Basic use of sum command help

Recall that a geometric sum is a sum of the form a + ar + ar2 + ar3 + . . ..

(a) Write a function file that accepts the values of r, a and n as arguments and uses a for loop to return the sum of the first n terms of the geometric series. Test your function for a = 3, r = 1/2 and n = 10.

S = 0;
a=3;
r=1/2;
for n=0:1:10;
  S=S+a*r^n;
end
S

This works, So I'm good here.

(b) Write a function file that accepts the values of r, a and n as arguments and uses the built in command sum to find the sum of the first n terms of the geometric series. Test your function for a = 3, r = 1/2 and n = 10. Hint: Start by defining the vector e=0:n-1 and then evaluate the vector R = r.^e. It should be easy to figure out how to find the sum from there

This is where I am stuck. Keep in mind that I have only been using MATLAB for about a week now. Any help is appreciated, I don't even know where I should start with this. Thank you.

Answer 1

[Image Analyst]

Declare the function

function theSum = yourFunctionName(r, a, n)

In the function get rid of the "for" loop - just have n=0:1:10;

Then use .^ instead of ^. Then a few more little things you can get rid of - hopefully you can figure those out.

That should be more than enough hints for you to complete it.

[Daniel Millam]

I did exactly what you said and ended up with

function [y] = f(r, a, n)
a=3;
r=1/2;
n=0:1:10
    y=sum(a*r.^n)
end

AND IT WORKED Thank you very much.

Answer 2

[John D'Errico]

Ok, you did a decent job on the loop. Now it is time to learn how NOT to use a loop. :)

What if you create a vector n?

r = 0.5;
a = 3;
n = 10;
e = 0:1:(n-1);

Now, suppose you considered the vector:

a*r.^e

What would the elements of that vector represent?

What would result if you applied sum to that vector? Thus,

S = sum(a*r.^e);

Effectively, we have avoided the explicit for loop, working on vectors of elements instead.

[Daniel Millam]

Very helpful, Thank you.