scala - Why do one of these split & filter work but not the other? - 


val c = s.split(" ").filter(_.startswith("#")).filter(x =>     x.contains("worcester") || x.contains("energy"))

works

but not

val c = s.split(" ").filter(_.startswith("#")).filter(_.contains("worcester") ||    _.contains("energy"))

i have not understood why latter not work - may have flaw in fundamentals

any appreciated sumit

using underscore known placeholder syntax. so, _.contains("x") equivalent x => x.contains("x"). can use each parameter once when using placeholder syntax. using multiple placeholders denotes multiple parameters of anonymous function (which used in order of underscores). so, when write:

o.filter(_.contains("worcester") || _.contains("energy"))

it theoretically equivalent to

o.filter((x, y) => x.contains("worcester") || y.contains("energy"))

which doesn't type check, since filter expects parameter of type array[string] => boolean.

multiple placeholders common when using reduce variants. example, factorial can computed (1 n).reduce(_ * _). works because reduce expects parameter of type (int, int) => int, _ * _, equivalent (x, y) => x * y fits expected type.

it's important note placeholder syntax applies smallest possible scope. so, example, f(g(_)) equivalent f(x => g(x)), , not x => f(g(x)), common mistake.

you can find comprehensive list of use of underscore in scala here, , little bit more placeholder syntax here.


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