to either increment or insert a value in a dictionary. If the key I'm incrementing doesn't exist I'd like to set its value to 1.

 public void IncrementCount(Dictionary<int, int> someDictionary, int id)
 {  
     int currentCount;
     if (someDictionary.TryGetValue(id, out currentCount))
     {
         someDictionary[id] = currentCount + 1;
     }
     else
     {
         someDictionary[id] = 1;
     }
 }

Is this an appropriate way of doing so?

Lulalulea

What are you trying to achieve with this? What problem are you solving? – Eyra

The Dictionary object doesn't really have a 'count' and it doesn't really make sense to 'decrement' it like you would an array. Having said that, the code you provided should work, though I think there are more succinct ways to handle the situation: check the Dictionary object's built in methods. – Rutharuthann

What have you meant by the best way? Thread safe, more succinct, the fastest, whatsoever, the most readable? – Corruptible

@Corruptible this question just received a late answer. The question is more than five years old, and the OP hasn't been here for more than two years. – Conlan

@Conlan yeah, now I see. But this question is the first in the search results for 'most efficient way to increment an element in a .net dictionary', so I've tried to improve it. – Corruptible

@Conlan thanks for the edit, it's better than trying to guess what was meant. – Corruptible

@Rutharuthann As I understand it, this question isn't about any concept of a 'count' in a dictionary. Rather, in this particular dictionary, the semantic of each value is some kind of count, and the OP was looking for the most elegant way to increment a count and in doing so create the count in the dictionary it if it isn't already present. – Giorgione

Answer 1

Your code is fine. But here's a way to simplify in a way that doesn't require branching in your code:

int currentCount;

// currentCount will be zero if the key id doesn't exist..
someDictionary.TryGetValue(id, out currentCount); 

someDictionary[id] = currentCount + 1;

This relies on the fact that the TryGetValue method sets value to the default value of its type if the key doesn't exist. In your case, the default value of int is 0, which is exactly what you want.

---

UPD. Starting from C# 7.0 this snippet can be shortened using out variables:

// declare variable right where it's passed
someDictionary.TryGetValue(id, out var currentCount); 
someDictionary[id] = currentCount + 1;

Simonesimoneau

This code will not work. You are correct that if the key doesn't exist in the dictionary, currentCount will be 0. But that also means someDictinary[id] will throw a KeyNotFoundException. – Pilloff

@JBSnorro: No, this will work fine; I encourage you to try it. Note that the setter of the indexer is being called, not the getter. From the documentation: " When you set the property value, if the key is in the Dictionary<TKey, TValue>, the value associated with that key is replaced by the assigned value. If the key is not in the Dictionary<TKey, TValue>, the key and value are added to the dictionary." – Simonesimoneau

A source for that setter-creation comment: msdn.microsoft.com/en-us/library/9tee9ht2.aspx "a set operation creates a new element with the specified key" – Wordless

I realize this is a bit late to the show, but thought I'd help others that hit this. The above code will fail in a truly multi-threaded environment. Between the TryGetValue and the incrementing function below it the Dictionary can be updated on another thread, which means currentCount will be out of sync and you created a race condition. – Leesaleese

All of the examples here are not thread safe. It was not part of the question, so making the assumption that it is so would be a mistake. – Nebo

Answer 2

As it turns out it made sense to use the ConcurrentDictionary which has the handy upsert method: AddOrUpdate.

So, I just used:

someDictionary.AddOrUpdate(id, 1, (id, count) => count + 1);  

Lulalulea

That is fine, but I imagine there is some overhead to deal with thread issues. – Meanie

Indeed, ConcurrentDictionary is optimized for an environment with many threads accessing the collection. – Homonym

Answer 3

Here's a nice extension method:

    public static void Increment<T>(this Dictionary<T, int> dictionary, T key)
    {
        int count;
        dictionary.TryGetValue(key, out count);
        dictionary[key] = count + 1;
    }

Usage:

var dictionary = new Dictionary<string, int>();
dictionary.Increment("hello");
dictionary.Increment("hello");
dictionary.Increment("world");

Assert.AreEqual(2, dictionary["hello"]);
Assert.AreEqual(1, dictionary["world"]);

Regain

I realize this is a bit late to the show, but thought I'd help others that hit this. The above code will fail in a truly multi-threaded environment. Between the TryGetValue and the incrementing function below it the Dictionary can be updated on another thread, which means count will be out of sync and you created a race condition. – Leesaleese

Love this. I used this and changed the return value to be the incremented value return (dictionary[key] = ++count); – Dimerous

Answer 4

It is readable and the intent is clear. I think this is fine. No need to invent some smarter or shorter code; if it doesn't keep the intent just as clear as your initial version :-)

That being said, here is a slightly shorter version:

public void IncrementCount(Dictionary<int, int> someDictionary, int id)
{
    if (!someDictionary.ContainsKey(id))
        someDictionary[id] = 0;

    someDictionary[id]++;
}

If you have concurrent access to the dictionary, remember to synchronize access to it.

Understate

@inflagranti - this does too. (It increments the zero before returning - it was an example of a way to get less conditionals in the method). – Understate

Doesn't this require an extra look up when id does not exist? – Meanie

You're right, sorry. So I'd say the original code is more readable then ;) – Marlysmarmaduke

Yes it does require an extra lookup when ID does not exist. I read "best way" as "shorter/simpler". If "best way" means "most performant", the original version should be slightly more efficient. However, I doubt it will be measurable, unless this is used in a tight loop. – Understate

I also prefer the ContainsKey approach with value types if i need to "modify" them. But your approach needs 3 lookups if the id is not contained. You should use if...else.... It's even more readable if you use if(someDictionary.ContainsKey(id)) someDictionary[id]++; else someDictionary.Add(id, 1);. – Georg

Answer 5

Just some measurements on .NET 4 for integer keys.

