what is causing this error?

PosVecP = zeros(nMembersP, 3);		%Initialize memory storage
UnitVecP = zeros(nMembersP, 3);
OppPosVecP = zeros(nMembersP, 3);

for (j = 1 : nMembersP)
        SN = MemberP(j,1);           %SN = start node
        EN = MemberP(j,2);           %EN = end node
       
        PosVecP(j,:)= NodeP(EN,:)-NodeP(SN,:);
        UnitVecP(j,:)=PosVecP(j,:)./norm(PosVecP(j,:));
        OppUnitVecP(j,:)= -PosVecP(j,:)./norm(PosVecP(j,:));

%Finish this section to compute the position vector, length, unit vector,
%and negative unit vector for each member in the truss.

end		% for j (members)

WHY DOES THIS ERROR MESSAGE KEEP POPPING UP WHEN I RUN SECTION

Index in position 1 is invalid. Array indices must be positive integers or logical values.

Error in testruncp3 (line 92)

PosVecP(j,:)= NodeP(EN,:)-NodeP(SN,:);

Answer 1

[KSSV]

Note that in matlab the indexing shoul dbe always a posittive integer or a logical. You are suing negative number, so error.

A = rand(10,1) ; 
A(1)  % no error 
A(10)  % no error 
A(A>0.5)    % logical allowed 
A(0)  % error, 0 double not allowed 
A(-1)   % error , negative not allowed 

Put your code in debug mode and check the indices.