How to code NESTED CYCLES

Question

pipor 2023-9-9

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2018826-how-to-code-nested-cycles

评论： Image Analyst 2023-9-10

f=[3 6 3 9]
b=[5 8 10 12]

a is a function with parameter f and b!

for l=1:numel(b)
for i=1:numel(f)
  a(f(i),b(l))        
end
end

but this is a problem:

if b is empty the function won't loop me with "f"

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Matt J 2023-9-9

That doesn't seem like a problem.

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Answer 1

Star Strider 2023-9-9

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https://ww2.mathworks.cn/matlabcentral/answers/2018826-how-to-code-nested-cycles#answer_1306081

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I would set the empty array to 1 (so that it iterates one time only) and be done with it —

a = @(x,y) [x y];
f = [3 6 3 9];                      % Neither Empty
b = [5 8 10 12];
if isempty(f)
    f = NaN;
elseif isempty(b)
    b = NaN;
end
for l=1:numel(b)
    for i=1:numel(f)
        q = a(f(i),b(l))        
    end
end
q = 1×2
     3     5
q = 1×2
     6     5
q = 1×2
     3     5
q = 1×2
     9     5
q = 1×2
     3     8
q = 1×2
     6     8
q = 1×2
     3     8
q = 1×2
     9     8
q = 1×2
     3    10
q = 1×2
     6    10
q = 1×2
     3    10
q = 1×2
     9    10
q = 1×2
     3    12
q = 1×2
     6    12
q = 1×2
     3    12
q = 1×2
     9    12
f = [];                             % 'f' Empty
b = [5 8 10 12];
if isempty(f)
    f = NaN;
elseif isempty(b)
    b = NaN;
end
for l=1:numel(b)
    for i=1:numel(f)
        q = a(f(i),b(l))        
    end
end
q = 1×2
   NaN     5
q = 1×2
   NaN     8
q = 1×2
   NaN    10
q = 1×2
   NaN    12
f = [3 6 3 9];                      % 'b' Empty
b = [];
if isempty(f)
    f = NaN;
elseif isempty(b)
    b = NaN;
end
for l=1:numel(b)
    for i=1:numel(f)
        q = a(f(i),b(l))        
    end
end
q = 1×2
     3   NaN
q = 1×2
     6   NaN
q = 1×2
     3   NaN
q = 1×2
     9   NaN

The ‘default’ value (here ‘NaN’) can be any value that makes sense in the context of whatever ‘a’ does. If a given argument is added, then set it to the identity element for that operation, so if it is added set it to 0 if the associated vector is empty, if it is multiplied, set it to 1.

.

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Answer 2

Bruno Luong 2023-9-9

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Reverse the 2 loops, then it loops on f

f=[3 6 3 9]
f = 1×4
     3     6     3     9
b=[]
b =

     []
for i=1:numel(f)
    f(i)
    for l=1:numel(b)
        a(f(i),b(l))        
    end
end
ans = 3
ans = 6
ans = 3
ans = 9

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pipor 2023-9-9

I wrote the code in general but I don't know which array is empty..it can be f or b

Bruno Luong 2023-9-9

编辑：Bruno Luong 2023-9-9

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Split in three loops if you have to

for i=1:numel(f)
    for l=1:numel(b)
        % ... do something with both f(i) and b(l)
        a(f(i),b(l))        
    end
end
for i=1:numel(f)
    % ... do something with f(i) ALONE
end
for l=1:numel(b)
    % ... do something with b(l)  ALONE
end

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Answer 3

Image Analyst 2023-9-9

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Check them in advance:

if isempty(b)
    % What to do
    return;
end
if isempty(f)
    % What to do
    return;
end
for l=1:numel(b)
    for i=1:numel(f)
     something = a(f(i),b(l))        
    end
end

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pipor 2023-9-9

移动：Dyuman Joshi 2023-9-9

在 MATLAB Online 中打开

if b=[] it don't execute cycle for

b=[];
f=[3 4 5];
if isempty(b)
    % What to do
    return;
end
if isempty(f)
    % What to do
    return;
end
for l=1:numel(b)
    for i=1:numel(f)
        fprintf("execure cycle \n");      
    end
end

Image Analyst 2023-9-10

Correct. Why would you want to continue if one is empty? If you do you'll just get an error. If you can "fix" the situation, do so inside the if -- for example Star set them to nan instead. If you can't fix the situation, you should just exit the code after alerting the user.

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