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
– Panagiotis Kanavos, Commented Nov 6, 2013 at 8:23

Patryk Ćwiek · Accepted Answer · 2013-11-06 07:46:41Z

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 6, 2013 at 7:46

Patryk Ćwiek

14.3k3 gold badges58 silver badges78 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Panagiotis Kanavos Over a year ago

Value is an IEnumerable<string>. Simply concatenating strings will result in "String[]" instead of the actual value

Patryk Ćwiek Over a year ago

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.

Panagiotis Kanavos Over a year ago

Not really, you need to combine a single header name with multiple values. A single operation is not enough.

Patryk Ćwiek Over a year ago

Doing kvp => kvp.Key + "->" + string.Join("-", kvp.Value) should be good enough then. The question was about map though...

Adam Kewley · Accepted Answer · 2013-11-06 07:47:17Z

1

LINQ comes with Select

Signature (Haskell style):

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

answered Nov 6, 2013 at 7:47

Adam Kewley

1,2347 silver badges16 bronze badges

Comments

Panagiotis Kanavos · Accepted Answer · 2013-11-06 08:06:35Z

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);

Community · Accepted Answer · 2017-05-23 10:29:50Z

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, 2017 at 10:29

CommunityBot

11 silver badge

answered Nov 6, 2013 at 7:52

Florian Gl

6,0142 gold badges19 silver badges30 bronze badges

6 Comments

Panagiotis Kanavos Over a year ago

HttpResponseHeaders is also a collection of KeyValuePair, actually it implements IEnumerable<KeyValuePair<string, IEnumerable<string>>>. No need for Dictionary, and string concatenation will return "String[]", not the header value

Florian Gl Over a year ago

Then call .Aggregate() on your value. Have a look at my edit.

Panagiotis Kanavos Over a year ago

A String.Join would be simpler and faster, because it wouldn't create any temporary strings

Florian Gl Over a year ago

Are you sure .Aggregate() does? Why shouldn't it use the StringBuilder also?

Panagiotis Kanavos Over a year ago

Apart from the DRY, just try it and see how ugly it looks

|

David Pilkington · Accepted Answer · 2013-11-06 07:46:28Z

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 6, 2013 at 7:46

David Pilkington

13.6k3 gold badges44 silver badges75 bronze badges

Collectives™ on Stack Overflow

Visual C# Collection "map" function?

5 Answers 5

4 Comments

Comments

Comments

6 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

4 Comments

Comments

Comments

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related