# Can parallel traversals be done in MATLAB just as in Python?

Using the `zip` function, Python allows for loops to traverse multiple sequences in parallel.

`for (x,y) in zip(List1, List2):`

Does MATLAB have an equivalent syntax? If not, what is the best way to iterate over two parallel arrays at the same time using MATLAB?

Asked by: Ryan906 | Posted: 27-01-2022

If x and y are column vectors, you can do:

``````for i=[x';y']
# do stuff with i(1) and i(2)
end
``````

(with row vectors, just use `x` and `y`).

Here is an example run:

``````>> x=[1 ; 2; 3;]

x =

1
2
3

>> y=[10 ; 20; 30;]

y =

10
20
30

>> for i=[x';y']
disp(['size of i = ' num2str(size(i)) ', i(1) = ' num2str(i(1)) ', i(2) = ' num2str(i(2))])
end
size of i = 2  1, i(1) = 1, i(2) = 10
size of i = 2  1, i(1) = 2, i(2) = 20
size of i = 2  1, i(1) = 3, i(2) = 30
>>
``````

Answered by: Kellan271 | Posted: 28-02-2022

Tested only in octave... (no matlab license). Variations of arrayfun() exist, check the documentation.

``````dostuff = @(my_ten, my_one) my_ten + my_one;

tens = [ 10 20 30 ];
ones = [ 1 2 3];

x = arrayfun(dostuff, tens, ones);

x
``````

Yields...

``````x =

11   22   33
``````

Answered by: Emma433 | Posted: 28-02-2022

If I'm not mistaken the zip function you use in python creates a pair of the items found in list1 and list2. Basically it still is a for loop with the addition that it will retrieve the data from the two seperate lists for you, instead that you have to do it yourself.

So maybe your best option is to use a standard for loop like this:

``````for i=1:length(a)
c(i) = a(i) + b(i);
end
``````

or whatever you have to do with the data.

If you really are talking about parallel computing then you should take a look at the Parallel Computing Toolbox for matlab, and more specifically at parfor

Answered by: First Name849 | Posted: 28-02-2022

I would recommend to join the two arrays for the computation:

``````% assuming you have column vectors a and b
x = [a b];

for i = 1:length(a)
% do stuff with one row...
x(i,:);
end
``````

This will work great if your functions can work with vectors. Then again, many functions can even work with matrices, so you wouldn't even need the loop.

Answered by: Leonardo150 | Posted: 28-02-2022

``````for (x,y) in zip(List1, List2):
``````

should be for example:

``````>> for row = {'string' 10
>>           'property' 100 }'
>>    fprintf([row{1,:} '%d\n'], row{2, :});
>> end
string10
property100
``````

This is tricky because the cell is more than 2x2, and the cell is even transposed. Please try this.

And this is another example:

``````>> cStr = cell(1,10);cStr(:)={'string'};
>> cNum=cell(1,10);for cnt=1:10, cNum(cnt)={cnt};
>> for row = {cStr{:}; cNum{:}}
>>    fprintf([row{1,:} '%d\n'], row{2,:});
>> end
string1
string2
string3
string4
string5
string6
string7
string8
string9
string10
``````

Answered by: Lenny184 | Posted: 28-02-2022

If I have two arrays al and bl with same dimension No 2 size and I want to iterate through this dimension (say multiply `al(i)*bl(:,i)`). Then the following code will do:

``````al = 1:9;
bl = [11:19; 21:29];

for data = [num2cell(al); num2cell(bl,1)]

[a, b] = data{:};
disp(a*b)

end
``````

Answered by: Lana622 | Posted: 28-02-2022

`for` loops in MATLAB used to be slow, but this is not true anymore.

So vectorizing is not always the miracle solution. Just use the profiler, and `tic` and `toc` functions to help you identify possible bottlenecks.

Answered by: Emma387 | Posted: 28-02-2022

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