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I don't know if matlab is calculating this properly

Asked by Patrick Smith on 11 Sep 2019
Latest activity Commented on by Patrick Smith on 11 Sep 2019
Is this calculating correctly? It returns my answer as 1. It is meant to represent this series 1 + 1⁄r + 1⁄r^2 + 1⁄r^3 + … + 1⁄r^n.
function sum = mysum(r,n)
sum = 1;
for i = 1:n;
sum = sum + 1/(r^n);
end
Please help.

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2 Answers

Answer by John D'Errico
on 11 Sep 2019
Edited by John D'Errico
on 11 Sep 2019

MATLAB is calculating what it calculated properly. The issue is, you told it to calculate the wrong thing. Computers are sooooo picky. :)
You don't tell us what values of r that you used. Or what n was when you ran this code. But if you used a large value for n, then yes, it SHOULD return 1. Or it might return inf. Really? What did you write, and why is that?
In fact, you wrote:
1 + 1⁄r^n + 1⁄r^n + 1⁄r^n + … + 1⁄r^n
Your exponent was fixed, at n.
The exponent in what you wrote was CONSTANT, at n. So if n was large, and abs(r ) was relatively large so that 1/abs(r ) is small, then you would see 1 as a result, because the powers will underflow. Or if r is itself small, then you would just get overflows.
The fix is easy, of course. Change n to i in the expression. That is, the exponent of r needs to be the index variable, not the number n itself.
function sum = mysum(r,n)
sum = 1;
for i = 1:n;
sum = sum + 1/(r^i);
end
And using the name sum for a variable is a really bad idea, as it will cause bugs in your code sometime, when you actually want to use the FUNCTION named sum.
Always avoid using existing function names as variable names.

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Answer by madhan ravi
on 11 Sep 2019
Edited by madhan ravi
on 11 Sep 2019

Result = mysum(2,10) % just call it like this
doc function % to know how to use functions
function SUM = mysum(r,n)
SUM = 1; % don’t name variable sum because it will shadow the MATLAB’s inbuilt function
for ii = 1:n;
SUM = SUM + 1/(r^ii);
% ^^-- have a look here
end
end

  11 Comments

I'm confused which n do I change to an i?
function sum = mysum(r,n)
sum = 1;
for i = 1:i;
sum = sum + 1/(r^n);
end
Do you mean this? or can you send back what you guys mean lol
function SUM = mysum(r,n)
SUM = 1;
for ii = 1:n;
SUM = SUM + 1/(r^ii);
% ^^-- have a look here
end
end
Perfectt! Thank you! I couldn't figure out why it kept giving me 1 which made not sense.

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