It’s the simplest things I always forget. Hopefully I’ll remember this article next time. When traveling to a conference I record and categorize my expenses in a Google Docs spreadsheet, usually creating a new worksheet for each trip. I have a separate worksheet which contains a cumulative summary of each category of expense. This is the formula I use to keep the running total for specific categories.
=SUM(FILTER('Worsheet name'!D2:D; 'Worksheet name'!E2:E="food"))
The SUM function is self-explanatory. The FILTER function selects all cells from a given range which match particular conditions. The range of cells to choose from is the first argument. Subsequent arguments describe the conditions. In the above example we select all cells from column D from row 2 onwards for which the value of the corresponding cell in column E equals “food”.
UPDATE: As is so often the case when I learn something, there is a better way.