if statement not working

Question

Marisabel Gonzalez 2019-4-5

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/454637-if-statement-not-working

评论： Marisabel Gonzalez 2019-4-5

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

2 个评论
显示无隐藏无

Stephen23 2019-4-5

"if statement not working"

Did you read the IF documentation (to know how it works with non-scalar conditions), before deciding that it is "not working" ?

Rik 2019-4-5

编辑：Rik 2019-4-5

在 MATLAB Online 中打开

@Stephen I think Matlab should throw a warning on non-scalar inputs, maybe even an error. I've many people surprised by the implicit call to all that happens here.

%actually, if does this:
if ~isempty(expression) && all(expression)

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Rik 2019-4-5

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/454637-if-statement-not-working#answer_369235

编辑：Rik 2019-4-5

在 MATLAB Online 中打开

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])