School work question, Having troubles adding onto a matrix

This is my problem:

%Imagine that you are a proud new parent. You decide to start a college saving plan for your child hoping to have enough money in 18 years to pay for cost of education. Suppose that your folks give you a 1000 dollars to start with and you have a 6% interest per year of 0.5% interest per month. Because of interest payments and your contributions, each month you balance will increase in accordance with the formula:

% New balance = old balance + interest + your contribution

% Write a for loop formula to find the saving account balance in 18 years. Plot the values on a time line with horizontal being time and dollar on the vertical

I'm having a problem getting the matrix a(old_balance) to add on each new value. Any advice for fixing this bug. Please and Thankyou.

old_balance = 1000;

interest = .005;

a = zeros(217);

   for time = 0:1/12:18

    old_balance = old_balance + 100 + (old_balance * interest);

    a(old_balance) = old_balance

end

Answer 1

[Adam]

Without having time to read through your full homework question (try to ask specific questions rather than post your entire homework question) there are a few obvious things wrong:

First:

a = zeros(217)

will produce a 2d 217*217 matrix. I assume you want:

a = zeros(1,217)

instead.

Second:

Why are you using a for loop parameterised in that way when you don't use that 'time' loop variable at all?

Finally:

What are you trying to achieve with the line:

a(old_balance) = old_balance;

? You are trying to index into an array starting at index 1000 which is odd to start with, but then subsequent times round the loop the old_balance variable will no longer be integer-valued so cannot be used as an index. But you almost certainly do not want to be using it as an index anyway.

I assume you want something more like:

    for n = 1:217

    old_balance = old_balance + 100 + (old_balance * interest);
    a(n) = old_balance;

    end

Almost certainly this isn't the most efficient way to do it, but get the right answer first, then look to efficiency if you need to.

[greenie]

Thankyou very much, I ended up putting that in and I got the values I needed.

Answer 2

[the cyclist]

You effectively want

a(time)

instead of

a(old_balance)

in the last line of your loop. But notice that your time variable is not an integer, so you cannot index with that directly.

You will need to figure out a way to count that time interval and fill in the array accordingly. (It's not too difficult.)

Answer 3

[Star Strider]

You have the correct idea, but there are two problems: first, MATLAB uses positive integers for array subscripts so fractional increments such as 1/12 will not work; and second you need to subscript ‘old_balance’. )You don’t need ‘a’, since it just replicates ‘old_balance’.)

I made those tweaks, but otherwise did not alter your code:

old_balance(1) = 1000;

interest = .005;

time = 0:1/12:18;

for k1 = 1:length(time)

      old_balance(k1+1) = old_balance(k1) + 100 + (old_balance(k1) * interest);

end

I will leave it to you to determine if it produces the result you want.