Calculation help

Beavis3215

What is the best way to execute this in C?

rpm =  1/(9 * (elapsed / 60,000,000));

elapsed is an integer number of microseconds of 3,000 to 25,000

Dave Hein

rpm = 60000000/(9*elapsed);

Beavis3215

Thanks again Dave, never made the connection.

jmg

or even
rpm = 6666667/elapsed;

kwinn

Always good programming practice to calculate the result of constants or variables that do not change within a loop before entering that loop. Makes for faster code.

Mark_T

Though a good compiler is often better at doing this than you are, if allowed to...

pmrobert

Mark_T wrote: »

Though a good compiler is often better at doing this than you are, if allowed to...

True - I recently spent several hours attempting to speed up a couple of time sensitive procedures by simplifying calcs, etc. and the compiler (PropGCC) spit out pretty much the same assembly despite multiple approaches. One important concept is when scaling variables up or down to facilitate integer math using a power of 2 for the mults and divs is very, very fruitful when looking for speed. This has been mentioned multiple times over many years here but is worth repeating.

Mike R...