irr_f<-function(cashflows,bigV=100000){
   
   irr<-2
   rates<-seq(-1,1,1e-4)
  
   for(r in rates){
        npv<-abs(npv_f(r,cashflows))
        #print(npv)
        if(npv< bigV){
           irr<-r
           bigV<-npv
     }
   }
   return(irr)
}


cashflows<-c(-400,100,200,200,200,200)
irr_f(cashflows) $=# [1] 0.3119