CODEWITHSUNDEEP
c#listfor-loopc#-4.0
User987
User987

Reputation: 3823

For loop with 20 items skip each time

I would like to write a piece of for loop which would go through an existing list and take 20 items out of that list each time it iterates.

So something like this:

I have written something like this:

var allResponses= new List<string>();
for (int i = 0; i < filteredList.Count(); i++)
{
    allResponses.Add(GetResponse(filteredList.Take(20).ToList()));
}

Where assuming filteredList is a list that contains 68 items. I figured that this is not a way to go because I don't want to loop to the collections size, but instead of 68 times, it should be 4 times and that I take 20 items out of the list each time... How could I do this?

Upvotes: 4

Views: 2514

Answers (4)

Mr.PoP
Mr.PoP

Reputation: 1

i needed to do same but i needed to skip 10 after each element so i wrote this simple code

List<Location2D> points = new List<Location2D>();
                points.ForEach(p =>
                {
                    points.AddRange(path.Skip((int)points.Count * 10).Take(1));
                });
                if (!points.Contains(path.LastOrDefault()))
                    points.Add(path.LastOrDefault());

Upvotes: 0

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726579

You are pretty close - just add a call to Skip, and divide Count by 20 with rounding up:

var allResponses= new List<string>();
for (int i = 0; i < (filteredList.Count+19) / 20; i++) {
    allResponses.Add(GetResponse(filteredList.Skip(i*20).Take(20).ToList()));
}

The "add 19, divide by 20" trick provides an idiomatic way of taking the "ceiling" of integer division, instead of the "floor".

Edit: Even better (Thanks to Thomas Ayoub)

var allResponses= new List<string>();
for (int i = 0 ; i < filteredList.Count ; i = i + 20) {
    allResponses.Add(GetResponse(filteredList.Skip(i).Take(20).ToList()));
}

Upvotes: 6

Dmitrii Bychenko
Dmitrii Bychenko

Reputation: 186688

I suggest a simple loop with page adding on 0, 20, 40... iterations without Linq and complex modular operations:

int pageSize = 20;

List<String> page = null;

for (int i = 0; i < filteredList.Count; ++i) {
  // if page reach pageSize, add a new one 
  if (i % pageSize == 0) {
    page = new List<String>(pageSize);   

    allResponses.Add(page); 
  }

  page.Add(filteredList[i]); 
}

Upvotes: 0

Patrick Hofman
Patrick Hofman

Reputation: 156978

You simply have to calculate the number of pages:

const in PAGE_SIZE = 20;
int pages = filteredList.Count() / PAGE_SIZE
            + (filteredList.Count() % PAGE_SIZE > 0 ? 1 : 0)
            ;

The last part makes sure that with 21, there will be added 1 page above the previously calculated page size (since 21/20 = 1).

Or, when using MoreLINQ, you could simply use a Batch call.

Upvotes: 2

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