How do i calculate a sum using a function?

Problem description:

Using previous examples make a function with one parameter n, number of integers to sum, and one output, the sum of first n integers: 1+2+3+...+n.

How do i use a function to calculate the sum?

[KL]

Did you read the documentation? Show us what have you tried, we'd be happy help you

[Jonas Fuglsang Hansen]

Yes, i've read the documentation. I haven't tried anything yet, as i have no idea where to start. I don't really understand how to write functions in matlab.

Answer 1

[Jan]

Please take the time to read the "Getting Started" chapters. Matlab's OnRamp helps also very efficient to learn the basics.

Did you heard something about how Gauss has solved this problem with pencil and paper when he was in school? You need some seconds only to get the result of the sum from from 1 to 1000. Remember: Write the list of numbers twice in two rows, one in backward direction:

1  2    3   ... n-1  n
n  n-1  n-2 ... 2    1

Now sum them up vertically at first: You get n times the value of n+1. Summing them is n*(n+1). But then you have counted each value twice, so take the half of the result:

n * (n+1) / 2

This takes the same computing time for n=10 or n=10^15. How nice.

Now you have to convert it into a function, but you have learned how to do this already. Welcome to the world of Matlab!

[KL]

+1 for bringing in Gauss summation here, Good one Jan!

Answer 2

[jean claude]

as i understand you want to sum 1+2+...n so you can use

function S = Ma_somme(n)

       S = 0 ;

    for i = 1 : n
           S = S + i ; 
    end

end

[John D'Errico]

Please don't give explicit code like this for obvious homework. That does not help the student. They need to learn, which means they need to make some effort.

[Jonas Fuglsang Hansen]

I've tried to run this code, but i get an error, as n is undefined. Edit: It works now. I just realised i didn't use the same symbol as in the input.

[Jan]

To run a function, you cannot just hit the "Run" button, but you save it to a file and call it from Matlab's command window with providing an input:

Result = Ma_somme(19)

@jean claude: Please consider John's advice in the future. It is an advantage for the student and for the forum. Thanks.

Answer 3

[Image Analyst]

Hints

function theSum = ComputeSum(n)
    theSum = .......

and check this out, and learn from what you see:

vec = 1 : 10;

Also, are you allowed to use the built-in sum() function? And never use "sum" for the name of your variable or you'll destroy the built-in function.

[Jonas Fuglsang Hansen]

I don't think i am allowed to use the built in sum function. I don't understand what you mean with

vec = 1 : 10

as it'll have to go from 1:n, which gives an error, as n is undefined. I know much of the basics of matlab (i think), but functions are a weak point of mine, so the hints don't help a lot.

[KL]

Check this link: https://de.mathworks.com/help/matlab/matlab_prog/create-functions-in-files.html

So this link shows you how to create and save the function as an m-file. As Image Analyst shows, this function should accept an "input" (n).

vec = 1:10

creates a series of numbers from 1 to 10, that is just an example. You use this like

 vec = 1:n

inside your function m-file.

[Jonas Fuglsang Hansen]

Yeah i got it to work. I just don't understand why 1:n workshop inside the function, when it doesn't work outside of the function.

[KL]

because you're passing 'n' to the function so MATLAB knows what 'n' means. If you do the same thing outside, where you haven't defined 'n' yet, then it's not possible for your program to know what 'n' means.

On your command ĺine, try the following,

 vec = 1:n

now you'll get an error since the workspace doesn't have a variable called 'n'. Now try

n = 10;
vec = 1:n;

No error, because you just defined 'n' in the previous line. Get it?

[Jonas Fuglsang Hansen]

Ah, okay. So functions in matlab, are like functions everywhere else in math. You can call on the function and write a number in as the input, and then it calculates it and returns the output. In this case, if

function S=sumInt(n)
  S=0
  for i=1:n
  S=S+i
end
end

Then it inserts your input in the line "for i=1:n" and calculates? Thanks for the answer! I think i understand functions better now!

[Image Analyst]

Yes, that should do it.

Since you already got one working version, I'll give my version:

function theSum = ComputeSum(n)
    theSum = sum(1:n);

That's it! It uses the built-in sum() function but you didn't specifically disallow it so I used it.

Hint: to fix up your indenting, in the MATLAB editor, type control-a (to select all the code) and then control-i (to fix/standardize the indenting of the lines).

Here's another snippet that might come in handy for you:

% Ask user for one integer number.
defaultValue = 45;
titleBar = 'Enter an integer value';
userPrompt = 'Enter the integer';
caUserInput = inputdlg(userPrompt, titleBar, 1, {num2str(defaultValue)});
if isempty(caUserInput),return,end; % Bail out if they clicked Cancel.
% Round to nearest integer in case they entered a floating point number.
integerValue = round(str2double(cell2mat(caUserInput)));
% Check for a valid integer.
if isnan(integerValue)
    % They didn't enter a number.  
    % They clicked Cancel, or entered a character, symbols, or something else not allowed.
    integerValue = defaultValue;
    message = sprintf('I said it had to be an integer.\nTry replacing the user.\nI will use %d and continue.', integerValue);
    uiwait(warndlg(message));
end

If we're done, then you can click "Accept this answer" on the one best answer (it only works on one).