Matlab FOR Loop help!

_ I'm trying to get this function to work. But i keep getting an error. I am trying to determine the amount in the savings account for next 18 years, which is represented by x(k) in an array. Problem is that only the first value for the first month shows up in the array._ * _ * _

clc,clear
% Variable declaration
% x will be used to store the value of the new balance
% a will be used to store the value of the old balance
% i will be used to store the value of the interest rate
% c will be used to store the value of the user's contribution
% Variable initialization
a = input('Enter a value for the initial balance: ');
i = input('Enter a value for the interest rate: ');
c = input('Enter a value that you will contribute: ');
% Calculation
month = 1:12:216;
x=zeros(length(month));
for k=1:length(month);
  x(k) = a + i(k) + c
end

[Sara]

is "i" an array? it doesn't look so in the input, but then you use i(k). In addition, x=zeros(length(month)); should be x=zeros(length(month),1); probably. Can you provide the inputs you are using?

[Dylan Flores]

a = 1000 i = 0.05 c = 100

also sorry, made an edit to the question above. Interest rate should be fixed since its per month.

[Sara]

All the number in the expression of x are constant so you get just one answer. Which eqn are you trying to implement?

[Dylan Flores]

Its an equation given by the question. Its mentioned above. Any ideas on how I can get around this problem? Because the question demands that i use the for loop to find the amount in the savings account each month for the next 18 years

[Sara]

Then I think you got the wrong eqn. How does this sound:

x(1) = a;
for k = 2:numel(month)
  x(k) = (x(k-1)+c)*(1+i);  % this assuming you make the contrib every month and that the interest rate is applied monthly
end

[Dylan Flores]

Ah but that equation was given by the question

[Sara]

What question? Can you post the text of the question? Your eqn makes no sense, interest rate multiply capital not sum it.

[Dylan Flores]

Yeah i thought it was weird as well how this formula was handled but...

[Sara]

Old balance is x(k-1) not a and I think interest means x(k-1)*i. The loop becomes:

x(1) = a;
for k = 2:numel(month)
  x(k) = x(k-1)+ x(k-1)*i + c; %or x(k-1)*(i+1) + c;
end

If the interest is %, divide i by 100.

[Dylan Flores]

Thanks! It works a lot better now!

Answer 1

[Ben11]

Maybe try this:

month = 1:12:216;

x=zeros(1,length(month)); % otherwise you have a 18*18 array
x(1) = a;
for k=2:length(month);

x(k) = x(k-1) + (i/100)*x(k-1) + c; % add the amount present during the previous month. Oh and I divided your interest rate by 100.

end

[Dylan Flores]

Hey Thanks for helping out!

[Ben11]

You're welcome! It looks like Sara and I arrived at pretty much a similar solution; although I incorrectly used a instead of x(k-1) :)