Example of array.map() in C#?

Question

Consider the following common JavaScript construct

var ages = people.map(person => person.age);

Giving the desired result, which is an array of ages.

What is the equivalent of this in C#? Please include a simple example. The documentation indicates select or possible selectAll but I can't find an example online or any other SO question which can be pasted in and works.

If possible, give an example which turns the following array {1,2,3,4} into the following {'1a','2a','3a','4a'}. For each element, append "a" to the end, turning it from an Integer to a String.

(new[]{1,2,3,4}).Select(v => String.Format("{0}a", v))

Alexei Levenkov
– Alexei Levenkov

2015-10-05 23:35:23 +00:00
Commented Oct 5, 2015 at 23:35 — Alexei Levenkov
– Alexei Levenkov, Commented Oct 5, 2015 at 23:35
@AlexeiLevenkov You missed the .ToArray() on the end.

Corey
– Corey

2015-10-06 01:47:56 +00:00
Commented Oct 6, 2015 at 1:47 — Corey
– Corey, Commented Oct 6, 2015 at 1:47

Daniel A. White · Accepted Answer · 2020-05-20 03:02:26Z

201

This is called projection which is called Select in LINQ. That does not return a new array (like how JavaScript's .map does), but an IEnumerable<T>. You can convert it to an array with .ToArray.

using System.Linq; // Make 'Select' extension available
...
var ages = people.Select(person => person.Age).ToArray();

Select works with all IEnumerable<T> which arrays implement. You just need .NET 3.5 and a using System.Linq; statement.

For your 2nd example use something like this. Notice there are no arrays in use - only sequences.

 var items = Enumerable.Range(1, 4).Select(num => string.Format("{0}a", num));

edited May 20, 2020 at 3:02

answered Oct 5, 2015 at 23:32

Daniel A. White

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2 Comments

Bart Calixto Over a year ago

on c#6 : var items = Enumerable.Range(1, 4).Select(num => string.Format($"{num}a"));

freefaller Over a year ago

@BartCalixto - you don't need the string.Format... var items = Enumerable.Range(1, 4).Select(num => $"{num}a");

Buzinas · Accepted Answer · 2015-10-06 00:00:59Z

33

Only for info, if people is a List<Person>, the ConvertAll method is pretty similar to JS's map, e.g:

var ages = people.ConvertAll<int>(person => person.age);

But if you have an Array and you want to use any List<T> methods, you can easily achieve that by converting your variable into a List from Array, e.g:

var ages = people.ToList().ConvertAll<int>(person => person.age);

And finally, if you really need an Array back, then you could convert it back, e.g:

var ages = people.ToList().ConvertAll<int>(person => person.age).ToArray();

But that last example is not as good as the other answers, and you should use Select if you're working only with Arrays. But if you can, I suggest you to move to List<T>, it's much better!

answered Oct 6, 2015 at 0:00

Buzinas

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1 Comment

V-SHY Over a year ago

convert to List and use ConvertAll allow me to refactor my large Select code that is not allow to have parameter constructor in Linq

pb2q · Accepted Answer · 2015-10-05 23:50:46Z

5

The LINQ extension methods on collections give you a host of really handy utilities. Select is one of them:

int[] arr = { 1, 2, 3 };
IEnumerable<string> list = arr.Select(el => el + "a");
string[] arr2 = list.ToArray();

foreach (var str in arr2)
    Console.Write(str + " ");

This should output:

1a 2a 3a

This can safely be condensed to a 1-liner:

string[] arr2 = arr.Select(el => el + "a").ToArray();

A working example:

https://ideone.com/mxxvfy

Related docs:

Enumerable.Select

Basic LINQ Query Operations (C#)

edited Oct 5, 2015 at 23:50

answered Oct 5, 2015 at 23:35

pb2q

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1 Comment

Prashanth Thurairatnam Over a year ago

How this is different from @Richard Schneider's answer. Also what's the need to do ToList()??

plewand · Accepted Answer · 2021-06-26 15:22:25Z

If you don't want to perform conversion to/from an array Array.ConvertAll can to the work. It is declared in Array class of System namespace (remember to add using System; at the top of the file) as:

public static TOutput[] ConvertAll<TInput,TOutput> (TInput[] array, Converter<TInput,TOutput> converter);

Example:

var items = new[] {"abc", "ab", "abcde"};

var result = Array.ConvertAll(items, p => p.Length);
            
Console.Out.WriteLine(string.Join(", ", result)); // Outputs 3, 2, 5

Method reference: https://learn.microsoft.com/en-us/dotnet/api/system.array.convertall?view=net-5.0

Check this section to see .NET version compatibility: https://learn.microsoft.com/en-us/dotnet/api/system.array.convertall?view=net-5.0#applies-to

Richard Schneider · Accepted Answer · 2015-10-05 23:34:39Z

2

Linq's .Select is the map equivalent and .Aggregate is the fold equivalent.

var nums = new[] {1,2,3,4};
string[] r = nums.Select(x => x + "a").ToArray();

answered Oct 5, 2015 at 23:34

Richard Schneider

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Comments

merlin · Accepted Answer · 2018-05-17 04:04:59Z

1

You can use the keywords from, select, in and while;
Or for your example:

 var ages = (from person in people select person.age).ToArray();

So essentially the syntax would be:

 <<list-output>> = (from <<var-name>> in <<list-input>> select <<operation>>);

answered May 17, 2018 at 4:04

merlin

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