Iterating over the sum of values using a loop

Question

I have a row matrix as follows

a=[1 2 3];

The for loop has been implemented as follows

 for i=1:a(1,1:size(a,2))
    disp(i);
    disp("Hello");
end

Based on the values of the row matrix Hello has to be printed 6 times(i.e 1+2+3) but it gets printed only once. How can I iterate along the row matrix and get Hello printed 6 times?

Adriaan · Accepted Answer · 2018-04-04 07:27:04Z

4

a=[1 2 3];
for ii=1:sum(a)
    disp("Hello")
end

1:a(1,1:size(a,2)) == 1:a(1,[1 2 3]) == 1:a(1,1) == 1:1 == 1 actually creates an array containing the number 1 (more specific: a(1), as 1:[1 2 3] will evaluate to 1:1, discarding all elements further on in the vector). Given the number 6 you mention I take it you want the sum of all elements in a which is given by sum.

Final word of caution: please refrain from using i and j as variable names, as they also denote the imaginary unit.

Reading your comment you probably need a nested loop, as the entries of a might not be monotonically increasing:

k = 1; % counter to show which iteration you're in
for ii = 1:numel(a) % for all elements of a do
    for jj = 1:a(ii) % do the following a(ii) times
        disp('Iteration %d', k)
        disp('Hello')
        k = k+1; % increase iteration count
    end
end

Note that both methods fail (obviously) when a does not contain strictly non-negative integer values.

edited Apr 4, 2018 at 7:27

answered Mar 29, 2018 at 13:02

Adriaan

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3 Comments

Kushan Peiris Over a year ago

not the sum I want to iterate it once, then twice and then thrice since I may need to implement such for loops in future. so how can I iterate in that manner?

Adriaan Over a year ago

Iterate once, then twice, then three times means you want to iterate on sum(a). I don't see what the problem is in that case?

Adriaan Over a year ago

@KushanPeiris I think I solved your problem, see the edit, but this smells of an XY problem so I think you're better of asking a new question with the actual code instead of an oversimplified example, so we can take a look whether the approach you're currently using is a good one (I suspect not, as this is a very contrived usage of loops).

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