# Convert matrix into CSV, with each matrix element as a separate line including indices of matrix element

2 views (last 30 days)
Daniel on 17 Sep 2022
Commented: Star Strider on 18 Sep 2022
Struggling with writematrix and having to resort to odd things like appending repeating vectors to achieve this... I am sure there must be an easier way. The size of the matrix is such that for loops etc are terribly slow.
I have a matrix like this:
0.5 0.1 0.6
0.9 0.8 0.3
I'd like to output this as a CSV in the format x, y, e - where x and y are the column and row numbers and e is the element, to give something like this:
1, 1, 0.5
2, 1, 0.1
3, 1, 0.6
1, 2, 0.9
2, 2, 0.8
3, 2, 0.3

Star Strider on 17 Sep 2022
Try something like this —
M = [0.5 0.1 0.6
0.9 0.8 0.3];
[r,c] = ndgrid(1:size(M,1), 1:size(M,2));
rv = reshape(r.',[],1);
cv = reshape(c.',[],1);
Mv = reshape(M.',[],1);
A = [cv rv, Mv]
A = 6×3
1.0000 1.0000 0.5000 2.0000 1.0000 0.1000 3.0000 1.0000 0.6000 1.0000 2.0000 0.9000 2.0000 2.0000 0.8000 3.0000 2.0000 0.3000
Then use the writematrix function with a .csv suffix on the file name.
.
##### 2 CommentsShowHide 1 older comment
Star Strider on 18 Sep 2022
As always, my pleasure!

Stephen23 on 17 Sep 2022
Edited: Stephen23 on 17 Sep 2022
M = [0.5,0.1,0.6;0.9,0.8,0.3]
M = 2×3
0.5000 0.1000 0.6000 0.9000 0.8000 0.3000
N = M.';
S = size(N);
[X,Y] = ndgrid(1:S(1),1:S(2));
writematrix([X(:),Y(:),N(:)],'test.txt')
Checking:
type test.txt
1,1,0.5 2,1,0.1 3,1,0.6 1,2,0.9 2,2,0.8 3,2,0.3
##### 2 CommentsShowHide 1 older comment
Stephen23 on 17 Sep 2022
Edited: Stephen23 on 17 Sep 2022
@Dyuman Joshi: thanks, fixed now. That neatens things up too.

Walter Roberson on 17 Sep 2022
There are two possibilities here.
If you need every location output because in practice the data will be read as a vector of numbers and reshaped under the assumption that it is complete and in standard order, then use techniques like what Star Strider showed.
But if the individual coordinates are actively used then
[r, c, s] = find(YourMatrix);
writematrix([r, c, s], filename);
this will not write any zeros.
You would recover the matrix by reading as the columns as variables and passing them to sparse()

Dyuman Joshi on 17 Sep 2022
You can try this -
y=[0.5 0.1 0.6;0.9 0.8 0.3];
z=y';
mat=[repmat(1:size(y,2),1,size(y,1))' repelem(1:size(y,1),1,size(y,2))' z(:)]
mat = 6×3
1.0000 1.0000 0.5000 2.0000 1.0000 0.1000 3.0000 1.0000 0.6000 1.0000 2.0000 0.9000 2.0000 2.0000 0.8000 3.0000 2.0000 0.3000
And use writematrix() as Star Strider mentioned.

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