假设我有一个这样的集合：
{ "_id" : 8751, "title" : "The Banquet", "author" : "Dante" }
{ "_id" : 8752, "title" : "Divine Comedy", "author" : "Dante", "copies" : 1 }
{ "_id" : 8645, "title" : "Eclogues", "author" : "Dante" }
{ "_id" : 7000, "title" : "The Odyssey", "author" : "Homer", "copies" : 10 }
{ "_id" : 7020, "title" : "Iliad", "author" : "Homer", "copies" : 10 }
如何生成仅包含具有“副本”字段的文档的新集合，以获得如下所示的新集合：
{ "_id" : 8752, "title" : "Divine Comedy", "author" : "Dante", …我有两个向量,我想要一个新的向量,其中元素是向量1的元素和向量2的元素的总和.
v1<-c(1,2,3,4,5,6)
v2<-c(0,1,1,2,2,1)
for(i in 1:length(v1)){
for(j in 1:length(v2)){
n<-vector()
n<-v1[i]+v2[j]
}
m<-NULL
m[n]<-m
}
当我运行循环时,我得到m=NULL并且n是数字类NA.任何的想法？