0

We know that if you have:

var aa = new [] { 1, 2, 3, 4, 5 };
for (int i = 0; i < aa.length; ++i)
{
    aa[i] = aa[i] + 1;
}

it's really

var aa = new [] { 1, 2, 3, 4, 5 };
Arrary.ForEach(aa, a => a + 1);

However, what if I had this:

var aa = new [] { 1, 2, 3, 4, 5 };
var ab = new [] { 1, 2, 3, 4, 5 };
for (int i = 0; i < aa.length; ++i)
{
    aa[i] = ab[i] + 1;
}

Can I convert this to use just one Array.ForEach? Or, how would you do it, if you wanna go all functional programming crazy? Clunky for loops just looks ugly.

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3
  • I can't get your first ForEach code to compile. "new Action(a => a + 1)" doesn't make sense, since "a => a + 1" is an Action<int>, and "a + 1" isn't a statement. Commented Jan 28, 2009 at 4:12
  • I'm sorry, I'm at work now, so I'm not on .Net framework 3.5. I do know that using a delegate would work though, I not so sure if it can be changed to use a lambda expression instead... Commented Jan 28, 2009 at 4:17
  • "Or, how would you do it, if you wanna go all functional programming crazy?" -- Use F# :) Commented Feb 6, 2009 at 4:05

6 Answers 6

Reset to default
4

How about:

aa = ab.ConvertAll(x => x+1);
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5 Comments

Ah heck there's plenty of ways to do it if you're just gonna reassgn aa! I figured he wanted to do it in place.
Yeah, doing it with a destination array is "interesting". It might be fun to add an extension method which is like ConvertAll but which takes an existing destination array.
Good answer btw. But actually I simply made up a toy problem (Not the real one I was trying to solve) where I'm in a situation where I needed the index to refer to a few other arrays. But Thanks anyway :)
Do you actually need the index, or do you just need to get a slice into all the other arrays at the same place? If it's the latter, see stackoverflow.com/questions/496704
Great, I got just what I wanted in your first question of that link.
2

It's not pretty, but this will work (using the Select() overload that returns the index of the element):

var aa = new[] { 1, 2, 3, 4, 5 };
var ab = new[] { 1, 2, 3, 4, 5 };

Array.ForEach(
    aa.Select((x, i) => i).ToArray(),
    i => aa[i] = ab[i] + 1);

IMHO a simple "for" loops is a lot easier to understand in this case.

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2 Comments

I can't seems to find the Select overload, is this part of Linq, or is a custom written method?
Yeah just part of Linq. Select can take a function which passes the index (an int) as the second parameter.
2

Another similar approach:

Enumerable.Range(0, ab.Length).Select(i => aa[i] = ab[i] + 1);
// Will not work if aa.Length != ab.Length
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Comments

1

This won't be fast, but it just might work

var aa = new [] { 1, 2, 3, 4, 5 };
var ab = new [] { 1, 2, 3, 4, 5 };
Array.ForEach(aa, new Action(a => ab[Array.IndexOf(a)] + 1));

Unfortunately thiswill not work for an array with duplicate elements.

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1 Comment

Because Array.IndexOf is an O(n) operation inside a ForEach which is an O(n) operation as well.
0 
var aa = new[] { 1, 2, 3, 4, 5 };
var ab = new[] { 1, 2, 3, 4, 5 };

var index = 0;
Array.ForEach(aa, a => { aa[index] = ab[index] + 1; index++; });

Would this not work? Its 1 Array.ForEach with the indexer held outside of the Array.ForEach.

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-1

Not a direct answer, but you want ForEach2, which iterates over two sequences/arrays. Unfortunately this is not included with the BCL, so you'll have to add it.

But if you're trying to do functional in C#, you'll probably have a helper library anyways, so it's not that big of a deal.

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