如何從R中的數據框構建倒排索引？

Question

說我有R中的數據幀：data.frame(x=1:4, y=c("a b c", "b", "a c", "c"))

x  y 
1 1 a b c 
2 2  b 
3 3 a c 
4 4  c 

現在我想建立一個新的數據幀，這是在IR或推薦系統很常見的倒排索引，從中：

y x 
a 1 3 
b 1 2 
c 1 3 4 

如何以有效的方式做到這一點？

Answer 1

conv <- function(x) { 
    l <- function(z) { 
    paste(x$x[grep(z, x$y)], collapse=' ') 
    } 
    lv <- Vectorize(l) 

    alphabet <- unique(unlist(strsplit(as.character(x$y), ' '))) # hard-coding this might be preferred for some uses. 
    y <- lv(alphabet) 
    data.frame(y=names(y), x=y) 
} 

x <- data.frame(x=1:4, y=c("a b c", "b", "a c", "c")) 
> conv(x) 
## y  x 
## a a 1 3 
## b b 1 2 
## c c 1 3 4 

Answer 2

的嘗試，轉換y字符後：

test <- data.frame(x=1:4,y=c("a b c","b","a c","c"),stringsAsFactors=FALSE) 

result <- strsplit(test$y," ") 
result2 <- sapply(unique(unlist(result)),function(y) sapply(result,function(x) y %in% x)) 
result3 <- apply(result2,2,function(x) test$x[which(x)]) 
final <- data.frame(x=names(result3),y=sapply(result3,paste,collapse=" ")) 

> final 
    x  y 
a a 1 3 
b b 1 2 
c c 1 3 4 

Answer 3

快速和骯髒的

original.df <- data.frame(x=1:4, y=c("a b c", "b", "a c", "c")) 

    original.df$y <- as.character(original.df$y) 

    y.split <- strsplit(original.df$y, " ") 

    y.unlisted <- unique(unlist(y.split)) 

    new.df <- 
    sapply(y.unlisted, function(element) 
     paste(which(sapply(y.split, function(y.row) element %in% y.row)), collapse=" ")) 

    as.data.frame(new.df) 

    > new.df 
    a 1 3 
    b 1 2 
    c 1 3 4