It's not quite an answer to your question, but for the sake of completeness I've measured the behavior of various classes useful for incrementing integers based on integer keys: simple Array, Dictionary (@Ani's approach), Dictionary (simple approach), SortedDictionary (@Ani's approach) and ConcurrentDictionary.TryAddOrUpdate.

Here is the results, adjusted by 2.5 ns for wrapping with classes instead of direct usage:

Array                 2.5 ns/inc
Dictionary (@Ani)    27.5 ns/inc
Dictionary (Simple)  37.4 ns/inc
SortedDictionary    192.5 ns/inc
ConcurrentDictionary 79.7 ns/inc

And that's the code.

Note that ConcurrentDictionary.TryAddOrUpdate is three times slower than Dictionary's TryGetValue + indexer's setter. And the latter is ten times slower than Array.

So I would use an array if I know the range of keys is small and a combined approach otherwise.

Corruptible

Answer 6

Here is a handy unit test for you to play with concerning ConcurrentDictionary and how to keep the values threadsafe:

     ConcurrentDictionary<string, int> TestDict = new ConcurrentDictionary<string,int>();
     [TestMethod]
     public void WorkingWithConcurrentDictionary()
     {
         //If Test doesn't exist in the dictionary it will be added with a value of 0
         TestDict.AddOrUpdate("Test", 0, (OldKey, OldValue) => OldValue+1);

         //This will increment the test key value by 1 
         TestDict.AddOrUpdate("Test", 0, (OldKey, OldValue) => OldValue+1);
         Assert.IsTrue(TestDict["Test"] == 1);

         //This will increment it again
         TestDict.AddOrUpdate("Test", 0, (OldKey, OldValue) => OldValue+1);
         Assert.IsTrue(TestDict["Test"] == 2);

         //This is a handy way of getting a value from the dictionary in a thread safe manner
         //It would set the Test key to 0 if it didn't already exist in the dictionary
         Assert.IsTrue(TestDict.GetOrAdd("Test", 0) == 2);

         //This will decriment the Test Key by one
         TestDict.AddOrUpdate("Test", 0, (OldKey, OldValue) => OldValue-1);
         Assert.IsTrue(TestDict["Test"] == 1);
     }

Leesaleese

Answer 7

Tried this in C# 7.0, and it works as expected

// var Nos = new Dictionary<int, int>();

if (!Nos.ContainsKey(id)) Nos.Add(id, 0);
var currentCount = ++Nos[id];

Langan

Answer 8

using CollectionsMarshal to avoid extra copy/lookup.

private static void IncrementCount(Dictionary<uint, uint> someDictionary, uint id)
{
    ref var value = ref CollectionsMarshal.GetValueRefOrNullRef(someDictionary, id);

    if (Unsafe.IsNullRef(ref value))
    {
        someDictionary[id] = 1;
        return;
    }

    value++;
}

and if you wanna be generic with initial value as extension method:

public static void IncrementCountAsInt32<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key, TValue initialValue) where TKey : notnull
{
    ref var value = ref CollectionsMarshal.GetValueRefOrNullRef(dictionary, key);

    if (Unsafe.IsNullRef(ref value))
    {
        dictionary[key] = initialValue;
        return;
    }

    Unsafe.As<TValue, int>(ref value)++;
}

Homochromatic

Might as well just use GetValueRefOrAddDefault if you're always going to add the value if it's missing. And there's no point in a "generic" solution if you're just going to cast the value to an int anyway, it'll just break if the value isn't an int. You can constrain it to INumber to increment any type of number, if you actually want a generic solution. – Ruthanneruthe

@Ruthanneruthe You're right, but GetValueRefOrAddDefault doesn't have more control over what initial value you want to increment from at runtime. The interface constraint causing too much... constraint. Custom types would require to have ISignedNumber`1 or INumber`1 it... for me that would ruin the genericness. I was thinking more of the Tvalue as a memory region representation with 1/2/4/8 byte length, not specifically a numerical value, but as numerical representation. But it's good to do if someone prefers more constraint on generics methods to avoid issues in case of misuse of the method. – Homochromatic

Currently you're checking for null, you'd be able to just replace it with default(T) if you want to account for the situations where the default value isn't the additive identity. And by constraining to INumber you have T.Zero to use, rather than needing a user-supplied value. Your solution of casting to int is what ruins the genericness. If the value in question is anything other than an int, this code will jut break. Constraining to INumber (or IIncrementOperators if you want) then you can safely support every type of number, including custom-user-numbers. – Ruthanneruthe

If you want to make a method that only works for ints, that's fine, that covers lots of use cases; but making the method claim to be generic, and then performing an unsafe cast to int and letting the code just break if the value isn't one, is way worse than just being truthful about the fact that it only supports ints. – Ruthanneruthe

Yeah the reinterpretation of TValue to int is a bit bad in a generic function, but I wasnt thinking about int or any numerical type, I was thinking of 32bit block of memory just for the example sake, hence the name "IncrementCountAsInt32" ASINT32. You want me to rewrite the increment part to a pseudo code? So you can stop whining about it? – Homochromatic

Again, if you want to write a function that only ever works for an int then only accept an int. If you want to make a generic function that works for any number Make it work for any number, which in this case, would be done by just constraining the generic value to INumber. If you don't know how to actually implement the generic version in a way that actually works, don't try to make a generic function, Writing a function that doesn't work and then whining when people tell you that it doesn't work is not helpful to anyone. – Ruthanneruthe