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Can anyone help me to find out what is wrong with this code?

func double(_ i: Int) -> Int { return i * 2 }

let doubler = double let doubledValues = [1,2,3,4].map(doubler)

let doubledValues2 = [1,2,3,4].map( {(_ i:Int) -> Int in return i * 2 }) let doubledValues3 = [1,2,3,4].map({_ i in return i * 2 }) let doubledValues4 = [1,2,3,4].map({_ i in i * 2}) let doubledValues5 = [1,2,3,4].map({\$0 * 2}) let doubledValues7 = [1,2,3,4].map { \$0 * 2 }

let doubledValue = [1,2,3,4].map() {\$0 * 2}

closures.swift
func double(_ i: Int) -> Int {
return i * 2
}

let doubler = double
let doubledValues = [1,2,3,4].map(doubler)

let doubledValues2 = [1,2,3,4].map( {(_ i:Int) -> Int in return i * 2 })
let doubledValues3 = [1,2,3,4].map({_ i in return i * 2 })
let doubledValues4 = [1,2,3,4].map({_ i in i * 2})
let doubledValues5 = [1,2,3,4].map({\$0 * 2})
let doubledValues7 = [1,2,3,4].map { \$0 * 2 }

let doubledValue = [1,2,3,4].map() {\$0 * 2}

I think you need to delete the opening and closing parenthesis () after .map . In swift, .map just takes a closure like this .map{} not like this .map({}).

Also, you don't need the external argument name in a closure. So you can get rid of these "_" in doubledValues3 and doubledValues4 then it should work.

func double(_ i: Int) -> Int {
return i * 2
}

let doubler = double
let doubledValues = [1,2,3,4].map(doubler)

let doubledValues2 = [1,2,3,4].map { (_ i:Int) -> Int in return i * 2 }
let doubledValues3 = [1,2,3,4].map { i in return i * 2 }
let doubledValues4 = [1,2,3,4].map { i in i * 2 }
let doubledValues5 = [1,2,3,4].map { \$0 * 2 }
let doubledValues7 = [1,2,3,4].map { \$0 * 2 }

let doubledValue = [1,2,3,4].map { \$0 * 2 }