Problem with symsum function: doesn't work with matrix elements?

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Habib
Habib 2019-3-7
编辑： Habib 2019-3-7
I am trying to sum elements of a matrix with the symsum function.
E = squareform(pdist(ConcernedNodes2(:,1:2)));
DistMin = min(DistRoadMetro,[],3);
N = length(ConcernedNodes2);
V = zeros(length(ConcernedNodes2),timeSteps);
for t = 1:timeSteps
C = perc_road_num(:,:,t);
for i=1:length(ConcernedNodes2)
V(i,t) = (1/(symsum(E(i,k),k,1,length(ConcernedNodes2))*(N-1)))*symsum(E(i,j).*C(i,j)/(DistMin(i,j)),j,1,length(ConcernedNodes2));
end
end
And I get this error message: Undefined function 'symsum' for input arguments of type 'double'.
Can someone help me please?
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Habib
Habib 2019-3-7
编辑：Habib 2019-3-7
for t = 1:timeSteps
C = perc_road_num(:,:,t);
for i=1:length(ConcernedNodes2)
V2(i,t) = (1./sum(E(i,1:length(ConcernedNodes2)~=i)*(N-1))).*sum(E(i,1:length(ConcernedNodes2)~=i).*C(i,1:length(ConcernedNodes2)~=i)./(DistMin(i,1:length(ConcernedNodes2)~=i)));
end
end
This works, I forgot to add ~=i in the sum

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回答（1 个）

Adam Danz
Adam Danz 2019-3-7
编辑：Adam Danz 2019-3-7
The first two inputs to symsum() should be symbolic. It's difficult to interpret what the code is supposed to do without knowing what's stored in the variables.
More info in symsum inputs:
https://www.mathworks.com/help/symbolic/symsum.html#btydjwt-2
Examples:
https://www.mathworks.com/help/symbolic/symsum.html#btydjwt-4
It seems like you don't need symsum() in the first place. In this segment below, what I think you're trying to do is sum part of a row of matrix 'E'. It looks like you're trying to take the sum of row 'i' from column 1 to column length(ConcernedNodes2).
symsum(E(i,k),k,1,length(ConcernedNodes2)) % <--- incorrect
If that's the case, what you really want is
sum(E(i,1:length(ConcernedNodes2))) % <--- correct
which will give you one value (the sum of those numbers). Or, perhaps you are looking for cumsum() which would be cumulative sum and would output a vector the same length as the segment you're isolating.
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Habib
Habib 2019-3-7
编辑：Habib 2019-3-7
Actually this worked well:
for t = 1:timeSteps
C = perc_road_num(:,:,t);
for i=1:length(ConcernedNodes2)
U1=0;
U2=0;
for j = 1:length(ConcernedNodes2)
if j ~=i
U1 = U1 + E(i,j)*C(i,j)/(DistMin(i,j));
U2 = U2 + E(i,j);
end
end
V(i,t) = 1/(U2*(N-1))*U1;
end
end
And it is quicker than using the sum function :)

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