Mismatch between 'formula' and graphed 'surface'

Hi guys,

I have a feeling that my code isn't generating the surface I want. I basically have this test function:

    x = [0:50];
    y = [0:50];

    ModY = @(x,y) [20*(y<20) + 80*(y>80) + y*((y>=20)&(y<=80))];

    Test1 = @(x,y) ((1300./ModY(x,y)-3.34)-(1.3./((1.3./(x))-((3.44)./998))));

    [X1,Y1] = meshgrid(x,y);
    Z1 = Test1(X1,Y1);
    s1 = surf(X1,Y1,Z1,'LineStyle','none');

But when I test the function as follows:

Test1(40,20)

ans =

     16.9144

I'm not convinced that it's a match. I feel like something is happening in my condensed code. Please help! Thank you!

Answer 1

[Star Strider]

I’m not convinced either:

The problem is here:

ModY = @(y) [20*(y<20) + 80*(y>80) + y.*((y>=20)&(y<=80))];
                                      ^  NEED ARRAY MULTIPLICATION HERE

You need to do array multiplication of ‘y’ and the logical array created by the ‘((y>=20)&(y<=80))’ inequality. (I recognised this because I’ve seen something like this code quite recently!)

With that, it works:

The reason it produced one value with the scalars and another with the matrices was due to the omitted element-wise array operator multiplication (.*).

[A]

You are so good! Thank you so much again! I can't believe I made a thread about a '.'

Thanks again!

[Star Strider]

My pleasure! Thank you for the compliment!

I didn’t see anything wrong with your code to calculate ‘Test1’, so I plotted ‘ModY’ and the problem immediately showed itself.

The dot operator differentiates matrix from element-wise operations (except for the transpose operator, where its presence denotes the ‘normal’ transpose and its absence denotes the complex-conjugate transpose). For details on the differences, the discussion of Array vs. Matrix Operations is a page I’ve bookmarked.

As always, my pleasure!