MATLAB: Summing nonsequential elements in a matrix

matrixsortsum

Sorry if this is a very simple question, but how would I go about finding the sum of a set of predetermined elements in a matrix. So if I was using

a=magic(50),

I found that my idea of using the command

x = sum(a, (1, [5 7 19 33 34 35 36 47 50]))

does not provide the output (the sum of the elements in those positions) that I wanted. I would appreciate any help.

Best Answer

magic(50) is a square, 50x50 matrix. What do you want to achieve? Is it summing elements [5 7 19 33 34 35 36 47 50] of row 1 of a? If so, you'll want to do

 s = sum( a(1,[5 7 19 33 34 35 36 47 50]) )

If you are unsure, work with smaller objects that can be easily displayed, e.g.

 >> a = magic(5)
 a =
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9
 >> s = sum( a, (1, [2,4,5]))        % First idea, is it right?
 s = sum( a, (1, [2,4,5]))
              |
 Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.
 >> a(1,[2,4,5])                     % Let's see if we can at least access the 
                                     % correct set of elements before summing.
 ans =
    24     8    15
 >> s = sum( a(1,[2,4,5]) )          % Let's see if we can sum them now.
 s =
    47

Related Solutions

MATLAB: Adding two matrixes with different row numbers.

A = [1,2,3,4,5,6,7,8,9,10; 10,11,12,13,14,15,16,17,18,19; 19,20,21,22,23,24,25,26,27,28];
B = [1,2,3,4,5,6,7,8,9,10; 2,4,6,8,10,12,14,18,18,20];
[Y,X] = ndgrid(1:size(B,1),1:size(A,1));
out = A(X(:),:) + B(Y(:),:)
out = 6×10
     2     4     6     8    10    12    14    16    18    20
     3     6     9    12    15    18    21    26    27    30
    11    13    15    17    19    21    23    25    27    29
    12    15    18    21    24    27    30    35    36    39
    20    22    24    26    28    30    32    34    36    38
    21    24    27    30    33    36    39    44    45    48

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

Best Answer

Related Solutions

MATLAB: Adding two matrixes with different row numbers.

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)

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)