Dividing all values of a dictionary by a value

I have the following code in C# where vector is a [string,double] type of Dictionary.I want to divide all values in this dictionary by a value, "magnitude". Now, my naive first code was the following:

foreach (var key in vector.Keys)
{
    vector[key] = vector[key] / magnitude;
}

Which throws an exception, saying the Collection has been modified in a foreach. I may create a second dictionary to write the resulting values into, but I do not want this way.

Is there any easier way to do that, for example by using methods which operate on all Dictionary values, like the following?

vector.Values().Aggreagate(), vector.Values().Average()
Jon Skeet
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The simplest way to do this is just to copy the list of keys before iterating:

foreach (var key in vector.Keys.ToList())
{
    vector[key] = vector[key] / magnitude;
}

Or:

foreach (var entry in vector.ToList())
{
    vector[entry.Key] = entry.Value / magnitude;
}

(That avoids a double lookup, but will copy more data of course.)

It's a bit of a shame that modifying the value of an existing entry is seen as a change that prevents you from continuing to iterate over the keys, admittedly.

An alternative would be to have a mutable wrapper type as the value, then you could use:

foreach (var wrapper in vector.Values)
{        
    wrapper.Value = wrapper.Value / 10;
}

That wouldn't be modifying the dictionary at all - just the objects that the dictionary refers to. I personally wouldn't do this in most cases, but it may be appropriate for you.

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