Invalid use of operator while trying to plot surface

I am a total rookie when it comes to MatLab so I got stuck right away when trying to plot a surface. I get the "invalid use of operator" error code when trying to write the expression for Z, which is supposed to look something like this: z=(4x^2+8xy-2y^2)e^{-(x^2+2y^2)}. MatLab seems to point out that the minus sign in front of the 2 is the issue. How should I code the expression?

>> x=linspace(-3,3);
y=linspace(-2,2);
[X,Y]=meshgrid(x,y);
 Z=(4*X.^2+8*X.*Y.-2*Y.^2)*exp^-(X.^2+2Y.^2);
                  ↑
Invalid use of operator.

Thanks in advance!

Answer 1

[Dave B]

A few issues:

• There's no .- in MATLAB, you use the dot to indicate you want a element-wise operation instead of a matrix operation, but for + and - there's only one option.
• exp is a function, so you can do exp(1)^n if you really want, but it's easier to just do exp(n)
• Inside your exp power you have 2Y, but you meant 2*Y.

x=linspace(-3,3);
y=linspace(-2,2);
[X,Y]=meshgrid(x,y);
Z=(4*X.^2+8*X.*Y-2*Y.^2)*exp(-X.^2+2*Y.^2)

Z = 100×100

1.0e+05 * 0.0007 0.0010 0.0014 0.0019 0.0027 0.0038 0.0053 0.0072 0.0098 0.0133 0.0178 0.0237 0.0313 0.0411 0.0535 0.0692 0.0889 0.1132 0.1433 0.1799 0.2242 0.2775 0.3409 0.4156 0.5031 0.6045 0.7211 0.8538 1.0036 1.1710 0.0007 0.0010 0.0014 0.0020 0.0028 0.0039 0.0053 0.0073 0.0099 0.0134 0.0180 0.0240 0.0317 0.0416 0.0542 0.0702 0.0901 0.1148 0.1452 0.1823 0.2273 0.2812 0.3454 0.4212 0.5099 0.6127 0.7308 0.8653 1.0171 1.1868 0.0007 0.0010 0.0014 0.0020 0.0028 0.0039 0.0054 0.0074 0.0101 0.0136 0.0183 0.0243 0.0321 0.0422 0.0550 0.0711 0.0912 0.1163 0.1471 0.1847 0.2302 0.2849 0.3499 0.4267 0.5165 0.6207 0.7403 0.8766 1.0304 1.2022 0.0007 0.0010 0.0014 0.0020 0.0028 0.0040 0.0055 0.0075 0.0102 0.0138 0.0185 0.0246 0.0325 0.0427 0.0556 0.0720 0.0924 0.1177 0.1489 0.1870 0.2331 0.2885 0.3544 0.4321 0.5230 0.6285 0.7496 0.8876 1.0433 1.2174 0.0007 0.0010 0.0015 0.0021 0.0029 0.0040 0.0055 0.0076 0.0103 0.0139 0.0187 0.0249 0.0329 0.0432 0.0563 0.0728 0.0935 0.1192 0.1507 0.1893 0.2360 0.2920 0.3587 0.4373 0.5294 0.6361 0.7588 0.8984 1.0560 1.2322 0.0007 0.0010 0.0015 0.0021 0.0029 0.0040 0.0056 0.0077 0.0104 0.0141 0.0189 0.0252 0.0333 0.0437 0.0570 0.0737 0.0946 0.1206 0.1525 0.1915 0.2387 0.2954 0.3629 0.4425 0.5356 0.6436 0.7677 0.9090 1.0685 1.2467 0.0007 0.0010 0.0015 0.0021 0.0029 0.0041 0.0057 0.0078 0.0106 0.0143 0.0191 0.0255 0.0337 0.0442 0.0576 0.0745 0.0957 0.1219 0.1542 0.1937 0.2414 0.2988 0.3670 0.4475 0.5417 0.6509 0.7764 0.9193 1.0806 1.2608 0.0007 0.0011 0.0015 0.0021 0.0030 0.0041 0.0057 0.0078 0.0107 0.0144 0.0194 0.0258 0.0341 0.0447 0.0583 0.0754 0.0967 0.1233 0.1559 0.1958 0.2441 0.3021 0.3710 0.4524 0.5477 0.6581 0.7849 0.9294 1.0925 1.2747 0.0007 0.0011 0.0015 0.0021 0.0030 0.0042 0.0058 0.0079 0.0108 0.0146 0.0196 0.0261 0.0344 0.0452 0.0589 0.0762 0.0978 0.1246 0.1576 0.1979 0.2467 0.3053 0.3750 0.4572 0.5535 0.6651 0.7933 0.9393 1.1041 1.2882 0.0008 0.0011 0.0015 0.0022 0.0030 0.0042 0.0058 0.0080 0.0109 0.0147 0.0198 0.0263 0.0348 0.0457 0.0595 0.0769 0.0988 0.1259 0.1592 0.1999 0.2492 0.3084 0.3788 0.4619 0.5591 0.6719 0.8014 0.9489 1.1154 1.3014

Pro tip: break things up into small pieces...it's much easier to debug (and catch if you've not made the order of operations you intended!). For example:

a=4*X.^2;
b=8*X.*Y;
c=2*Y.^2;
d=-X.^2;
e=2*Y.^2;

Z2=(a+b-c)*exp(d+e);

ans = logical
   1

[August Lindberg]

Thank you so much!

[August Lindberg]

I noticed that I missed some information. I was supposed to use x=linspace(-3,3,61) and y=linspace(-2,2,41).

Tried to break it up into smaller parts according to your comment and added a . infront of the * due to the difference in dimensions. However, I don't seem to get the surface right. What am I doing wrong?

>> x=linspace(-3,3,61);
y=linspace(-2,2,41);
[X,Y]=meshgrid(x,y);
a=4*X.^2;
b=8*X.*Y;
c=2*Y.^2;
d=-X.^2;
e=2*Y.^2;
Z=(a+b-c).*exp(d+e);
surf(X,Y,Z)

[August Lindberg]

Never mind. I found my mistake: e should also be negative. Thanks for your help!

[RITWIK SINGH]

i have a similar issue in this line

EbN0Lin = 10.^(EbN0dB/10);
theoryBer_nRx1 = 0.5.(1-1(1+1./EbN0Lin).^(-0.5)); 
p = 1/2 - 1/2.(1+1./EbN0Lin).^(-1/2);
theoryBerMRC_Rx2 = p.^2.(1+2(1-p)); 

any idea how to fix?