Area of qudrilateral from input vertices (condition error when 3+ points on the same line)

Hello everyone, I want to implement a condition for the while loop where after checking some other details (vytest), it will check if at least one of the last two vertices (C, D) are on top of the line drawn by the first two (A, B).

else
        coef(cnt,:)=polyfit([vx(1) vx(cnt)], [vy(1) vy(cnt)], 1);
        while vytest==0||(vy(cnt)==coef(cnt,1)*vx(cnt)+coef(cnt,2) && vy(cnt-1)==coef(cnt-1,1)*vx(cnt-1)+coef(cnt,2))
            if vytest==0
                fprintf('ERROR: no1\n');
            else
                fprintf('ERROR: no2\n');
            end
            temp = input('  Y=');
            vytest=isnumeric(temp);
            vy(cnt) = temp;
        end
end

As to this point I have done the above but with no error turning up when inserted 3+ points of the same line.

A=[vx(1) vy(1)]
B=[vx(2) vy(2)]
C=[vx(3) vy(3)]
D=[vx(4) vy(4)]

and

cnt=1:4

Thanks very much in advance!!

[KSSV]

Why did you use polyfit to get area? What exactly you want?

[Petros Tsitouras]

The coefficients for y=a*x+b made by the two points.

I believe that the first output value of the polyfit is "a" and the second is "b".

[KSSV]

Yes, when you use degree 1, in polyfit; the first one would be a and second one is b.