Financial: Savings plan calculation

[cortopar]

Pseudo Pascal-ish code for the brute force method...

Code:


EXPORT FindInterest(years,monthly,bonus,goal);

LOCAL interest:=0.001;
LOCAL increment:=0.002;
LOCAL i:=0;
LOCAL month:=0;
LOCAL balance:=0;

WHILE balance<goal DO
  balance:=0;
  FOR i:=1 to years DO
    FOR month:=1 to 12 DO
      balance:=balance+(balance * (interest / 12))+monthly;
    END;
  
    balance:=balance+bonus;
  END;

  interest:=interest+increment;
END;

RETURN interest;
END;

Now this is going to return the interest at which balance exceeds goal. To get it exact would need additional processing at finer precision.

I’m sure there’s a formulaic way to solve this easier, but it exceeds my math abilities.

Edited to Prime-compliant HP-PPL. Tested on my Prime. It is slow as you’d expect. It could be sped up by providing or calculating a starting guess and larger increment before refining.