Graph,Smallnetwrok,WattsStrogatz, confused command

Question

Jan 2018-3-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/387331-graph-smallnetwrok-wattsstrogatz-confused-command

评论： Jan 2018-3-9

What is an intuitive difference between/purpose of these two lines in the following program?

newTargets(source) = 0;
newTargets(s(t==source)) = 0;

PROGRAM

 function h = WattsStrogatz(N,K,beta)
% H = WattsStrogatz(N,K,beta) returns a Watts-Strogatz model graph with N
% nodes, N*K edges, mean node degree 2*K, and rewiring probability beta.
%
% beta = 0 is a ring lattice, and beta = 1 is a random graph.
% Connect each node to its K next and previous neighbors. This constructs
% indices for a ring lattice.
s = repelem((1:N)',1,K);
t = s + repmat(1:K,N,1);
t = mod(t-1,N)+1;
% Rewire the target node of each edge with probability beta
for source=1:N    
    switchEdge = rand(K, 1) < beta;
    newTargets = rand(N, 1);
    newTargets(source) = 0;
    newTargets(s(t==source)) = 0;
    newTargets(t(source, ~switchEdge)) = 0;
    %
    [~, ind] = sort(newTargets, 'descend');
    t(source, switchEdge) = ind(1:nnz(switchEdge));
end
h = graph(s,t);
end
% Copyright 2015 The MathWorks, Inc.

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Answer 1

Geoff Hayes 2018-3-9

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Jan - source is a scalar from 1 to N and is used in

newTargets(source)

as an index. So

newTargets(source) = 0;

will set the element in newTargets at index source to zero. So a single element of the array is set to zero.

For the other, presumably, s and t are of the same dimension of newTargets. The line

t==source

will compare each element of t to the value of the scalar source and return a logical array of zeros and ones where a one indicates a match and a zero indicates no match. This logical array can then be used to retrieve all those elements of s where we have a one in the result t==source (see Using Logicals in Array Indexing for more details).

s(t==source)

The set of values of s that satisfy the above are then used as indices into newTargets to set all those elements of newTargets at indices s(t==source) to zero.

So

 newTargets(source) = 0;       % sets element of newTargets at index source to zero
 newTargets(s(t==source)) = 0; % sets multiple elements of newTargets at indices s(t==source) to zero

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Geoff Hayes 2018-3-9

Jan - I'm not familiar with the algorithm...I assumed that you wanted an explanation of what the difference was between the two lines of code. Have you looked at other implementations or a reference for the algorithm?

Jan 2018-3-9