Visual C# Collection "map" function?

Question

In Haskell we have the function

map (a -> b) -> [a] -> [b]

to change the type of a Collection using a function.

Is there something similar in C#?

Or what else is the fastest way to put a complete collection of KeyValuePairs into a debug message, for example?

I thought of sth like

debugmsg("http response is " + service.HttpResponseHeaders
                                      .Map<string>((k, v) => k + "->" + v)
                                      .Aggregate((s, sx) => s + "," + sx)
                             + ". ");

Note that a header can have multiple values. Simply concatenating the value won't work — Panagiotis Kanavos, Nov 06 '13 at 08:23

score 13 · Accepted Answer · answered Nov 06 '13 at 07:46

13

In LINQ, map is named Select. Also, please note that any collection of KeyValuePair<TKey, TValue> will have only one argument in Select; you'll have to pull the Key and Value out of it.

service.HttpResponseHeaders.Select(kvp => kvp.Key + "->" + kvp.Value)
                           .Aggregate((s, sx) => s + "," + sx);

answered Nov 06 '13 at 07:46

Patryk Ćwiek

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Value is an IEnumerable. Simply concatenating strings will result in "String[]" instead of the actual value – Panagiotis Kanavos Nov 06 '13 at 08:19
If it's the case (that the `Value` from key-value pair is a collection), then doing `SelectMany` a.k.a `flatMap` on the value should solve the issue. – Patryk Ćwiek Nov 06 '13 at 08:19
Not really, you need to combine a single header name with multiple values. A single operation is not enough. – Panagiotis Kanavos Nov 06 '13 at 08:22
Doing `kvp => kvp.Key + "->" + string.Join("-", kvp.Value)` should be good enough then. The question was about `map` though... – Patryk Ćwiek Nov 06 '13 at 08:24

score 1 · Answer 2 · answered Nov 06 '13 at 07:47

1

LINQ comes with Select

Signature (Haskell style):

IEnumerable<TSource>.Select(Func<TSource->TResult>)->IEnumerable<TResult>

answered Nov 06 '13 at 07:47

Adam Kewley

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score 1 · Answer 3 · edited May 23 '17 at 10:29

1

You could use Dictionary, which is basically a collection of KeyValuePairs and do something like:

service.HttpResponseHeaders
                  .Select(kvp => kvp.Key + " -> " + String.Join(" - ", kvp.Value))
                  .Aggregate((s, sx) => s + ", " + sx);

Optimizing Aggregate: Optimizing Aggregate for String Concatenation

edited May 23 '17 at 10:29

Community

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answered Nov 06 '13 at 07:52

Florian Gl

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HttpResponseHeaders is also a collection of KeyValuePair, actually it implements IEnumerable>>. No need for Dictionary, and string concatenation will return "String[]", not the header value – Panagiotis Kanavos Nov 06 '13 at 08:20
Then call `.Aggregate()` on your value. Have a look at my edit. – Florian Gl Nov 06 '13 at 08:35
A String.Join would be simpler and faster, because it wouldn't create any temporary strings – Panagiotis Kanavos Nov 06 '13 at 08:51
Are you sure .Aggregate() does? Why shouldn't it use the `StringBuilder` also? – Florian Gl Nov 06 '13 at 08:53
Apart from the DRY, just try it and see how ugly it looks – Panagiotis Kanavos Nov 06 '13 at 09:12
Calling .Aggregate() two times doesn't hurt the DRY-principle, imo. And the result is the same as using String.Join(). – Florian Gl Nov 06 '13 at 09:25

score 1 · Answer 4 · answered Nov 06 '13 at 08:06

As others have noted, you can map one type to another using Select. Creating the final string is best done using String.Join though, to avoid creating useless temporary strings.

Strings in .NET are immutable so adding two strings creates a new string. String.Join on the other hand uses a StringBuilder internally to add data to a mutable buffer and return the final result.

You should note though that HttpResponseHeaders contains multiple values for each key. Just converting the value to a string will not work.

The following will create a comma-separated list of values from the response headers. If the header has multiple values, they are separated by '|':

var headerStrings=from header in service.HttpResponseHeaders
            let headerValue=String.Join("|",header.Value)
            select String.Format("{0} -> {1}",header.Key,headerValue);
var message=String.Join(",",headerStrings);

score 0 · Answer 5 · answered Nov 06 '13 at 07:46

0

You can use .Select() or if it is a List you can use .ConvertAll()

Here is the MSDN Documentation

Enumerable.Select

List.ConvertAll

answered Nov 06 '13 at 07:46

David Pilkington

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Visual C# Collection "map" function?

5 Answers5