This document discusses Kotlin's built-in higher-order functions including minByOrNull, maxByOrNull, sumBy, sumByDouble, filter, map, forEach, groupBy, and fold. It provides examples of how to use these functions to find the oldest child, calculate total donations, filter lists, transform collections, and more. The document also covers concepts like function call chaining and closures as it relates to functional programming in Kotlin.

1. Built-in
higher-order functions
Kotlin Study Group #3
Andy Lu
2021.06.17

2. Who am I
Andy Lu
Android Engineer in FutureDial
第二屆 Kotlin 讀書會導讀者
❤ Kotlin
❤ Flutter
Medium: https://medium.com/@andyludeveloper
2

3. Agenda
● Recap higher-order fuction
● Built-in higher-order functions
● Functional programming
● Q&A
3

4. Recap higher-order function
4

5. What’s higher-order function?
5

6. What’s higher-order function?
6
Function as
arguments
(one or more)

7. What’s higher-order function?
7
Function as
arguments
(one or more)
Returns
function as a
results.

8. Why higher-order function?
8

9. Why higher-order function?
9
No temporary variables

10. Why higher-order function?
10
No temporary variables
Function call chaining

11. Why higher-order function?
11
No temporary variables
Function call chaining
Higher level of abstraction

12. How to use higher-order function?
Ex: { a, b -> a+b }
● { }
● Parameters (a, b)
● Body (after ->)
12

13. Built-in higher-order functions
13

14. minBy, maxBy
14

15. minByOrNull
15

16. maxByOrNull
16

17. Thinking
How to find the oldest child?
17

18. Use maxByOrNull
18

19. sumBy, sumByDouble
19

20. sumBy
20

21. sumByDouble
21

22. Thinking
22
How to calculate all the donate?

23. Use sumBy
23

24. Use for-loop
24
Temporary variable

25. filter
25

26. filter
26
Predicate
Filtered list
● Filtered new list

27. filter
27
[“three”, “four”]

28. Family of filter
● filter
● filterTo
● filterNot
● filterIndexed
● ...
28

29. map
29

30. map
30
● Collection transformation.
● Build new collection
For each subsequent element

31. map
31
[6, 12, 18, 24]

32. Family of map
● map
● mapNotNull
● mapKeys
● mapValues
● mapIndexed
32

33. forEach
33

34. forEach
34
● for-loop like
● Support function call chaining
● No return value

35. forEach - V.S for-loop
35

36. Function call chaining
36

37. Function call chaining
37

38. Closure
38

39. groupBy
39

40. groupBy
40
● Grouping collection elements

41. groupBy
{3=[one, two], 5=[three], 4=[four]}
41

42. fold
42

43. fold
43
● Operator and accmulate result

44. fold
20
44

45. Functional Programming
45

46. Three types of functions
46
Transform
map zip
flatten ...
Filter
filter
partition
...
Combine
fold
reduce
max
sum
...

47. Q & A
47