# Compact way to write matrices containing indices

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Sim on 6 Apr 2022
Commented: Sim on 6 Apr 2022
Hi, I have an array A and a matrix B.
(Task 1) First, I would like to check which elements of A are members of the first column of B.
(Task 2) Then, I would like to return (i.e. write down) my matrix B with the same order of the elements of A, and leaving a NaN (or zero) for those elements of A that were not found in B. Is there a compact way to accomplish the Task 2 ?
OK, a little bit messy to explain, but with this example it should be easier to understand what I need:
% Input
A = [3
4
9
1
5];
B = [3 5
6 2
4 8
5 7
9 7];
[~,i] = ismember(A,B(:,1))
% Task (2) - First naive attempt gives an error
>> B(i,:)
Index in position 1 is invalid. Array indices must be positive integers or logical values.
% Task (2) - A second naive attempt works, but the 4th row of A is missing
>> B(i(i~=0),:)
ans =
3 5
4 8
9 7
5 7
% Desired Output - Here the 4th row of A is present with a NaN in the
% second column
>> B(?,?)
ans =
3 5
4 8
9 7
1 NaN
5 7
% Is there a compact way to get my desired output?
% Maybe just writing something like: B(?,?)
Sim on 6 Apr 2022
Edited: Sim on 6 Apr 2022
Basically, I would like to avoid this loop:
[~,i] = ismember(A,B(:,1));
% Task (2) - Desired Output
for j = 1 : length(i)
if i(j)~=0
B_des_output(j,:) = B(i(j),:);
else
B_des_output(j,:) = [A(j) NaN];
end
end
% Result
>> B_des_output
3 5
4 8
9 7
1 NaN
5 7

Stephen23 on 6 Apr 2022
Edited: Stephen23 on 6 Apr 2022
A = [3;4;9;1;5];
B = [3,5;6,2;4,8;5,7;9,7];
[X,Y] = ismember(A,B(:,1));
M = A;
M(:,2) = NaN;
M(X,2) = B(Y(X),2)
M = 5×2
3 5 4 8 9 7 1 NaN 5 7
Sim on 6 Apr 2022
All great solutions @David Hill and @Stephen, but this one looks like even more compact than other ones!
Many thanks to everyone for your contribution, I would accept all the answers :)

David Hill on 6 Apr 2022
[~,idx]=ismember(A,B(:,1));
f=find(idx==0);
idx(idx==0)=1;
C=B(idx,:);
C(f,:)=[A(f),nan(length(f),1)];
Sim on 6 Apr 2022
@David Hill Coooooolll !!! Many thanks :)