Matlab, load adjacency lists (irregular file)

Question

I have already looked here and here , but I'm not sure I found what I need.

I have an irregular file (that represents neighbors of particles 1 to 5) that looks like that

2 3 5
1 3
1 2

1

I am want to figure out a way to load it (as 'something' called A ) and do the following things:

1. Count the number of elements on one line (for instance size(A(1,:)) shall give me 3 )
2. Given an array B (of size 5) select the elements of B corresponding to the indices given by a line (something like B(A(1,:)) shall give me [B(2) B(3) B(5)] )

Answer 1

Since you want arrays with size depending on their first index, you're probably left with cell s. Your cell A could be such that A{1} equals to [2 3 5] and A{2} to [1 3] in your example etc. To do this you can read your file infile by

fid=fopen(infile,'rt');
A=[];
while 1
    nextline=fgets(fid);
    if nextline==-1 %EOF reached
        break;
    end
    A{end+1}=sscanf(nextline,'%d');
end
fclose(fid);

%demonstrate use for indexing
B=randi(10,5,1);
B(A{3}) %2-element vector
B(A{4}) %empty vector

Then A{i} is a vector corresponding to the i th line in your file. If the line is empty, then it's the empty vector. You can use it to index B as you want to, see the example above. Note that you should not add a newline at the very end of your infile , otherwise you'll have a spurious empty element for A . If you know in advance how many lines you need to read, this is not an issue.

And the number of entries in line i are given by length(A{i}) , with i=1:length(A) .