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How to add value at end of row in a matrix

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etudiant_is
etudiant_is 2016-4-11
编辑： Stephen23 2016-4-12
It is a very basic question but I could not figure how to do it without a for loop.
I have two vectors R and C of same length
R'=[2 4 5 2]
C'=[1 3 6 7]
I want to have
A=
[0 0
1 7
0 0
3 0
6 0]
Basically R is for the rows and I want to have the content of C in the corresponding row in matrix A
If I do
A(R)=C
it is going to be a vector not a matrix and the newest value will erase the oldest value in each row
A(R,:) will assign the value to the whole line which is not also what I want.
how can I increase the index of the column if there is more than one entry for each row?
  1 个评论
Muhammad Usman Saleem
Muhammad Usman Saleem 2016-4-11
"Basically R is for the rows and I want to have the content of C in the corresponding row in matrix A"
What is meany by R is for rows, is this means you want A matrix rows as no of rows in R? and remaining portion of line is totally unable to understand ,plz

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采纳的回答

Stephen23
Stephen23 2016-4-11
编辑：Stephen23 2016-4-11
Without any loops:
R = [2;4;5;2];
C = [1;3;6;7];
[~,X] = sort(R);
[~,~,Y] = unique(R);
Z = accumarray(Y,ones(size(R)),[],@(v){v});
Y(X) = cell2mat(cellfun(@cumsum,Z,'UniformOutput',false));
A = zeros(max(R),max(Y));
A(sub2ind(size(A),R,Y)) = C
creates this output:
A =
0 0
1 7
0 0
3 0
6 0
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Stephen23
Stephen23 2016-4-11
编辑：Stephen23 2016-4-12
@etudiant_is: here is the code with comments to help you:
% sort the values of R:
[~,X] = sort(R);
% get indices/groups of unique values in R:
[~,~,Y] = unique(R);
% collect each group's values into a vector in a cell array:
Z = accumarray(Y,ones(size(R)),[],@(v){v});
% create consecutive column indices for each value in each group:
Y(X) = cell2mat(cellfun(@cumsum,Z,'UniformOutput',false));
% preallocate an output matrix of the correct size:
A = zeros(max(R),max(Y));
% use linear indices to allocate the original values into the output matrix:
A(sub2ind(size(A),R,Y)) = C
And in particular the lines that you ask about. The vector Y contains a list of unique groups/rows. For your example this is
>> [~,~,Y] = unique(R)
Y =
1
2
3
1
We use these group indices to collect some 1's together in a cellarray, they will be later converted into the column indices:
>> Z = accumarray(Y,ones(size(R)),[],@(v){v});
>> Z{:}
ans =
1
1
ans =
1
ans =
1
To convert these vectors of 1's into consecutive column indices we use cumsum, which has to be encapsulated in a cellfun because the groups of ones are held in a cell array:
>> tmp = cellfun(@cumsum,Z,'UniformOutput',false);
>> tmp{:}
ans =
1
2
ans =
1
ans =
1
These are joined together using cell2mat to give one numeric vector:
>> cell2mat(tmp)
ans =
1
2
1
1
Now we have to right column indices, but in the wrong order. We can use the sort index X to put them back into the correct order:
>> Y(X) = cell2mat(tmp)
Y =
1
1
1
2
I reused the variable Y because this allow us to keep its shape (column vector), otherwise a reshape would be required. Now we have a list of row indices R and column indices Y. These can be used with sub2ind to get the linear indices, and these then used to allocate the value allocated to the output matrix.
The @ is used to define a function handle, which allows the function to be passed around like a variable. This is quite handy!
etudiant_is
etudiant_is 2016-4-11
Thanks for the detailed explanation. I will take my time to understand it.

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更多回答（2 个）

Andrei Bobrov
Andrei Bobrov 2016-4-11
R=[2 4 5 2]
C=[1 3 6 7]
[b,ii] = histc(R(:),unique(R));
out = zeros(max(R),max(b));
for jj = 1:numel(b)
out(R(find(jj==ii,1,'first')),1:b(jj)) = C(jj==ii);
end
Muhammad Usman Saleem
编辑：Muhammad Usman Saleem 2016-4-11
I unable to understand what you want in your final matrix.
First see if you wanting column vectors these are not correct format in matlab, see it below
>> R'=[2 4 5 2]
C'=[1 3 6 7]
output
R'=[2 4 5 2]
|
Error: The expression to the left of the equals sign is not a
valid target for an assignment.
Now try this if you want column vectors
>> R=[2 4 5 2]'
R =
2
4
5
2
>> C=[1 3 6 7]'
C =
1
3
6
7
Now
>> A(R)=C
A =
0 7 0 3 6
  1 个评论
etudiant_is
etudiant_is 2016-4-11
For the transpose, I just wanted to write the vector here horizontally to be more clear, that is all.
What I want is to take the number of the rows from vector R and then see what is in C and put it in these corresponding rows. In my example, I have
R(1)=2 and C(1)=1
so I am going to have
A(2, 1)=1
then at the end of R, I have
R(4)=2 and
C(4)=7 again
so I will have
A(2,2)= 7 and so on.
Hope it is clearer.

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