aggregate columns

[vera-atomic]

Trying to make sense of how I could use expressions to create a custom aggregate cumulative sum column for each of my split_by pivot columns.
I've seen the issue opened here: #2624
And I tried with an example from the library that doesn't have live data:
https://perspective.finos.org/block/?example=editable

This is the starting config object:

{
  "version": "3.4.0",
  "plugin": "Datagrid",
  "plugin_config": {
    "columns": {},
    "edit_mode": "EDIT",
    "scroll_lock": false
  },
  "columns_config": {},
  "settings": true,
  "theme": "Pro Light",
  "title": null,
  "group_by": [
    "Segment"
  ],
  "split_by": [
    "Ship Date"
  ],
  "columns": [
    "Quantity"
  ],
  "filter": [],
  "sort": [],
  "expressions": {},
  "aggregates": {}
}

Then I tried adding the code you've suggested in issue #2624
But adapting it to this example

var cum_quantity := 0;
for (var idx := 0; idx < index(); idx += 1) {
    cum_quantity += vlookup('Quantity', idx);
};

cum_quantity

here's the config post setting up the new column:

{
  "version": "3.4.0",
  "plugin": "Datagrid",
  "plugin_config": {
    "columns": {},
    "edit_mode": "EDIT",
    "scroll_lock": false
  },
  "columns_config": {},
  "settings": true,
  "theme": "Pro Light",
  "title": null,
  "group_by": [
    "Segment"
  ],
  "split_by": [
    "Ship Date"
  ],
  "columns": [
    "Quantity",
    "New Column 1"
  ],
  "filter": [],
  "sort": [],
  "expressions": {
    "New Column 1": "var cum_quantity := 0;\nfor (var idx := 0; idx < index(); idx += 1) {\n    cum_quantity += vlookup('Quantity', idx);\n};\n\ncum_quantity"
  },
  "aggregates": {}
}

But the new column is showing 0 everywhere instead of cumulative sum.
I would have expected the totals row, for first split by and new column 1 to show 5, then for second split by column and new column 1 to show 5 + 11 = 16 etc for subrows too

What am I doing wrong here please?
TIA!

[texodus]

Your expression does not work because the "editable" example does not set an index, and the implicit index type is "integer" (#2624 talks about this distinction in the paragraph following the code snippet you copied):

var cum_quantity := 0;
for (var idx := 0; integer(idx) < index(); idx += 1) {
    cum_quantity +=  vlookup('Quantity', integer(idx)); 
};

cum_quantity

Note:

1. The superstore example has 10k rows in it so this expression conservatively evaluates this expression ~50,000,000 times.
2. Expressions are calculated before aggregates.