CODEWITHSUNDEEP
c#lambda
Thomas
Thomas

Reputation: 34188

Why lambda here in the code?

i found a sample code where lambda was used the code as follows

var sumOfgreaterThanSeven = numbers.Sum(n => n > 7 ? n : 0);

but the above code can be written as 

var sumOfgreaterThanSeven = numbers.Sum(n > 7 ? n : 0);

so why user write lambda. please help me to understand why user write lambda here. also tell me what is the advantage of lambda. thanks

Upvotes: 2

Views: 379

Answers (3)

Bill Forney
Bill Forney

Reputation: 796

When you write a lambda you are calling a method or evaluating an expression. The left side of the => is a set of parameters passed into the method/expression on the right side.

IEnumerableThing.Where(a => DoSomeStuff(a))

It's like writing this:

foreach (var a in IEnumerableThing)
{
   DoSomeStuff(a);
}

In the case of the Sum() method you are really doing something like this:

int mysum = 0;

foreach (var n in whatever)
{
   if (n > 7) { mysum += n; }
}

Upvotes: 2

Marc Gravell
Marc Gravell

Reputation: 1062780

The lambda is because you want to evaluate the conditional expression per item n. The version you added (Sum(n > 7 ? n : 0)) can't work - n isn't defined anywhere (the compiler message should be "The name 'n' does not exist in the current context").

The lambda can be read as:

given a term n, if n is greater than 7 return n, else return 0

and then sum over that logic.

Re the advantage - firstly, convenience - but also composition. For example, with LINQ-to-SQL I would absolutely expect that to issue something like:

 select sum(case when row.col > 7 then row.col else 0 end)
 from sometable row

of course, it might be better to use:

 var sumOfgreaterThanSeven = numbers.Where(n => n > 7).Sum();

which would map to 

 select sum(row.col)
 from sometable row
 where row.col > 7

which might hit an index more accurately

Upvotes: 12

Aliostad
Aliostad

Reputation: 81660

You need to think of Lambda Expressions as methods. 

n => n > 7 ? n : 0

Can in fact be written as 

(n) => {
    if(n > 7)
        return n;
    else
        return 0;
    }

Lambda expression will be converted to an anonymous method and an instance of Func<> will be created from it.

UPDATE

As Marc pointed out, this conversion to anonymous method and instance of Func<> or Action does not always happen -as rightly pointed out in Linq-to-sql. ... - but here it does, hence I pointed out the underlying.

Upvotes: 2

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