Functional Programming Concepts

Introduction to functional programming

Earth Engine uses a parallel processing system to carry out computation across a large number of machines. To enable such processing, Earth Engine takes advantage of standard techniques commonly used by functional languages, such as referential transparency and lazy evaluation, for significant optimization and efficiency gains.

The main concept that sets functional programming apart from procedural programming is the absence of side effects . What it means is that the functions that you write doesn’t rely on or update data that is outside of the function. As you will see in the examples below, it is possible to re-structure your problem so that it can be solved using functions without side-effects - which are much better suited to be executed in parallel.

For Loops

The use of for-loops is discouraged in Earth Engine. The same results can be achieved using a map() operation where you specify a function that can be independently applied to each element. This allows the system to distribute the processing to different machines.

The example below illustrates how you would take a list of numbers and create another list with the squares of each number using map() :

Code Editor (JavaScript)

 // This generates a list of numbers from 1 to 10. 
 var 
  
 myList 
  
 = 
  
 ee 
 . 
 List 
 . 
 sequence 
 ( 
 1 
 , 
  
 10 
 ); 
 // The map() operation takes a function that works on each element independently 
 // and returns a value. You define a function that can be applied to the input. 
 var 
  
 computeSquares 
  
 = 
  
 function 
 ( 
 number 
 ) 
  
 { 
  
 // We define the operation using the EE API. 
  
 return 
  
 ee 
 . 
 Number 
 ( 
 number 
 ). 
 pow 
 ( 
 2 
 ); 
 }; 
 // Apply your function to each item in the list by using the map() function. 
 var 
  
 squares 
  
 = 
  
 myList 
 . 
 map 
 ( 
 computeSquares 
 ); 
 print 
 ( 
 squares 
 ); 
  
 // [1, 4, 9, 16, 25, 36, 49, 64, 81]