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A higher-order function is a function that takes one or more functions as arguments or returns a function as its result. Here are some swift higher-order functions — forEach, map, CompactMap, flatMap, filter, reduce, sort, and sorted.

To understand functions and closures in Swift you really need to understand three things, in roughly this order of importance:
1. Functions can be assigned to variables and passed in and out of other functions as arguments, just as an Int or a String can be.
2. Functions can “capture” variables that exist outside of their local scope.
3. There are two ways of creating functions: with the func keyword, or with { }. Swift calls the latter “closure expressions”.
Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.
Closures can capture and store references to any constants and variables from the context in which they’re defined. This is known as closing over those constants and variables. Swift handles all of the memory management of capturing for you.

ForEach
forEach will iterate through all elements in an array and will not return anything.
“forEach” works like “for in” but the basic difference is, you can’t use break and
continue statement to exit from the closure for forEach.

mapmap will iterate through all elements in an array and will return an updated array.

compactMapcompactMap will iterate through all elements in an array and will return an updated array only with the elements which satisfied the condition written inside the body of compactMap. Any element which resulting in a nil value will be excluded from the updated array.

flatMapflatMap converts 2D array to one dimensional array.

filterfilter will iterate through all elements in an array and will return an updated array only with the elements which satisfied the condition written inside the body of filter.

reducereduce will iterate through all elements in an array and returns an object with a combined value of all elements.

Three types of sort functions are available in swift(5.4) :

1. sort() :When you don't want to store sorted array result in another array (only changes should be reflect in to same array).

2. sorted() : it will give a copy of fresh sorted array and existing array will be in original state always.

3. sorted(by:) : By this function you can sort ascending/descending both