Start a loop from information in a feature property

Question

I have a feature collection of polygons that contains a property ("yrbegun") indicating the year I want to use to do some manipulation. I would like to start a loop from the year before to the year after, i.e. if the property indicates 2005, I want to loop from 2004 to 2006

// Determine the year the project was started 
var projectyear = ee.Number(inFeature.get("yrbegun"))

// Define the "boundaries" of the loop
var startyear = projectyear.subtract(1)
var endyear = projectyear.add(1)

// Run the loop
for (var yr= projectyear; yr <= endyear; yr = yr + 1) {

  print('This is year ' + yr)
  print(yr)

}

However, the loop never runs. I tried to define projectyear without the ee.Number() function, but it also does not work.

Answer 1

You have to read and understand this before you start coding in GEE: https://developers.google.com/earth-engine/client_server

You are trying to execute a client side loop using a server side object (ee.Number). That is impossible.

You have 3 options: (I set projectyear to 2000 just to illustrate)

client side:

var projectyear = 2000
var startyear = projectyear - 1
var endyear = projectyear + 1

// Run the loop
for (var yr= projectyear; yr <= endyear; yr = yr + 1) {

  print('This is year ' + yr)
  print(yr)
}

server side: (can't print because it is a client side function)

var projectyear = ee.Number(2000)

// Define the "boundaries" of the loop
var startyear = projectyear.subtract(1)
var endyear = projectyear.add(1)

var seq = ee.List.sequence(startyear, endyear)

var result = seq.map(somefunction)

mixed (not recommended):

// Determine the year the project was started
var projectyear = ee.Number(2000)

// Define the "boundaries" of the loop
var startyear = projectyear.subtract(1)
var endyear = projectyear.add(1)


// Run the loop
for (var yr= projectyear.getInfo(); yr <= endyear.getInfo(); yr = yr + 1) {

  print('This is year ' + yr)
  print(yr)

}

Answer 2

Client-side looping is rarely the right approach when using Earth Engine, but it is usually possible to get equivalent results without loops. The following example creates a feature property that contains a range of years without client-side looping:

var myFeatures = ee.FeatureCollection([
  ee.Feature(ee.Geometry.Polygon([[[-108.9, 42.7],
               [-108.9, 42.6],
               [-108.8, 42.6]]]), {name:'poly1', yrbegun: 2014}),
  ee.Feature(ee.Geometry.Polygon([[[-108.7, 42.5],
               [-108.7, 42.4],
               [-108.6, 42.4]]]), {name:'poly2', yrbegun: 2015})  
]);

var addYearRange = function(feat) {
  var projectyear = ee.Date.fromYMD(
    ee.Feature(feat).get('yrbegun'), 1, 1
  );
  // Add a year range property.
  feat = feat.set('year_range', ee.List([
    projectyear.advance(-1, 'year'),
    projectyear,
    projectyear.advance(1, 'year')  
  ]));
  return feat;
};
var myFeaturesWithYearRange = myFeatures.map(addYearRange);

print('myFeatures', myFeatures);
print('myFeaturesWithYearRange', myFeaturesWithYearRange);

Similarly, if you want to create properties based on image statistics for each polygon (as indicated by your comment), you can add additional properties to the polygon:

var images = ee.ImageCollection('LANDSAT/LC8_SR');

feat = feat.set('mean_year_before',
  ee.Number(
      images.filterDate({
      start: projectyear.advance(-1, 'year'),
      end: projectyear
    }).mean()
      .reduceRegion({
        reducer: 'mean',
        geometry:feat.geometry(),
        scale:30,
        bestEffort: true
      }).get('B4')
  )
);