portfolio_vol<-function(ret_matrix,weight){
    m<-ncol(ret_matrix)
    m2<-length(weight)
    if(m!=m2) stop(" dimensions are not matched!!!")
    COV<-cov(ret_matrix)
    sum<-0
    for(i in 1:m)
        for(j in 1:m)
           sum<-sum + weight[i]*weight[j]*COV[i,j]
    return(sqrt(sum))
}