Append element to an array in Google Earth Engine

Question

I am trying to write a for loop to append elements dynamically to an array. However, the dimensions of the array change on every loop run, throwing off error.

var arr = 1
for (var i=0;i<4;++i){
  var newval = somefunction()
  arr = ee.Array.cat([newvar,arr],0)
}
print(arr)

Error: Array.cat: Arrays must have same dimensionality. Expected 0, found 1 for array 1.

EDIT: The new value to be appended is coming out of a for loop, so I can't use a list.map or iterate function

Answer 1

Your code is returning "unexpected" results because you are mixing client-side Javascript control structures with server-side Earth Engine methods. In general, you should avoid doing this. Review the Earth Engine Client vs. Server docs page for a complete discussion.

For your simple example, you can use an ee.List() object to accumulate values, and later convert it to an ee.Array():

var first = ee.List([1]);

var list = ee.List.sequence(0, 4);
var iter_function = function (num, list){
  num = ee.Number(num);
  var new_value = num.multiply(num);
  return ee.List(list).add(new_value);
};

var resulting_list = list.iterate(iter_function, first);
print('resulting_list', resulting_list);

var resulting_array = ee.Array(resulting_list);
print('resulting_array', resulting_array);

However, if your actual use case is too complex for ee.List() objects, you can iterate using ee.Array() objects:

var first = ee.Array([1]);

var list = ee.List.sequence(0, 4);
var iter_function = function (num, list){
  num = ee.Number(num);
  var new_value = ee.Array([num.multiply(num)]);
  return ee.Array.cat([list, new_value]);
};

var resulting_array = list.iterate(iter_function, first);
print('resulting_array', resulting_array);

If processing each element in your list is independent of the others, you can map your function across this list using ee.List.map() instead:

var list = ee.List.sequence(0, 4);
var map_function = function (num){
  num = ee.Number(num);
  var new_value = num.multiply(num);
  return new_value;
};

var resulting_list = list.map(map_function);
print('resulting_list', resulting_list);

var resulting_array = ee.Array(resulting_list);
print('resulting_array', resulting_array);

The moral of this story is: "Friends don't let friends use loops with big data."