One issue that recently reared it's head again at Sprezz Towers Support Central was the problem of using the Basic+ Rnd() function to generate a random number sequence - apparently the same series of numbers was being produced every time.

On first investigation it appeared that the customer was not using the InitRnd() function to seed the Rnd() generator before asking for the random numbers. This is a documented requirement - Calling InitRnd() is an absolute necessity before using the Rnd() function.

Even so, using InitRnd() as specified in the online help failed to produce sufficiently random results so we decided to take a closer look at the process to see what the problem was and how we could improve it.

Under the hood the Basic+ InitRnd() and Rnd() functions are little more than thin wrappers around the srand() and rand() functions in the Windows C-runtime library. The rand() function actually produces a pseudorandom sequence of numbers, so it is not truly random at all - instead the srand() function is used to seed the starting point for the pseudorandom sequence so it begins in a different place after each initialisation. Also, if srand() is called again with the same seed the same sequence of numbers is generated by rand()!

Going back to the problem, the OpenInsight online help gives an example of using InitRnd() with a seed based on the current time and date (which is what our customer tried next):


Unfortunately for this example using the date results in a seed value that doesn't show much variance in it's low order value (it only increments by one per day), and this appears to have the effect of generating a identical Rnd() sequence between each initialisation (especially if called multiple times on the same day which is something that should be avoided). We then thought about swapping the date and time around so that the low order value would hopefully be more varied, but here we find ourselves in danger of generating predictable sequences if we started our application at the same time every day (like at 9:15am for example).

What we needed was some way to fulfil the following criteria:

• Ensure that InitRnd() is only called once per session to avoid the same seed being used more than once.
• Provide InitRnd with a varied seed value that is less predictable and has a greater "spread" than a simple date/time value.

We quickly put together the following function to do this that gave much better results:


(Note that we have a global variable in place to ensure that initRnd is only called once per session, unless we explicitly override this)

You can download a text version of zz_InitRnd here.

Disclaimer

Labels: Basic+, OpenInsight

Following on from our recent post on how to remove the row-selection from an EditTable we came across a related issue last week that we think you might like to know about.

Basically we designed a form with several EditTables, all row-select enabled, but during testing it became difficult to know where the actual input focus was as so many controls had a selection highlighted. We could have used the technique in the aforementioned blog post to clear the row-selection during the LOSTFOCUS event, but in this case we still needed to see which row was selected even though the focus was on another control.

The solution we used was to change the highlight color of the EditTable during the LOSTFOCUS event, toning it down to a lighter shade than normal. We then reset it during the GOTFOCUS event. This is actually quite easy to achieve with the COLOR_BY_POS message, the only real challenge is to devise an algorithm to fade the highlight color.

As usual we've saved you the trouble - here's small function that you can use as a starting point for your own application should you wish to use a similar technique:



In your EditTable LOSTFOCUS event you call this:




And to reset the colors in your EditTable GOTFOCUS event you call this:




(You'll notice that the function uses two "WinAPI" $insert records, both of which can be found the WinAPI Library that we recently posted.)

You can download a text version of edt_FadeSelection here.

Disclaimer

Labels: EditTable, EditTable Cookbook, OpenInsight

There's a quick way to select or deselect all rows in a multi-select edit table, and that's via the DTM_SELALLROWS message. It takes a single wParam argument which is TRUE$ to select all rows, or FALSE$ to deselect them.

Here's an example:

Labels: EditTable, EditTable Cookbook, OpenInsight