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.

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Labels: Basic+, OpenInsight