differentiating function & getting different answer

hey! I was trying to differentiate this function g(x)=3x-1 and despite the fact that I followed the code provided on the matlab website to a "t", the answer I got within matlab was incorrect. I included a link to the example as well as screenshots of the code they provided and how it appears in my command window.

here's how the code appears on the site:

I typed the same code into my command window but got a different answer...

I'm really confused at this point because now whenever I try to differentiate ANY function I get the same answer!

[Aquatris]

What version of Matlab are you using? I typed the exact code in Matlab 2013a and 2017a and it gave me the correct answer.

[James Tursa]

What does this show:

which diff

[Noah Barrow]

when I type this in it says "diff is a variable"

[Noah Barrow]

i'm using 2018b. is the code drastically different?

Answer 1

[James Tursa]

Perhaps you are shadowing the MATLAB function diff with a function of your own. Make sure diff is pointing to the MATLAB function.

Answer 2

[Noah Barrow]

can you explain what shadowing is? The code I entered in my command window is directly above-does it appear that my "diff" is pointing to the matlab function?

[Aquatris]

Shadowing means that your workspace has a variable called "diff" so when you run the command "diff(g)", instead of going to the built-in diff() function of Matlab, it tries to run the "diff" in the workspace (or a function in your working directory). That is why it is common practice to not name variable or custom functions with the same name as Matlab's built-in ones.

When I run which diff I get;

which diff
built-in (C:\Program Files\MATLAB\R2013a\toolbox\matlab\datafun\diff)

[Noah Barrow]

ah ha! okay, because when I type in diff it says "diff is a variable". So how should i proceed? Am I simply using the wrong command?

[Walter Roberson]

Execute the command

clearvars

and then run your code again. If the same thing happens, then track down where in your code you assign something to a variable named diff and use a different variable name instead.

[Noah Barrow]

FIXED!

Thank you so much :)