How to find the row numbers for the pairwise entries from one matrices to another?

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KieranSQ
KieranSQ 2019-6-4
回答： Mehdi Ben Slama 2019-6-4
I am trying to compare two matrices of length 2xn and 2xm for n>m. For example consider n=19,m=16. Then we have the following two vectors, nodes (2x19) and old_nodes (2x16).
%Nodes Old_nodes
0 0 0 0
0 0.333333333 0 0.333333333
0 0.666666667 0 0.666666667
0 1 0 1
0.333333333 0 0.333333333 0
0.333333333 0.333333333 0.333333333 0.333333333
0.333333333 0.666666667 0.333333333 0.666666667
0.333333333 1 0.333333333 1
0.666666667 1 0.666666667 0
1 0.666666667 0.666666667 0.333333333
1 1 0.666666667 0.666666667
1 0.166666667 0.666666667 1
0.666666667 0 1 0
1 0 1 0.333333333
0.833333333 0.166666667 1 0.666666667
0.666666667 0.666666667 1 0
0.666666667 0.333333333
1 0.333333333
0.833333333 0.333333333
From here I am then trying to find out which rows in nodes are in old_nodes, there should be 16 of them. However, my attempt does not yield any effective results. I have tried the following, by taking four elements that i know are in old_nodes (rows: 6 7 10 11)
j=0
r=0
old_free=[6 7 10 11]
rows=zeros(length(old_free),1);
for i=1:length(e)
% i
for jj=1:length(old_free)
[row,~]=find(nodes(i,1)==old_nodes(old_free(jj),1) & nodes(i,2)==old_nodes(old_free(jj),2))
if isempty(row)
r=r+1
else
j=j+1
rows(j)=row;
end
end
end
Essentially what i would like is a column with the indices of the coordinates from nodes which match the indices of the entries for old_nodes, which will all be unique pairs:
e.g.
1 4
5 8
11 3
If anyone can be of help, I'd greatly appreciate it.

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采纳的回答

Mehdi Ben Slama
Mehdi Ben Slama 2019-6-4
Hi, it looks to me that the function 'ismember' does what you want. Specifically:
ismember(nodes, old_nodes, 'rows')
In this case, you're interested not only in whether the rows of 'nodes' are contained in 'old_nodes', but also their location in 'old_nodes'. You can obtain both in this manner:
[ LIA, LOCB ] = ismember(nodes, old_nodes, 'rows')
LIA is a vector with values 1 or 0 depending on whether the row of A is a member of B.
LOCB is a vector containing the location (row index) in B when a row in A is a member of B, 0 otherwise.
You should be able to get what you need by fiddling around with LOCB to get it into your desired format!
PS: do check the help section on 'ismember', it's probably more exhaustive

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