Use forLoop to manipulate values of elements in a specific column

JZ
2015 12 5
2 个回答
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更新时间：2015 12 6
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0 个投票

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

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Star Strider
Star Strider 2015-12-5

1 个投票

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

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JZ
JZ 2015-12-5
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
Star Strider 2015-12-5
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
JZ 2015-12-5
编辑：JZ 2015-12-5
That is indeed a much simpler solution - thank you!
And thanks for the clarification on cell/element. No offense taken.

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更多回答（1 个）

lsutiger1
lsutiger1 2015-12-5

0 个投票

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;

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lsutiger1
lsutiger1 2015-12-5
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
JZ 2015-12-5
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
lsutiger1 2015-12-6
That's because you used i-4 rather than i-1.

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