MATLAB: Find the sum of the non perimeter elements of a matrix using a for loop

loopmatrixnon perimetersum

I have this matrix

 B=[-3 4 7;9 11 -13;-17 19 -23;-29 31 37;39 41 47]

and I need to find the sum of the non perimeter elements using a for loop. I can find the sum of a single column or row or of the whole matrix, but I don't know how to write a loop to only sum the 3 non perimeter numbers. Any help is appreciated, thank you.

Best Answer

Try it this way:

B=[-3 4 7;9 11 -13;-17 19 -23;-29 31 37;39 41 47]
[rows, columns] = size(B)
theSum = 0;
for col = 2 : columns - 1
  for row = 2 : rows-1
    theSum = theSum + B(row, col);
  end
end
fprintf('The sum = %f\n', theSum);

Related Solutions

MATLAB: I need to know how to sum to the values of a specific column (identified by an index that can vary) of a generic matrix, a given column vector. For instance, how i decrease by 1 all the values of a column in a matrix (the rest remains the same)

Your question is rather confusing, as you seem to pose two different questions. I address both of these below:

>> A = magic(10)
A =
    92    99     1     8    15    67    74    51    58    40
    98    80     7    14    16    73    55    57    64    41
     4    81    88    20    22    54    56    63    70    47
    85    87    19    21     3    60    62    69    71    28
    86    93    25     2     9    61    68    75    52    34
    17    24    76    83    90    42    49    26    33    65
    23     5    82    89    91    48    30    32    39    66
    79     6    13    95    97    29    31    38    45    72
    10    12    94    96    78    35    37    44    46    53
    11    18   100    77    84    36    43    50    27    59
>> col = 6;
>> sum(A(:,col)) % the sum of one column
ans =  505
>> A(:,col) = A(:,col)-1 % decrease all values in one column
A =
    92    99     1     8    15    66    74    51    58    40
    98    80     7    14    16    72    55    57    64    41
     4    81    88    20    22    53    56    63    70    47
    85    87    19    21     3    59    62    69    71    28
    86    93    25     2     9    60    68    75    52    34
    17    24    76    83    90    41    49    26    33    65
    23     5    82    89    91    47    30    32    39    66
    79     6    13    95    97    28    31    38    45    72
    10    12    94    96    78    34    37    44    46    53
    11    18   100    77    84    35    43    50    27    59

MATLAB: I have this error msg (Matrix index is out of range for deletion. Error in tray1 (line 31) x1(:,rows) = []; ) what is the problem ???

You are deleting columns of your matrix x1 in the loop, so its size has changed (moreover, you skip some columns).

To avoid this, you can store the index of the column you want to delete in the loop and delete them only when the loop is finished:

colToDelete = [];
[rows, columns] = size(x1); 
for col = 1 : columns
    sum = 0;
    for row = 1 : rows % Now get the mean over all values in this column.
        columnMeans(col) = sum / rows;
        if columnMeans(col)> avgcut || columnMeans(col)< avgcut
            colToDelete = [colToDelete,col];
        end
    end
end
x1(:,colToDelete) = [];

Best Answer

Related Solutions

MATLAB: I need to know how to sum to the values ​​of a specific column (identified by an index that can vary) of a generic matrix, a given column vector. For instance, how i decrease by 1 all the values ​​of a column in a matrix (the rest remains the same)

MATLAB: I have this error msg (Matrix index is out of range for deletion. Error in tray1 (line 31) x1(:,rows) = []; ) what is the problem ???

Related Question

MATLAB: I need to know how to sum to the values of a specific column (identified by an index that can vary) of a generic matrix, a given column vector. For instance, how i decrease by 1 all the values of a column in a matrix (the rest remains the same)