How to print the sum of the middle array's columns ?

Here is an past exam question

a) read the dimensions of the matrix from the user and prompt the user for the data needed to fill the array (10 marks)

Sample Output:

How many rows?: 2How many columns?: 3

Enter the values for row 1, column 1: 5

Enter the values for row 1, column 2: 4

Enter the values for row 1, column 3: 6

Enter the values for row 2, column 1: 1

Enter the values for row 2, column 2: 2

Enter the values for row 2, column 3: 3

b) sum the elements of the middle column. The MATLAB function sum() must not be used in your solution (10 marks)

Sample Output: Given A = [ 5, 4, 6, 7, 3 ; 1, 2, 3, 4, 5 ; 5, 6, 4, 2, 4 ; 4, 5, 3, 2, 1]

The sum of all the elements of the middle column is: 16

Here is my code.

% (a)

% create two variables to store the number of rows and columns

numRow = input('How many rows?: ');

numCol = input('How many Cols?: ');

% create a variable as the empty vector

P = [];

for r = 1:numRow

for c = 1:numCol

fprintf('Enter the value for row %d column %d',r,c);

P(r,c) = input(': ');

end

end

% (b)

% ensure the number of row and col of P

[rows,cols] = size(P);

% create an variable sumMiddle

sumMiddle = 0;

for r = 1:rows

for c = 1:cols

if (mod(c,3) == 0)

sumMiddle = sumMiddle + P(r,c); % add all the elements together

end

end

end

fprintf('The sum of all the elements of the middle column is: %d ',sumMiddle);

The comment of the test result is that part 2 does not work properly. How can I improve the code of Part (b)?

Thank you.

[dpb]

You don't need a double loop although the above code should compute the right number -- how can you calculate the middle column number and then loop over just the rows for only the one column?

[Shuoze Xu]

I think should use round() here, such as middle_column = round(cols/2)

[dpb]

Not best algebra/numerics, no...think some more...there's a direct way with no rounding required to compute the middle value (if total number of columns is odd, of course)

Answer 1

[Varun]

Hi,

I understand that you are trying to calculate the sum of middle column of a matrix (odd) without using “sum” function. You gave one solution where you have used 2 nested loops, time complexity is O(r*c).

You can achieve this task using single loop in time complexity O(r). Also, you can find the middle column index without using round. Refer the following code:

middle_column = (cols+1)/2;
middleColumnSum = 0;
  
for i = 1:rows
    middleColumnSum = middleColumnSum + matrix(i, middle_column);
end