A for loop that counts the negatives of a 1D array

Question

Eric Brown 2022-4-19

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https://ww2.mathworks.cn/matlabcentral/answers/1700215-a-for-loop-that-counts-the-negatives-of-a-1d-array

回答： Vatsal 2023-11-17

My function is to recieve a 1D array of numbers as the input and uses a for loop to find the location of all negatives in the array and return it as the output. If given [-2 3 0 -5 4] the output would be [1 4]. I'm not sure what to do in order for it to count, does y have to be a array before the for loop? If so, how do I add my indexing variable to y inside the for loop?

function y = WhereNeg(x)
[xrow, xcol] = size(x);
for ii = 1:xcol
    if x(ii) < 0
        y = ii;
    end
end

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Eric Brown 2022-4-19

Exactly so does that mean y has to be an array before the for loop adn if so how do I keep ii in y without updating ii during each loop?

Riccardo Scorretti 2022-4-19

Dear Eric, this look like an assigment (= you have to do, not us) so I'll give you some hints, but not the solution.

your function accepts as input argument a vector, and it is expected to return a vector: in your code, it returns a scalar (because of the line y = ii). A possible, but very inefficient way to fix it, would be to define y as an empty vector before entering the loop, and then to put new values (= ii) at the end of y. Remember that you can access to the last element of a vector by using the syntax y(end), so y(end+1) would be the next unoccupied position. I would like to stress that this is a very inefficient way to do.
In Matlab, normally this kind of job can be done in a single line by using the function find (read the doc). Your program is written in a kind of C style.
I strongly suggest you to use numel to get the number of elements of a vector, not size. The point is that your code assumes that x is a row-vector, which is an unnecessary hypothesis.
It is a good practice (at least, I consider that it is) to close all functions by end, so I advice you to add an ending keyword end to close your function.

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Answer 1

Vatsal 2023-11-17

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Hi,

In your current function, the variable 'y' is being overwritten each time a negative number is found. To store all the indices of negative numbers, you should initialize 'y' as an empty array before the loop and then append to it each time a negative number is found.

Here's how you can modify your function:

function y = WhereNeg(x) 
    y = []; % Initialize y as an empty array 
    [xrow, xcol] = size(x); 
    for ii = 1:xcol 
        if x(ii) < 0 
            y = [y ii]; % Append the index to y 
        end 
    end 
end

In this modified function, 'y' is an array that gets populated with the indices of negative numbers in 'x'. The line ‘y = [y ii]’ appends the current index 'ii' to the array 'y' when a negative number is found. To count the number of negative numbers, you can simply use the above code and return the length of 'y'.

I hope this helps!