function [newpop]=roulette(oldpop,popsize,stringlength)
totalfit=0;
popsize=size(oldpop,1);
for i=1:popsize
    totalfit=totalfit+oldpop(i,stringlength+2);
end
prob=oldpop(:,stringlength+2)/totalfit;
prob=cumsum(prob);
rns=sort(rand(popsize,1)); %按元素的從小到大排成一列
% rns=rand(popsize,1);
fitin=1;newin=1;
while (newin<=popsize)
    if rns(newin)<prob(fitin)
        newpop(newin,:)=oldpop(fitin,:);
        newin=newin+1;
    else
        fitin=fitin+1;
    end
end