why the command "figure(gcf+1)" gives error?????

doas=[-30 -5 40]*pi/180; 
P=[1 1 1]; 
N=10; %Number of array elements
K=1024; %Number of data snapshots
d=0.5; %Distance between elements in wavelengths
noise_var=1; %Variance of noise
r=length(doas); %Total number of signals
% Steering vector matrix. Columns will contain the steering vectors
% of the r signals
A=exp(-i*2*pi*d*(0:N-1)'*sin([doas(:).']));
% Signal and noise generation
sig=round(rand(r,K))*2-1; % Generate random BPSK symbols for each of the
 % r signals
noise=sqrt(noise_var/2)*(randn(N,K)+i*randn(N,K)); %Uncorrelated noise
X=A*diag(sqrt(P))*sig+noise; %Generate data matrix
R=X*X'/K; %Spatial covariance matrix
[Q ,D]=eig(R); %Compute eigendecomposition of covariance matrix
[D,I]=sort(diag(D),1,'descend'); %Find r largest eigenvalues
Q=Q(:,I); %Sort the eigenvectors to put signal eigenvectors first
Qs=Q(:,1:r); %Get the signal eigenvectors
Qn=Q(:,r+1:N); %Get the noise eigenvectors
% MUSIC algorithm
% Define angles at which MUSIC “spectrum” will be computed
angles=(-90:0.1:90); 
%Compute steering vectors corresponding values in angles
a1=exp(-i*2*pi*d*(0:N-1)'*sin([angles(:).']*pi/180)); 
for k=1:length(angles)
 %Compute MUSIC “spectrum”
 music_spectrum(k)=(a1(:,k)'*a1(:,k))/(a1(:,k)'*Qn*Qn'*a1(:,k)); 
end
figure(1)
plot(angles,abs(music_spectrum))
title('MUSIC Spectrum')
xlabel('Angle in degrees')
%ESPRIT Algorithm
phi= linsolve(Qs(1:N-1,:),Qs(2:N,:));
ESPRIT_doas=asin(-angle(eig(phi))/(2*pi*d))*180/pi;
%MVDR
IR=inv(R); %Inverse of covariance matrix
for k=1:length(angles)
 mvdr(k)=1/(a1(:,k)'*IR*a1(:,k));
end
figure(gcf+1)
plot(angles,abs(mvdr))
xlabel('Angle in degrees')
title('MVDR')
%Min norm method
alpha=Qs(1,:);
Shat=Qs(2:N,:);
ghat=-Shat*alpha'/(1-alpha*alpha');
g=[1;ghat];
for k=1:length(angles)
 minnorm_spectrum(k)=1/(abs(a1(:,k)'*g));
end
figure(gcf+1)
plot(angles,abs(minnorm_spectrum))
xlabel('Angle in degrees')
title('Min-Norm')
%Estimate DOA’s using the classical beamformer
for k=1:length(angles)
 Classical(k)=(a1(:,k)'*R*a1(:,k));
end
figure(gcf+1)
plot(angles,abs(Classical))
xlabel('Angle in degrees')
title('Classical Beamformer')

Answer 1

[Rik]

The gcf get the handle to the current figure. In old releases this used to be a double, but since several years ago it has been changed to an object. +1 is defined for double, but not for all objects.

You can probably use the number property to the same effect, but this is fragile programming. You should consider other ways to create new figures.

figure(get(gcf,'Number')+1)

[Sadiq Akbar]

Thank you very much dear Rik.

[Steven Lord]

There's no need to use gcf if you want a new figure. Just call the figure function with no inputs. As stated in the first paragraph of the help text:

help figure

 FIGURE Create figure window.
    FIGURE, by itself, creates a new figure window, and returns
    its handle.
  
    FIGURE(H) makes H the current figure, forces it to become visible,
    and raises it above all other figures on the screen.  If Figure H
    does not exist, and H is an integer, a new figure is created with
    handle H.
 
    GCF returns the handle to the current figure.
 
    Execute GET(H) to see a list of figure properties and
    their current values. Execute SET(H) to see a list of figure
    properties and their possible values.
 
    See also SUBPLOT, AXES, GCF, CLF.

    Documentation for figure
       doc figure

[Rik]

@Steven Lord there are edge cases where you might treat the next n figures differently (e.g. if your first figure is 10, that figures 11, 12 and 13 are related to it, and you want those to change to 21,22,23 if you start with figure 20).

I don't think that is good programming, but that would be a use case.

[Steven Lord]

Suppose you had figure 12 left over from a previous run of your code or some other section of code and you wanted to preserve it (for comparison with a figure in the new collection of figures)?

If you need a collection of related figures I likely would store them as an array of figure handles or maybe in a struct.

f = [figure('Name', 'A'); figure('Name', 'B'); ...
     figure('Name', 'C'); figure('Name', 'D')]

f = 
  4×1 Figure array:

  Figure    (1: A)
  Figure    (2: B)
  Figure    (3: C)
  Figure    (4: D)

g = [figure('Name', 'E'); figure('Name', 'F'); ...
     figure('Name', 'G'); figure('Name', 'H')]

g = 
  4×1 Figure array:

  Figure    (5: E)
  Figure    (6: F)
  Figure    (7: G)
  Figure    (8: H)

f(3).Name

ans = 'C'

g(2).Name

ans = 'F'

When I attempt to retrieve the second figure in g I don't need to know or care whether or not I'd created the array of figure handles f beforehand.