How to create a surface plot of a function that uses colon operators?

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Rahul Bhadani
Rahul Bhadani 2021-4-7
移动：VBBV 2025-5-2
I have a following function:
function p = dist(n, theta, M, Ns, Nb, G, kappa)
Ntheta = 55;
logterm1 = sum( log(1: n + M - 1) ) - sum( log (1: M - 1)) - sum( log (1:n));
logterm2 = n.*(log (Ntheta) - log(1 + Ntheta));
logterm3 = M*log(1 + Ntheta);
total = logterm1 + logterm2 - logterm3;
p = exp(total);
end
function PE = Error(x, y)
p0 = x;
p1 = 1-x;
Ns = 1e-2;
Nb =10;
kappa = 1e-2;
G = 1.01;
M = 1e3;
Ntheta0 = 150;
Ntheta_pi = 33;
sigma0 = sqrt( Ntheta0.*( Ntheta0 + 1));
sigma_pi = sqrt( Ntheta_pi.*( Ntheta_pi + 1));
narray = 1:floor(y);
PX0LessthanTh = 0;
for na = narray
pp = dist(na, 0, M, Ns, Nb, G, kappa);
PX0LessthanTh = PX0LessthanTh + pp;
end
PXpiLessthanTh = 0;
for na = narray
pp = dist(na, pi, M, Ns, Nb, G, kappa);
PXpiLessthanTh = PXpiLessthanTh + pp;
end
PE = p0.*PX0LessthanTh + p1.*(1 - PXpiLessthanTh);
end
I am looking to create a surface plot of Error with respect to x and y. However, I because I am using a statement narray = 1:floor(y); I am getting an error:
The following error was reported evaluating the function in FunctionLine
update: Colon operands must be real scalars.
How can I modify my Error function above so that I can use 3D functions like surf.

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回答（2 个）

Harshavardhan
Harshavardhan 2025-4-29
Hi @Rahul Bhadani
The current “Error” function looks to be designed for scalar inputs. When used with “meshgrid” for surface plots, MATLAB passes arrays, leading to issues with expressions like “narray = 1:floor(y);”.
To resolve this, use “arrayfun” to evaluate the function for each (x,y) pair separately as shown below.
%Example x,y arrays
x = linspace(0, 1, 30);
y = linspace(1, 20, 30);
[X, Y] = meshgrid(x, y); %Make the X,Y grid
Z = arrayfun(@(xi, yi) Error(xi, yi), X, Y);% Using “arrayfun” to calculate Z(error)
figure
surf(X, Y, Z);
xlabel('x'); ylabel('y'); zlabel('Error');
title('Surface plot of Error(x, y)');
colorbar;
For more information on “arrayfun” and “meshgrid”, type the following commands in a MATLAB command window
doc meshgrid
doc arrayfun
Hope this helps.
VBBV
VBBV 2025-4-30
移动：VBBV 2025-5-2
Ran in:
@Rahul Bhadani It seems you have not defined the y variable. However, If its an external input value to the function Error then you need to ensure that its a scalar. In your case its doesnt seem to be scalar but rather a vector or a matrix. Since you have a for loop inside the function, the colon operator does not parse the vector inputs to a for loop index The solution would be to reshape the matrix and use numel instead of floor as below to get rid of the warning/error message
x = linspace(0, 1, 30);
y = linspace(1, 20, 30);
[X, Y] = meshgrid(x, y); %Make the X,Y grid
PE = Error(X,Y);
narray = 1×900
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
y = 1×900
1.0000 1.6552 2.3103 2.9655 3.6207 4.2759 4.9310 5.5862 6.2414 6.8966 7.5517 8.2069 8.8621 9.5172 10.1724 10.8276 11.4828 12.1379 12.7931 13.4483 14.1034 14.7586 15.4138 16.0690 16.7241 17.3793 18.0345 18.6897 19.3448 20.0000
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
figure
surf(X,Y,PE)
function PE = Error(x, y)
p0 = x;
p1 = 1-x;
Ns = 1e-2;
Nb =10;
kappa = 1e-2;
G = 1.01;
M = 1e3;
Ntheta0 = 150;
Ntheta_pi = 33;
sigma0 = sqrt( Ntheta0.*( Ntheta0 + 1));
sigma_pi = sqrt( Ntheta_pi.*( Ntheta_pi + 1));
narray = 1:numel(y)
y = reshape(y,1,[]) %
PX0LessthanTh = 0;
for na = narray
pp = dist(y(na), 0, M, Ns, Nb, G, kappa);
% ^^
PX0LessthanTh = PX0LessthanTh + pp;
end
PXpiLessthanTh = 0;
for na = narray
pp = dist(y(na), pi, M, Ns, Nb, G, kappa);
% ^^
PXpiLessthanTh = PXpiLessthanTh + pp;
end
PE = p0.*PX0LessthanTh + p1.*(1 - PXpiLessthanTh);
end
function p = dist(n, theta, M, Ns, Nb, G, kappa)
Ntheta = 55;
logterm1 = sum( log(1: n + M - 1) ) - sum( log (1: M - 1)) - sum( log (1:n));
logterm2 = n.*(log (Ntheta) - log(1 + Ntheta));
logterm3 = M*log(1 + Ntheta);
total = logterm1 + logterm2 - logterm3;
p = exp(total);
end

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