Can anyone help? I cannot figure out how to use a for loop to check elements in part of a matrix.

Create a 5x4 matrix of random integers between -10 and 10 with the randi function. Write a for loop that will check each element in column 1 of the matrix. If that element is less than 0, change every element in that row to NaN.

I can make the actual matrix, but after that I don't know what comes next.

r=randi([-10,10],5,4)

Answer 1

[Stephen23]

"I can make the actual matrix, but after that I don't know what comes next."

Your assignment tells you exactly what you need to do. Fill in the ??? yourself:

for k = 1:size(???)  % loop over all row indices
    if r(k,1)<???    % check if first column value is <0
        r(k,:)=???;  % change entire row to NaN
    end
end

[Samantha Tomkowich]

Thank you so much! I was doing well with matlab until I got to for loops, now I feel like a bit of an idiot. Thank you for your help!!

[Stephen23]

@Samantha Tomkowich: I strongly recommend doing the introductory tutorials:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

https://www.mathworks.com/learn/tutorials/matlab-onramp.html

Answer 2

[Bhaskar R]

No need of for loop

r=randi([-10,10],5,4);
if any(r(:,1)<0)
    r(1,:) = nan;
end

[Stephen23]

The question states that "If that element is less than 0, change every element in that row to NaN", but this code always changes the first row to NaN, not the row/s where the negative values were found in the first column.

It is not clear how if is supposed to provide the requested behavior.

[Bhaskar R]

I am sorry, I misunderstood the question. I thought like there is to change row with respect to given column number.

Answer 3

[Samantha Tomkowich]

Thank you! That got me exactly what I was looking for answer wise, unfortunately the assignment needs me to use a for loop to get the same answer. But thank you!

[Bhaskar R]

It is not recommended in general programming, anyway

r=randi([-10,10],5,4);
for row = 1:size(r, 1)
    if r(1, row) <0
        for col = 1:size(r, 2)
            r(1, col) = nan;
        end
        break;
    end
end

[darova]

Why are you considering only first row?

[Samantha Tomkowich]

how would I consider more than one row?

[Stephen23]

Why so complex?