Issues defining dot product function

I'm working on my very first MATLAB assignment with no previous experience and I have to create a function called 'dotcheck', which checks whether the dot product of two column vectors can be found, and then it calculates the dot product.

I'm pretty sure I have the first part of my code correct, but I keep running into an error that says "Unrecognized function or variable 'dot'." when I try to run (for example):

u=randi(10,5,1), v=randi(10,5,1) 
[d1,d2]=dotcheck(u,v);

in my Live Editor with my dotcheck function:

function [d1,d2] = dotcheck(u,v)
format compact
m=length(u);
n=length(v);
if isequal(m,n)
    fprintf('both vectors have %i entries\n',n)
    else
    disp('the dot product is not defined')
    d1=[];
    d2=[];
    return
end
    d1=transpose(u)*v;
    d2=0;
    for i=1:n
        d2=d2+u(i)*v(i);
    end
    d=dot(u,v);
    if d1==dot(u,v), d2==dot(u,v)
        fprintf('the code is correct\n')
    else
        disp('check the code!')
        return;
    end
        if d>0
            disp('the angle between the vectors is either acute or zero')
            dot(u,v)=norm(u)*norm(v)
            dot(u,v)/(norm(u)*norm(v))-1==0
        end
        if d<0
            disp('the angle between the vectors is obtuse or 180 degrees')
            dot(u,v)=norm(u)*norm(v)*cos(180)
            dot(u,v)/(norm(u)*norm(v))+1==0
        end
        if d==0
            disp('the angle between the vectors is 90 degrees')
            dot(u,v)=norm(u)*norm(v)*cos(90)
        end
end

For some reason, it has an issue with the line:

d=dot(u,v);

because it doesn't understand that I'm trying to define the variable d to be the operation dot(u,v).

I tried using "dotprod" and other functions to test it out, and it had no problems with those. I can also use the operation dot(u,v) for some u and v in the Live Editor, so I know the function is defined.

I'm not sure if my coding afterwards is correct or not (because it won't let me get past this error), so I would also appreciate thoughts on how I can fix that to display the correct dot products for each particular condition.

Any advice or help is appreciated.

Thank you so much!

[Sindar]

dot is a built-in function, so it should be recognized (and you shouldn't redefine it in your code with dot(u,v)=)

Check out anonymous functions: https://www.mathworks.com/help/matlab/matlab_prog/anonymous-functions.html

Answer 1

[Walter Roberson]

In newer versions of MATLAB, if you use a name as both a function and a variable, then it will treat it as always being a variable and will fail at the point where you try to use it as a function. You are using dot as both a function and a variable, so you are running into problems.

Note by the way that in

dot(u,v)=norm(u)*norm(v)

you would encounter problems because your u and v are not expected to be positive integers suitable for use as subscripts.

See also my discussion of formula versus indexing at

https://www.mathworks.com/matlabcentral/answers/500679-how-to-add-code-in-each-cell-of-the-gui-table#comment_787442