sum of cubes question

Please let me know how to fix my code. I am unable to make mat lab run.

Here is my code so far:

I created a file and saved it as sum_of_cubes.m

for n=1:20

[sum]=sum_of_cubes(n)

end

I created a new live script, then ran it (or attempted to).

sum=0;

[sum]=sum_of_cubes(20)

Please see the homework prompt below.

Answer 1

[James Tursa]

This homework question is poorly written:

• It seems like the intent of the n is to use it as the upper limit of the for-loop, but the directions then state to hard-code this upper limit at 20, making n useless.
• It uses the variable name "sum" which causes the code to shadow the MATLAB sum( ) function. Best practice is to use a different variable name that doesn't shadow a MATLAB function.

Because of these apparently contradictory instructions, I can see how a beginner would get confused. I would advise you to ignore the direction in the first part 3 and use n as the upper limit of the for-loop. So the body of your function code would look like this:

sum = 0;  % <-- 2nd part 3 direction
for i=1:n  % <-- using n as the upper limit of the for-loop, ignoring 1st part 3 direction
    sum = sum + _______;  % <-- you fill this in, 2nd part 3 direction
end

Again, "sum" is a poor choice for a variable name, but I am using it because the directions explicitly told you to do so.

The fill-in-the-blank part is not going to be sum_of_cubes(n) as you have currently written it. See if you can figure out what it should be instead. Look at the sum that is written out in the instructions and ask yourself what you should be adding into sum at each iteration. The instructions state "... keep adding the cube of every number ...", so that is your clue as to what should be here.

[N/A]

Thank you for starting me on this, yet I struggle with this part: sum = sum + _______ .I don't know what to do. If I understood you correctly, said to do the following:

input this information into the sum_of_cubes.m file

for i=1:n

sum = sum + ????;

end

move to the new live script and input this information

[sum]=sum_of_cubes(20)

Where would this information go?

[sum]=sum_of_cubes(n)

[James Tursa]

The ???? part above needs to be "... keep adding the cube of every number ...".

What is the cube of the i'th number? If I gave you a variable named i, how would you calculate the cube of it?

This line is how you call your function (e.g., from the command line):

[sum] = sum_of_cubes(20)

This line doesn't go anywhere in your function. Remove it:

[sum]=sum_of_cubes(n)

[Image Analyst]

Or

% Test Script:
n = 5;
theSum = sum_of_cubes(n);
fprintf('For n = %d, sum_of_cubes(%d) = %d.\n', n, n, theSum);

% Function definition:
function sum_of_cubes(n)
theSum = 0;  % <-- 2nd part 3 direction
for i = 1 : n  % <-- using n as the upper limit of the for-loop, ignoring 1st part 3 direction
    theSum = theSum + n _______;  % <-- you fill this in, 2nd part 3 direction
end

[N/A]

Ok, I am reading through your answers now and trying them. Will share results shortly.

[N/A]

James I tried what you recommended (or what I think you recommended) and this did not work. See screen shot.

[James Tursa]

The code you posted above is supposed to be inside a function with this signature:

function sum = sum_of_cubes(n)

Then n will be defined by passing in an argument when you call it.

[N/A]

This is what I have and it is not working.

[James Tursa]

Why did you get rid of the sum=0 line? That needs to be there. Also, why did you suddenly switch variable name from n to number? Just use n.

[N/A]

I changed those things to make it work better. Here is me trying it again:

[James Tursa]

The sum=0 line needs to go inside your function. Make it the first line after the function line. You initialize this variable to 0 as your first step in the function. Then you add to it with the for-loop code.

[N/A]

function [sum=0]=sum_of_cubes(n) ?

[N/A]

This still does not work.

[N/A]

Is this what you meant?

[James Tursa]

You are very close, but you are still struggling. The code for the function is the following, which is what you already have but in the proper order:

% This code goes in a file called sum_of_cubes.m
function sum = sum_of_cubes(n)
sum = 0;  % <-- 2nd part 3 direction
for i = 1 : n  % <-- using n as the upper limit of the for-loop, ignoring 1st part 3 direction
    sum = sum + i^3;  % <-- you fill this in, 2nd part 3 direction
end
end

Then you call it e.g. from the command line:

>> sum_of_cubes(20)

[N/A]

When I do that it does not work.

[N/A]

[James Tursa]

Please post all of your current code so we can look to see what is going wrong.

[N/A]

That is all of it. Here is the typed version if you must have it:

function sum_of_cubes(n)

sum = 0;

for i = 1 : n

sum = sum + i^3;

end

>> sum_of_cubes(20)

[N/A]

The other stuff you see is from my last code for this problem that worked. My last code was this:

function[sum]=sum_of_cubes(n)

sum=0

For i=1:n

sum=sum+i^3

end

end

>> [sum]=sum_of_cubes(20)

sum =

44100

[James Tursa]

This function signature of your earlier post:

function sum_of_cubes(n)

doesn't return anything to the caller. That is, it does some calculations to generate the sum variable, but then it throws it away and doesn't return it. You have to have syntax that shows you want to return the sum variable:

function sum = sum_of_cubes(n)

That being said, it looks like you already have code that does this and works? I am not sure what problems you are currently having.

[N/A]

Yes, I have a code that works. For some reason, your code does not work when I copy and paste it. Not sure why.

[N/A]

Thank you for your help! :)