Modify a dictionary in a for loop in C#

Question

I am experienced in C/C++ but pretty much a newbie in C#. My question is pretty simple. Say we have a hash table with integer keys and values and we want to increment all the values in the hash table by 1. We prefer to accomplish this with O(1) extra memory.

Below is one solution, which, to my opinion, is somehow ugly. Is there other way to make it looks more decent?

        Dictionary<int, int> dict = new Dictionary<int, int>();
        for (int i = 0; i < dict.Count; ++i)
        {
            dict[dict.Keys.ElementAt(i)]++;
        } 

PS: I heard that foreach is read-only in C#. But, is there any way like for(auto it& : dict) it.second++ in C++ that I can use to still accomplish this task in C#?

Answer 1

Dictionary<,> itself doesn't provide a good way of doing this - because updating the value associated with a key counts as a change that invalidates any iterator. ConcurrentDictionary<,> does allow this though, and even has an AddOrUpdate method which will help you:

using System;
using System.Linq;
using System.Collections.Concurrent;

class Test
{
    static void Main()
    {
        var dict = new ConcurrentDictionary<int, int>
        {
            [10] = 15,
            [20] = 5,
            [30] = 10
        };
        foreach (var key in dict.Keys)
        {
            dict.AddOrUpdate(key, 0, (k, v) => v + 1);
        }
        Console.WriteLine(string.Join("\r\n", dict.Select(kp => $"{kp.Key}={kp.Value}")));
    }
}

Answer 2

You could use foreach in this instance, but we'd need to do it in a way that doesn't use the dictionary, since updating it changes the internal iterator (which is how foreach keeps up with where it is while iterating, check out this SO for more details: https://stackoverflow.com/a/398996/3874503) In this implementation we'll iterate over a list of the keys and then update each key's value as we iterate.

 Dictionary<int, int> dict = new Dictionary<int, int>();
 foreach (var i in dict.Keys.ToList())
 {
     dict[i]++;
 }

Update as Servy pointed out, I failed to mention that this solution is not O(1), but is instead O(N).

Answer 3

Why is this disallowed? Trying to assigning to it probably wouldn't do what you want - it wouldn't modify the contents of the original collection. This is because the variable x is not a reference to the elements in the list - it is a copy.

Because KeyValuePair have Read-Only Propreties for the Key and the Value to do this in a foreach you Would need a second Dictionnary to add these items and dispose the original one

        Dictionary<int, int> dict = new Dictionary<int, int>();
        dict.Add(0, 1);
        dict.Add(1, 2);
        dict.Add(2, 3);

        Dictionary<int, int> dictOutput = new Dictionary<int, int>();
        foreach (KeyValuePair<int,int> item in dict)
        {
            dictOutput.Add(item.Key, item.Value + 1);
        }
        dict.Dispose();

Answer 4

You don't need the dict.Keys.ElementAt(i) index in your loop. You can treat it like an array. Dictionarys are the C# equivalent to a Map in C++. All you need to do to increment your values is

for (int i = 0; i < dict.Count; i ++)
    dict[i]++;

To add to that, if you're using a Dictionary<int, int>, you can simply use

int[] dict = new int[count];

since your key is the index anyway; But I would really just recommend using a List<int>. This way you can adjust the size dynamically without having to do all the extra work.

This way, you can create your List<int> like so:

List<int> lstDict = new List<int>;
int value; /*assign to whatever value you need to insert*/
int count; /*however many elements you need*/

for (int i = 0; i < count; i ++) 
    lstDict.Add(value);

//
//  Whatever else you need to do
//

for (int i = 0; i < count; i ++)
    lstDict[i]++;

There's tons of way to do what you're trying to accomplish. You can use any of these options to achieve the same thing. If you want to stick with Dictionarys, just use that first snippet I put up there. I just, personally, wouldn't use them unless I needed to use two different data types, like a Dictionary<string, int>.