How to code NESTED CYCLES

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"

[Matt J]

That doesn't seem like a problem.

Answer 1

[Star Strider]

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.

.

Answer 2

[Bruno Luong]

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

[pipor]

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

[Bruno Luong]

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

Answer 3

[Image Analyst]

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

[pipor]

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]

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.