Ruby on Rails

If you want to have a 'quick' solution to the average problem why not
do it like this:

a table like:
id | number_of_days | total_cars

At the end of every day you add +1 to the number of days column and
add the number of cars for that day. So as an example after two weeks
it would look like this:

id | number_of_days | total_cars
1 | 14 | 43832

So if you wanted an average of the last 7 days, pull record 1 and
then:
43832 / 14 * 7 = 21916

Its statistically not quite true because on some days you will
inevitably have more traffic than on other days but its certainly the
quickest solution without having to have a row for ever day. (if you
have 3000 streets after 10 days you have 30000 rows...)

Of course you can also keep a record for every day and make a table
like this so you dont always have to fetch all the records just for a
quick and dirty solution.

Another idea is keeping a record for every day for every street like
this:

id | street_id | datetime | cars | total_weekly_cars |
1 | 12 | 5-21-2011 (rapture) | 244 | 5444 |

where the total_weekly_cars are all the cars of that week accumulated.
And then on Monday it resets to 0. This way you could keep a weekly
average as well as a daily number of cars and only fetch 1 record for
7 days. or 4 records for 1 month. 48 records for 1 year. And it means
you have accurate data for every day if you desire.

Just to give you an idea how I could approach this. (I'm not a
professional programmer nor am I a mathematician. I'm 'just' and
engineer ;) )

Regards
Stefano

On May 30, 7:19 am, Colin Law <clan...@googlemail.com> wrote:
> On 30 May 2011 04:51, Matt Harrison <iwasinnamuk...@genestate.com> wrote:
>
> > I'm working on a community app that allows the residents of a village to log and
> > monitor the traffic going past their houses.
>
> > There are multiple locations which have many log entries with a traffic count and a
> > time block.
>
> > I want to let users view a location and see the average vehicles per day, as well as
> > the average for a given month.
>
> > I'm thinking it won't be very efficient to run though every log entry and calculate a
> > running average every time a user views the statistics page. I've never dealt with
> > data in this manner before so I'm not sure the best practice. I suppose I could
> > maintain a table with a record for each location. On creation or modification of a log
> > entry I could calculate the average and store it there for viewing. Again, not sure if
> > I'm heading the right direction.
>
> Rails has several schemes for handling this sort of problem using
> caching.  Have a good look at the Rails Guide on caching.  It is quite
> complex so make sure you understand how the various methods work in
> order to work out which one is right for your particular problem.
> Come back and ask again if you are still not sure after understanding
> the guide.
>
>
>
> > The second part is calculating average for a give month, or indeed a year. I could
> > present the appropriate year/month select box but how can I pull records that only
> > relate to the given options?
>
> > I could probably pull every single record and compare the entry date to the given
> > parameters, then act on it if it fits the time period. Once more I'm thinking this
> > isn't the most efficient way to proceed.
>
> Asssuming your records have a column called measured_at, for example,
> then you can query with comparison operators for these.  So you can
> fetch records in a time range using something like the following in
> the query
> :conditions => ['measured_at >= ? and measured_at < ?', start_time, end_time]
> where start_time and end_time are derived from the values from the form.
>
> Colin

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