Perhaps it would be easier to look at this problem in terms of present value of investments. Ask yourself, "I will invest $500 three years from now. What would I have to invest today to be worth $500 three years from now?" Taking the liberty to add a year to the interest and investments: m=:1000 0 0 500 0 0 NB. m for money i=.0 0.05 0.04 0.06 0.05 0.05
]g=:*/\>:i NB. Calculate growth of an investment over the years 1 1.05 1.092 1.15752 1.2154 1.27617 m%g NB. Convert investments to present value 1000 0 0 431.958 0 0 NB. What would I have to invest today to be worth $500 in 3 years? NB. $431.958 +/\m%g NB. Accumulate investments at present values 1000 1000 1000 1431.96 1431.96 1431.96 g*+/\m%g NB. Convert to future value 1000 1050 1092 1657.52 1740.4 1827.42 c=: 13 : 'g*+/\y%g=.*/\>:x' NB. Putting this together i c m 1000 1050 1092 1657.52 1740.4 1827.42 5!:5<'c' ] (] * [: +/\ %) [: */\ [: >: [ There is probably a neater tacit expression, but to me looking at the problem in terms of present value makes so much clearer the thought process. Okay, so I cheated a little to get a tacit expression! ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
