MATLAB: If statement not working

Question

Hi, my following piece of code does not work as it expected to and I don't know how to fix it yet. Any suggestions? Thanks!

A = [5;5;5;3;3;2];
B = [10;4;10;10;4;4];
if A == 5
    plot(A,B,'rx')
    
elseif A == 3
    hold on
    plot(A,B,'bx')
    
else
    hold on
    plot(A,B,'gx')
end

Answer 1

What you probably think should happen is in the code below, however, there is a better way.

A = [5;5;5;3;3;2];
B = [10;4;10;10;4;4];
figure(1)
clf(1),axes;
for n=1:numel(A)
if A(n) == 5
    plot(A(n),B(n),'rx')
elseif A(n) == 3
    hold on
    plot(A(n),B(n),'bx')
else
    hold on
    plot(A(n),B(n),'gx')
end
end
axis([1 6 3 11])

The better way is to use logical indexing:

A = [5;5;5;3;3;2];
B = [10;4;10;10;4;4];
figure(1),clf(1)
already_plotted=false(size(A));
L= A == 5;
plot(A(L),B(L),'rx')
already_plotted(L)=true;
hold on
L= A == 3 & ~already_plotted;
plot(A(L),B(L),'bx')
already_plotted(L)=true;
L= ~already_plotted;
plot(A(L),B(L),'gx')
axis([1 6 3 11])
axis([1 6 3 11])