Google Earth Engine中具有map（）函数的双循环

Question

In Google Earth Engine Developer's Guide , there is a recommendation to avoid for() loops. They recommend to use map() function as this example:

// to avoid
var clientList = [];
for(var i = 0; i < 8; i++) {
  clientList.push(i + 1);
}
print(clientList);

// to use
var serverList = ee.List.sequence(0, 7);
serverList = serverList.map(function(n) {
  return ee.Number(n).add(1);
});
print(serverList);

I'm trying to select MODIS scenes from each month/year prior to compute VCI. So, the approach I'd take is with a double loop:

modis = ee.ImageCollection("MODIS/MYD13A1");

var modis_list = [];
for(var i = 1; i <13; i++) {
  for(var j = 2000; j <2018; j++){
    modis_list.push(modis.filter(ee.Filter.calendarRange(i, i, 'month'))
                          .filter(ee.Filter.calendarRange(j, j, 'year')));
  }
}
print(modis_list);

Is there a way to replicate a double loop like this with map() function to reach a server-side approach?

Answer 1

Assuming that you are just trying to understand GEE's map() function, and how would be the equivalent of a normal js for loop , the code would be:

var map_m = function(i) {
  i = ee.Number(i)
  var years = ee.List.sequence(2000, 2017)
  var filtered_col = years.map(function(j) {
    var filtered = modis.filter(ee.Filter.calendarRange(i, i, 'month'))
                        .filter(ee.Filter.calendarRange(j, j, 'year'))
    return filtered
  })
  return filtered_col
}

var months = ee.List.sequence(1, 12)
var modis_list2 = months.map(map_m).flatten()

This code replicates a normal for loop . First, it goes item by item of the years list , and then item by item of the months list , and then, once you have year and month, filter the collection and add it to a list ( map does that automatically). As you use 2 map functions (one over years and the other over months), you get a list of lists, so to get a list of ImageCollection use the flatten() function. Somehow the printed objects are a bit different, but I am sure the result is the same.

Answer 2

The easy way to do this is with a single map over the "months" you care about.

// Collect images for each month, starting from 2000-01-01.
var months = ee.List.sequence(0, 18*12).map(function(n) {
  var start = ee.Date('2000-01-01').advance(n, 'month')
  var end = start.advance(1, 'month')
  return ee.ImageCollection("MODIS/MYD13A1").filterDate(start, end)
})
print(months.get(95))

This will return a list of ImageCollections. Most months will have only 1 image, since MYD13A1 contains 16-day images, but some will have two. Month 95 is Jan of 2008 and has two.

Alternatively, you could join the collection with a collection of dates, but this is simpler.

And you should prefer filterDate over calendarRange when possible, as it's optimized.

Answer 3

Let me start by saying I know nothing about Google Earth Engine and my info is from functional programming knowledge.

---

map is unique in that it doesn't generate the things it loops over. You start with a list and map iterates over each item in that list and transforms it. If you don't have that list then map isn't a great fit.

It looks like you are creating a list with each month/year combo represented. I would break this into a few steps. Build the month and year lists, build the list that represents cartesian product of the 2 lists then transform to the ee objects.

var range = (from, to) => new Array(end-start+1).fill(0).map((_,i)=>i+from)
var cartesianProduct = (a, b) => // not gonna do this here but it returns pairs [ [ a[1], b[1] ], ... ]
var getEE = ([month, year]) => modis
    .filter(ee.Filter.calendarRange(month, month, 'month'))
    .filter(ee.Filter.calendarRange(year, year, 'year'));

var months = range(1,12);
var years = range(2000, 2017);
var data = cartesianProduct(months, years)
    .map(getEE)

There are likely better ways(like not iterating throught the whole list of modis each time we want a single month object (use a dictionary)) but the gist is the same.