Invalid use of operator while trying to plot surface

Question

August Lindberg 2021-11-20

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https://ww2.mathworks.cn/matlabcentral/answers/1591474-invalid-use-of-operator-while-trying-to-plot-surface

评论： RITWIK SINGH 2021-12-9

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!

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Answer 1

Dave B 2021-11-20

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https://ww2.mathworks.cn/matlabcentral/answers/1591474-invalid-use-of-operator-while-trying-to-plot-surface#answer_836259

编辑：Dave B 2021-11-20

在 MATLAB Online 中打开

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

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August Lindberg 2021-11-20

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

RITWIK SINGH 2021-12-9

在 MATLAB Online 中打开

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?