Question

我有这样的数据框架:

user1,product1,0
user1,product2,2
user1,product3,1
user1,product4,2
user2,product3,0
user2,product2,2
user3,product4,0
user3,product5,3

The data frame has millions of rows. I need to go through each row, and if the value in the last column is 0, then keep that product number, otherwise attach the product number to the previous product number that has value = 0, then write to a new data frame.

例如,由此形成的矩阵应当

user1,product1
user1,product1product2
user1,product1product3
user1,product1product4
user2,product3
user2,product3product2
user3,product4
user3,product4product5

I wrote a for loop to go through each row, and it works, but is very very slow. How can I speed it up? I tried to vectorize it, but I m not sure how because I need to check the value of previous row.

Answer 1

请注意,您实际上没有matrix/em>。矩阵只能包含一种原子类型(数字、分类、特性等)。确实有数据。

What you want to do is easily done with na.locf from the zoo package and the ifelse function.

x <- structure(list(V1 = c("user1", "user1", "user1", "user1", "user2", "user2", "user3", "user3"), V2 = c("product1", "product2", "product3", "product4", "product3", "product2", "product4", "product5"), V3 = c("0", "2", "1", "2", "0", "2", "0", "3")), .Names = c("V1", "V2", "V3"), class = "data.frame", row.names = c(NA, 8L)) library(zoo) # First, create a column that contains the value from the 2nd column # when the 3rd column is zero. x$V4 <- ifelse(x$V3==0,x$V2,NA) # Next, replace all the NA with the previous non-NA value x$V4 <- na.locf(x$V4) # Finally, create a column that contains the concatenated strings x$V5 <- ifelse(x$V2==x$V4,x$V2,paste(x$V4,x$V2,sep="")) # Desired output x[,c(1,5)]

由于你重新使用数据基,你需要确保“产品”一栏是特性而不是因素(如果“产品”一栏是因素,上述代码将产生奇数结果)。

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