# Graphing the compound interest formula correctly

To preface, I'm very, very new to Mathematica. I've written a few functions to calculate compound dividends from stocks.

compoundDivsInterest[shares_, divyield_, years_, compoundrate_] :=
((shares (1 + (divyield/compoundrate))^(compoundrate*years) - shares))

compoundDivsTotal[shares_, divyield_, years_,compoundrate_] :=
((shares (1 + (compoundrate))^(compoundrate*years)))


TL;DR: I can't seem to get these functions to graph their correct exponential form. I've tried using a sum, using Show[Table[Plot[..., For[...Plot[..., and a "recursive" function. None have worked so far. What should I do to get them to plot correctly?

As they are now, they will give the right value for a given x, but any other values are incorrect (as the function doesn't technically compound off the new principal). If I graph the functions, they are linear in nature.

A friend better versed in Mathematica than I suggested I use a sum to graph them. Plot[Sum[compoundDivsTotal[x, 0.0659, 1, 4], {x, 1, n, 1}],{n, 0,1000}]

This does yield an exponential curve as would be expected, but none of the values are correct.

In order to plot the correct, exponential graph of compounded interest, I figured it would probably have to be done recursively or iteratively. I've tried many different ways from google, such as Show[Table[Plot[..., and For[...Plot[.... Finally, I tried doing the closest thing I could find to recursion, which was to re-implement the function as:

recCompoundDivsTotal[shares_,divyield_,years_,compoundrate_] :=
recCompoundDivsTotal[shares] =
((shares (1 +(divyield/compoundrate))^(compoundrate * years)))


It gives the right value, but it's still linear in nature even though, from what I understand, it ought to recursively iterate using the previous value of shares

I'm at a loss for what to do. Any ideas?

• what is the parameters divyield? – Alex Trounev Jan 23 '19 at 19:53
• It's the dividend yield of the stock as a decimal percentage. In effect, the interest rate of the stock. – vlca Jan 23 '19 at 20:37
• And divyield=? – Alex Trounev Jan 23 '19 at 20:40
• My bad. Use .0659. I've been calling the function using compoundDivsTotal[x, 0.0659, 1, 4] – vlca Jan 23 '19 at 20:45
• So compoundrate = 4? It's a lot. – Alex Trounev Jan 23 '19 at 21:20

This is an option if the functions are defined correctly.

compoundDivsInterest[shares_, divyield_, years_,
compoundrate_] := ((shares (1 + (divyield/
compoundrate))^(compoundrate*years) - shares))

compoundDivsTotal[shares_, divyield_, years_,
compoundrate_] := ((shares (1 + (compoundrate))^(compoundrate*
years)))

{Plot[Evaluate[
Table[compoundDivsTotal[10^n, 0.0659, t, .4], {n, 2, 3}]], {t, 0,
10}, AxesLabel -> {"t, years", ""},
PlotLabel -> "Compound Dividends Total", PlotLegends -> Automatic],
Plot3D[Evaluate[
Table[compoundDivsTotal[s, 0.0659, t, .4], {n, 2, 3}]], {t, 0,
10}, {s, 0, 1000}, AxesLabel -> {"t, years", "shares", ""},
PlotLabel -> "Compound Dividends Total", Mesh -> None,
ColorFunction -> "Rainbow"]} 