How to use sum series in Matlab

I need to use sum series in some assignments. But don't know the syntax

I'm just trying to write sum series as below

Code

   k = sum(i^2, i = 1..4)

But I'm getting this error message

The expression to the left of the equals sign is not a valid target for an assignment.

[Stephen23]

The documentation for MATLAB is really rather good, and includes lots of complete, working examples:

http://www.mathworks.com/help/matlab/ref/sum.html

You should look at it one day. Looking at the documentation would immediately give a much better idea of how to use the function max than just inventing some syntax. After all, the people who wrote that function are probably the best ones to describe how it can be used.

Answer 1

[Jonathan Campelli]

I'm providing another solution to address the longer summation you have posted in my first answer. Please let me know if this suits your needs:

data = load('ex1data1.txt');
y = data(:, 2);
m = length(y);
X = [ones(m, 1), data(:,1)];
theta = zeros(2,1);

i=1:m;
J=sum(((theta(1)+theta(2).*X(i,2))-y(i)).^2)

Best,

Jonathan

[Image Analyst]

Atinesh's "Answer" moved here since it's a comment to Joseph, not an "Answer" to the originally posted question:

Hello @Jonathan Campelli, solution that you have given is producing different result. On internet I found one solution which is producing correct result, But I don't understand what code is doing can you please explain to me whats the different between your code above and below code

 J = (1/(2*m)) * (X*theta-y)' * (X*theta-y)

[Atinesh S]

Ohh little mistake, forgot to multiply the whole expression with "(1/(2*m))"

[Jonathan Campelli]

I apologize for the mistake, and I'm glad you found it even though I was away.

Answer 2

[Image Analyst]

If you want to use a for loop, try this:

theSum = 0;
for i = 1 : 4
  theSum = theSum + i ^ 2;
end

Or if you want a vectorized solution, try this:

i = 1 : 4;
theSum = sum(i .^ 2)

[Atinesh S]

Here's the complete code I'm using

 data = load('ex1data1.txt');
% 'ex1data1.txt' text file containing list of numbers in two 
% columns separated by commas
y = data(:, 2); % Storing column 2 of text file in vector array y
m = length(y);
X = [ones(m, 1), data(:,1)]; 
% Storing column 1 of text file in vector array X and appending all 1's 
% column to the vector array
theta = zeros(2,1);
J = symsum(((theta(1)+theta(2)*X(i,2))-y(i))^2, i, 1, m)

[Image Analyst]

OK, but that looks totally different than your original post where you mentioned nothing about y, m, X, J, data, or theta. All you mentioned was "i" and the sum that you called "k".

I don't have any symbolic toolboxes. Is that a requirement that you do it that way rather than the simple numerical way?

Answer 3

[Jonathan Campelli]

Hello Atinesh

syms i %Creat symbolic variable 'i'
symsum(i^2, i, 1, 4) %Use symsum to run your summation operation

Best regards,

Jonathan Campelli

Answer 4

[Atinesh S]

Hello Jonathan Campelli, It worked but actually I've to write this below series

While using the code below

 J = symsum(((theta(1)+theta(2)*X(i,2))-y(i))^2, i, 1, m)

I'm getting new error message

_Subscript indices must either be real positive integers or logicals.

Error in Test (line 7) J = symsum(((theta(1)+theta(2)*X(i,2))-y(i))^2, i, 1, m)_

[Jonathan Campelli]

Taking a look.

[Jonathan Campelli]

This error message indicates that you are trying to access either a negative or not a logical valued index in an array. I suspect that your variable 'i' may be throwing off either your 'X' or 'y' indexing, or both.

Can you submit the first 6 lines of code as well, containing your initialize variables?

[Atinesh S]

Here's the complete code I'm using

 data = load('ex1data1.txt');
% 'ex1data1.txt' text file containing list of numbers in two 
% columns separated by commas
y = data(:, 2); % Storing column 2 of text file in vector array y
m = length(y);
X = [ones(m, 1), data(:,1)]; 
% Storing column 1 of text file in vector array X and appending all 1's 
% column to the vector array
theta = zeros(2,1);
J = symsum(((theta(1)+theta(2)*X(i,2))-y(i))^2, i, 1, m)

[Jonathan Campelli]

I am unable to call X(i) using symsum. If you are able, Image Analyst has provided vectorized computations that are a lot cleaner.