Swift commonly used higher-order functions

var arr = [1, 2, 3]
//The map function has a return value. If you want the value in arr, map in the past, you need arr to receive a new value again. { (a : Int) -> Int in
  return a * 2
}
//This writing is just the abbreviation for trailing closures.  .arr =  {
  $0 * 2
}

//FloatMap function can reduce dimensionalityvar arr1 = [[1, 2], [4, 5], [6, 7]]
var aaa =  {
  $0
}
//The floatMap function removes the empty function and will also unpack the datavar arr2 = ["swift", nil, "java"]

var bbb =  {
  $0
}

//Filter function is used to filter elements in containervar arr3 = ["swift", "java", "oc", "python"]
arr3 =  {
  guard $ > 2 else {
    return false
  }
  return true
}

//Reduce function is used to operate on elements in containers and splice element stringsvar arr4 = [2, 3, 4, 5]
//("", {
//  return $0 == "" ? String($1) : $0 + String($1)
//})
//Sum with reduce(0) {
  return $0 + $1
}
// Stitch each string in the array with ','let arr5 = ["Objective-C", "Swift", "HTML", "CSS", "JavaScript"]

// $0 represents the calculated result, $1 represents each element in the array("", {
  return $0 == "" ? $1 : $0 + "," + $1
})