How to obtain percentiles of classes in classified image in Google Earth Engine?

Question

https://code.earthengine.google.com/e430e2a79aecb65e3efbce23779a0abc

I have written this code for supervised classification using the cart classifier. Now I want to extract the no. of pixels in each class i.e. water, urban, vegetation etc.

var rgb_vis = {min: 0, max: 0.3, bands: ['B4', 'B3', 'B2']};
var image =  ee.ImageCollection('LANDSAT/LT05/C01/T1_TOA')
  .filterBounds(ganga)
  .filterDate('2009-10-01', '2009-10-31')
  .median();
var region = image.clipToCollection(ganga);
// Map.addLayer(region, rgb_vis,'map');
//Data
// print(region);
var fc = water.merge(vegetation).merge(urban).merge(fallow);
var bands = ['B2','B3','B4','B5','B6','B7'];
//Train
var training = region.select(bands).sampleRegions({
  collection:fc,properties: ['landcover'],scale : 30});
var classifier = ee.Classifier.cart().train({features: training, classProperty: 'landcover', inputProperties: bands});
//Classification
var classified = region.select(bands).classify(classifier);
// print(classified);
//Display
Map.centerObject(fc, 11);
// Map.addLayer(region, rgb_vis, 'Landsat image');
// Map.addLayer(classified, {min: 0, max: 3, palette: ['0FD1F8','0E8205', 'EAF10B','EE0BF1']}, 'classification');
var output = classified.visualize({min:0,max:3,palette: ['0FD1F8','0E8205', 'EAF10B','EE0BF1'],forceRgbOutput:true});
Export.image.toDrive({image:output, region:geometry,description:'2009', scale: 30,crs:'EPSG:4326'});
Map.addLayer(output)

Answer 1

You can calculate the number of pixels per class by applying the frequencyHistogram reducer to your study region:

https://code.earthengine.google.com/f8e3d94f3b1af7f859f8203f135b3f0a

// calculate number of pixels per class
var classCount = classified.reduceRegion({
  reducer: ee.Reducer.frequencyHistogram(),
  geometry: ganga,
  scale: 30,
  maxPixels: 10e9})