accumulator textitem in data »

[Hassan]

how can I make textitem as an accumulator. how to use function sum in itemtext in data not in footer

[Arin Alex]

if you need to make some calculations within one record, you can simply add the values in the script something like this $S{$D{datasource.field1}+$D{datasource.field2}}

[davidovv]

I think the question was about running sum, i cant find answer on rest of forum, this question is closest and i didn't want to start a new topic.
if you have a datasource that has for example four rows with some value 10, 10, 100, 1, next to value at each row i would like a running sum. *the values for the example would be 10, 20, 120, 121. For each row accumulated values of previous rows.
How to achieve that?