C# Format foreach loop as Lambda

Question

I am getting started with C# and got the following class:

using System;
using System.Collections.Generic;

class PrefixMapSum : Dictionary<String, int> {

    public bool insert(String key, int value) {
        return base.TryAdd(key, value);
    } 

    public int sum(String prefix) {
        int sum = 0;

        foreach (String key in base.Keys) {
            if (key.StartsWith(prefix)) {
                sum = sum + base[key];
            }
        }

        return sum;
    }
}

Now I'd love to shorten the following part of the code with lambda-expressions:

        foreach (String key in base.Keys) {
            if (key.StartsWith(prefix)) {
                sum = sum + base[key];
            }
        }

I tried with:

new List<String>(base.Keys).ForEach(key => key.StartsWith(prefix) ? sum = sum + base[key] : sum = sum);

Yet i am running into this error: CS0201

I am coming from Java and I can't quite figure out why it does not work. Can anyone explain what I should do differently (and why)?

You'd get a similar error in Java. You tried to use the ternary operator as if it were an if block. Assignments are expressions, so this call will return a value BUT ForEach expects statements that do something, not expressions — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Feb 4, 2020 at 9:54
There are better ways to filter and aggregate values in Java, using streams for example. The syntax itself is quite similar to LINQ's — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Feb 4, 2020 at 9:54

kkica · Accepted Answer · 2020-08-10 17:01:48Z

6

This will only work when there is at least one element after the filtering.

base.Keys
   .Where(key=> key.StartsWith(prefix))
   .Sum(base[key])

If there can be none(The method cannot sum), you can use this DefaultIfEmptyso you will have the sum as 0.

base.Keys
   .Where(key=> key.StartsWith(prefix))
   .Select(key=> base[key])
   .DefaultIfEmpty(0)
   .Sum()

For performance reasons you would probably like to avoid using the indexer, and iterate the dictionary yourself.

var defaultZero = new KeyValuePair<string, int>(string.Empty, 0);
var sum = this
          .Where(pair => pair.Key.StartsWith(prefix))
          .DefaultIfEmpty(defaultZero)
          .Sum(pair => pair.Value);

If you need to handle the case where there is no element separately you can do something like

var elements =this
   .Where(pair => pair.Key.StartsWith(prefix))
   .Select(x=>x.Value);
if(!elements.Any()){
    // your logic here
}else{
    sum= elements.Sum();
    // other stuff
}

edited Aug 10, 2020 at 17:01

answered Feb 4, 2020 at 9:51

kkica

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1 Comment

Rüdiger Over a year ago

thanks for the explanation, the example above tells me CS0103 (key is not known for the select-part)

Pavel Anikhouski · Accepted Answer · 2020-05-08 16:03:34Z

4

You can achieve it with System.Linq, use Where method to filter the keys, then get Sum of values for filtered keys by passing Func<string, int> selector

public int sum(string prefix)
{
    return base.Keys.Where(key => key.StartsWith(prefix)).Sum(key => base[key]);
}

edited May 8, 2020 at 16:03

answered Feb 4, 2020 at 9:52

Pavel Anikhouski

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Comments

Dmitrii Bychenko · Accepted Answer · 2020-02-04 09:54:56Z

3

You can put it as a simple Linq query:

using System.Linq;

...

int sum = this
  .Where(pair => pair.Key.StartsWith(prefix))
  .Sum(pair => pair.Value);

edited Feb 4, 2020 at 9:54

answered Feb 4, 2020 at 9:54

Dmitrii Bychenko

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1 Comment

Orace Over a year ago

base.Where didn't work. You should work on this.

Dan T. · Accepted Answer · 2020-02-04 09:55:38Z

0

You can use Linq:

            var sum = base.Keys.Where(key => key.StartsWith(prefix))
                          .Sum(key => base[key]);

edited Feb 4, 2020 at 9:55

answered Feb 4, 2020 at 9:51

Dan T.

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Comments

Orace · Accepted Answer · 2020-02-04 10:01:08Z

0

For performance, avoid the indexer (ie: calls to base[key]) if you can. Here you can directly work on the KeyValuePair enumeration:

public int Sum(string prefix) => this.Where(kvp => kvp.Key.StartsWith(prefix)).Sum(kvp => kvp.Value);

edited Feb 4, 2020 at 10:01

answered Feb 4, 2020 at 9:53

Orace

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