Short Circuit Evaluation - F to C in Microsoft Access


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Short Circuit Evaluation

Maggie M

2 years ago

Thank you for all your great lessons; I am truly a better programmer thanks to you. When I learned to program using punched cards (remember them? LOL), we were taught to do IfThenElse by using the most common option a user would need 1st so it wouldn't have to evaluate the 2nd option to save processing cycles. In your example, you check for Null 1st so it always has to process that option. Shouldn't you write it as follows . . .
    If Not IsNull(TempF) Then
      FtoC = Round((TempF - 32) * 5 / 9, DecimalPlaces)
    Else
      FtoC = Null
    End If
. . . or doesn't it matter with today super-fast computers?

Sami Shamma @Reply

2 years ago

Hi Maggie,

I also did my early programming work on punch cards. The computers were so limited that even the programs were on punch cards. We needed to load the program, link it, compile it, run it, and take it out of memory before moving on to the next program. So, I know exactly what you're talking about.

Having said that, with today's computers, you can't measure the difference, not even in milliseconds. So, it's far better to write the "if-then-else" statement to make it more readable than to gain 1 millionth of a second in speed.

Richard Rost @Reply

2 years ago

What you're talking about is something called short circuit evaluation. Basically, the processor stops evaluating the rest of the statement as soon as it reaches something that allows it to be done. So, in your case, if you have mostly null values, you might want to check for those first. As far as saving processor cycles go, it doesn't matter for average everyday stuff. But, if you're running code against 100,000 records and you have to check each one of them, then yes, you might want to do some short circuit evaluation.

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