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What's an efficient way to do exponentials of numbers with decimals, such as when compounding interest rate calculations?

For example: 1.001 ** 69

As of now, I naively implement it like this:

uint mantissa = 1e3;
uint rate = 1;
uint periods = 69;
uint out = mantissa;
for(uint i=0; i < periods; i++){
  out = out * (mantissa + rate);
  out = out / mantissa;
}

My worry is if the number of periods get big then I'm running a pretty inefficient loop. Anyone have any better ideas? Are there any libraries that do this implementation for me? Thanks!

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  • Can your number of period vary or is it constant ?
    – hroussille
    Nov 30, 2021 at 7:39

1 Answer 1

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Just do not store the accumulated interest on the main account, but store them separately.

Or use a simplified formula for small percentages: (1+x)^y = 1+x*y.

For values x=0.001 and y=100, the true value will be 1.105, and the simplified 1.100

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