Use forLoop to manipulate values of elements in a specific column

I created the following 5x5 matrix with no data:

mat = zeros(5:5)

mat =

       0     0     0     0     0
       0     0     0     0     0
       0     0     0     0     0
       0     0     0     0     0
       0     0     0     0     0

I would like to use a for loop to generate the following result

mat =

       0     0     0     0      0
       0     0     0     10     0
       0     0     0     20     0
       0     0     0     30     0
       0     0     0     40     0

I attempted using the following code, but the operation was performed on the first column - not the 4th, as I had intended.

for i = 2:length(mat(:,4))
mat(i) = mat(i-1) + 10;
end

This was the actual result

mat =

       0     0     0     0     0
      10     0     0     0     0
      20     0     0     0     0
      30     0     0     0     0
      40     0     0     0     0

I imagine this is a simple syntax problem, but I am not finding the solution in any of the forums. Any help is appreciated.

Thanks

Answer 1

[Star Strider]

This works:

mat = zeros(5:5);
mat(:,4) = 0:10:10*(size(mat,1)-1);

mat =
     0     0     0     0     0
     0     0     0    10     0
     0     0     0    20     0
     0     0     0    30     0
     0     0     0    40     0

[JZ]

Thanks for the solution, but it is slightly different than I needed.

I was actually after a way to subtract a constant value from all cells in the 6th column of of a 32x7 matrix. My example (of a 5x5 zeros) was for simplicity sake.

I figured out a solution based on your previous answers - a solution that allows me to progressively add 10 to each cell of a column, but also allows me to subtract or perform any other operation (which is why I was working with a for loop):

for i=2:length(mat)
mat(i,4) = mat(i,4) + 10;
end

I am still trying to understand the syntax of your proposed solution:

mat(:,4) = 0:10:10*(size(mat,1)-1);

How could I use that same structure to subtract a constant value from the cells of column 4?

[Star Strider]

If you want to subtract a constant value from the elements in column 4, it is almost as easy:

mat(:,4) = mat(:,4) - pi

giving:

mat =
            0            0            0      -3.1416            0
            0            0            0      -3.1416            0
            0            0            0      -3.1416            0
            0            0            0      -3.1416            0
            0            0            0      -3.1416            0

MATLAB automagically expands the value (here pi) to the dimensions needed to subtract it from every element in the column, vector, or matrix, as necessary.

Parenthetically, the values in a vector or matrix are referred to as ‘elements’. A cell is a particular MATLAB data type (see the documentation on Cell Arrays for details) and using ‘cell’ to mean ‘element’ can lead to confusion here on MATLAB Answers. Please understand that’s a clarification, not a criticism!

[JZ]

That is indeed a much simpler solution - thank you!

And thanks for the clarification on cell/element. No offense taken.

[Star Strider]

My pleasure!

Answer 2

[lsutiger1]

You have only specified the row that you want to manipulate; you need to specify both the row and column.

mat(i) = mat(i-1,4) + 10;

[Walter Roberson]

Perhaps you mean

mat(i,4) = mat(i-4,4) + 10;

?

[lsutiger1]

Yep. I left a comment on his question, rather than an answer, so as I was writing an actual answer I missed it.

Life of a coder.

[JZ]

Thanks, but I am still getting errors with both of your suggestions:

mat = zeros(5:5)

mat =

       0     0     0     0     0
       0     0     0     0     0
       0     0     0     0     0
       0     0     0     0     0
       0     0     0     0     0

mat(i,4) = mat(i-4,4) + 10;
Index exceeds matrix dimensions.

mat(i) = mat(i-1,4) + 10;
Index exceeds matrix dimensions.

Suggestions?

[lsutiger1]

That's because you used i-4 rather than i-1.