LINQ where vs takewhile

Question

I want to get a difference between TakeWhile & Where LINQ methods .I got the following data from MSDN .But It didn't make sense to me

Where<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>) 
Filters a sequence of values based on a predicate.

TakeWhile<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>)
Returns elements from a sequence as long as a specified condition is true.

All opinions are welcomed.

Good question - the Intellisense on TakeWhile still says "Returns elements ... as long as a specified condition is true". This could very well be interpreted to be the same as a Where. The wording should rather be something like "Returns elements ... until the condition evaluates as false". — Peter, Jul 17 '18 at 14:55

score 201 · Accepted Answer · edited Sep 04 '20 at 21:46

TakeWhile stops when the condition is false, Where continues and find all elements matching the condition

var intList = new int[] { 1, 2, 3, 4, 5, -1, -2 };
Console.WriteLine("Where");
foreach (var i in intList.Where(x => x <= 3))
    Console.WriteLine(i);
Console.WriteLine("TakeWhile");
foreach (var i in intList.TakeWhile(x => x <= 3))
    Console.WriteLine(i);

Gives

Where
1
2
3
-1
-2
TakeWhile
1
2
3

score 39 · Answer 2 · edited Sep 04 '20 at 21:47

39

Where can examine the whole sequence looking for matches.

Enumerable.Range(1, 10).Where(x => x % 2 == 1)
// 1, 3, 5, 7, 9

TakeWhile stops looking when it encounters the first non-match.

Enumerable.Range(1, 10).TakeWhile(x => x % 2 == 1)
// 1

edited Sep 04 '20 at 21:47

Neuron

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answered Feb 17 '11 at 16:35

Amy B

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short and sweet, perfect answer – Neuron Sep 04 '20 at 21:48

score 10 · Answer 3 · answered Feb 17 '11 at 16:39

10

Say you have an array that contains [1, 3, 5, 7, 9, 0, 2, 4, 6, 8]. Now:

var whereTest = array.Where(i => i <= 5); will return [1, 3, 5, 0, 2, 4].

var whileTest = array.TakeWhile(i => i <= 5); will return [1, 3, 5].

answered Feb 17 '11 at 16:39

Jim Mischel

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I think whileTest will return only 1 depends to @David B and @Albin Sunnanbo answers – Mohammed Thabet Feb 17 '11 at 17:42
No, it will return items until the condition isn't met. In this case, 1, 3, and 5 meet the condition (they are <= 5). – Jim Mischel Feb 17 '11 at 18:05

score 10 · Answer 4 · answered Feb 17 '11 at 16:44

MSDN says

Enumerable.TakeWhile Method

Returns elements from a sequence as long as a specified condition is true, and then skips the remaining elements.

Enumerable.Where

Filters a sequence of values based on a predicate.

The difference is that Enumerable.TakeWhile skips the remaining elements from the first non-match whether they match the condition or not

score 8 · Answer 5 · answered Oct 29 '15 at 15:21

While the existing answers are correct, none of them point out why you'd want to use TakeWhile if the results would be the same: Performance. Suppose you have an ordered list with 2 billion items in it, and you want the ones that (probably 10 or 15 items) less than a given vallue. The Where clause will examine all 2 billion items, while the TakeWhile will stop as soon as it finds a value equal or greater than your supplied value

StuartLC · Answer 6 · 2015-06-23T06:57:22.713

The order of the sequence passed is absolutely critical with TakeWhile, which will terminate as soon as a predicate returns false, whereas Where will continue to evaluate the sequence beyond the first false value.

A common usage for TakeWhile is during the lazy evaluation of large, expensive, or even infinite enumerables where you may have additional knowledge about the ordering of the sequence.

e.g. Given the sequence:

IEnumerable<BigInteger> InfiniteSequence()
{
    BigInteger sequence = 0;
    while (true)
    {
        yield return sequence++;
    }
}

A .Where will result in an infinite loop trying to evaluate part of the enumerable:

var result = InfiniteSequence()
    .Where(n => n < 100)
    .Count();

Whereas a .TakeWhile, and armed with the knowledge that the enumerables is ascending, will allow the partial sequence to be evaluated:

var result = InfiniteSequence()
    .TakeWhile(n => n < 100)
    .Count();

Indeed. It's also really useful for parsing a sequence of tokens. — Aluan Haddad, Sep 04 '20 at 21:50

LINQ where vs takewhile

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