Adding valies from a table into a zero matrix

2016 4 16
2 个回答
更新时间：2021 8 20
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0 个投票

Hello,
I've created a zero matrix array of the size that I need, and I want to add values from a table to it.
My table is in an excel spreadsheet with three columns. Column 1 denotes the x co-ordinate, column 2 the y co-ordinate, and column 3 the value I want to add. I'm trying to create a loop that will allow me to find the matrix position (x, y) as specified in the table, and then add the third column value to that position only.
I know roughly how to specify positions in each, but not how to then say, 'use the numbers in here to find the position in the array'.
If my matrix is X and table is A, I'm thinking the code needs to be similar to:
i=1
b=A[i,1]
c=A[i,2]
d=A[i,3]
X[b,c] + d
i=i+1
Any help on how to smooth this out would be fantastic as it's been forever since I did anything with tables and matrixes other than pulling values out.
Cheers.

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Azzi Abdelmalek
Azzi Abdelmalek 2016-4-16
Make your question clear and brief. Have you a problem to read your Excel file? If not, how your are data are stored? like this ?
M=[1 2 3;
4 5 6;
7 8 9]
Then What is the expected result from M?
Maeve
Maeve 2016-4-16
编辑：Azzi Abdelmalek 2016-4-16
My excel file has read in fine an example is:
M=[8 26 240; 39 6 52; 600 530 211]
I want to use this excel file to find X(8,26) and add 240 to that 0, then find X(39,6) and add 52 to that zero.
But to use a loop to specify the x coordinates based off of the M table as I have hundreds of these co-ordinates to find.

回答（2 个）

Azzi Abdelmalek
Azzi Abdelmalek 2016-4-16

0 个投票

M=[8 26 240; 39 6 52; 600 530 211]
idx=ismember(M(:,1:2),[8,26],'rows')
out=M(idx,3)

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Maeve
Maeve 2016-4-16
编辑：Azzi Abdelmalek 2016-4-16
Thank you, that's helpful but I don't think it's going to solve my problem. I'm looking at how to use the table to change my zero array.
What I want to do is take
X=[0 0 0; 0 0 0; 0 0 0]
Use
A=[1 2 12; 1 3 8; 2 2 5; 3 3 16]
To turn X into
out=[0 12 8; 0 5 0; 0 0 16].
Azzi Abdelmalek
Azzi Abdelmalek 2016-4-16
Ok, but you need to explain how to get out from X and A
Maeve
Maeve 2016-4-16
That is what I need help with, as I said in my original question.
A specifies co-ordinates in X and a value with which to replace that zero, which will then give me the modified X Out.
Azzi Abdelmalek
Azzi Abdelmalek 2016-4-16
We can't provide the criterion at your place. From the example, explain how to get the result, then we can help you to code it
Maeve
Maeve 2016-4-16
If I have A [1 2 12] (1,2) refers to a co-ordinate that I want to find in X, 12 refers to the number I want to add to the zero at X(1,2).
I don't know any other way to explain what I'm looking to do.
Azzi Abdelmalek
Azzi Abdelmalek 2016-4-16
编辑：Azzi Abdelmalek 2016-4-16
X=zeros(3)
A=[1 2 12; 1 3 8; 2 2 5; 3 3 16]
idx=sub2ind(size(X),A(:,1),A(:,2))
X(idx)=A(:,3)
Maeve
Maeve 2016-4-16
That looks like it could work, cheers! I'll give it a try.
Guillaume
Guillaume 2016-4-16

0 个投票

A loop is totally overkill for this. There are two simple ways of obtaining your final output:
1. sub2dind which will convert two vectors of row, column coordinates into a vector of linear indices. You then use this linear index to directly assign all the values in one go:
X = zeros(3, 3);
X(sub2ind(size(X), A(:, 1), A(:, 2)) = A(:, 3);
2. Create a sparse matrix with your vectors of coordinates and vector of values and convert to full matrix:
%no need for zero
X = full(sparse(A(:, 1), A(:, 2), A(:, 3), 3, 3));

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Maeve
Maeve
2016-4-16

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