Pull specific rows | 易学教程

问题

Let's say that I have a data frame that looks like this...

City <- c("x","x","y","y","z","z","a","a")
Number <-c(1,2,3,4,5,6,7,8)

mat <- cbind.data.frame(City ,Number)

"City"  "Number"
     x          1
     x          2  
     y          3
     y          4
     z          5
     z          6
     a          7
     a          8

Now I want to be able to pull the data for...

list <- c("x","y", "a")

And the desired out come would look something like this...

x  y a 
1  3 7 
2  4 8

I tried using which(list%in%City) to help find the indices to pull that data from the index but that does not produce the rows that I want.

UPDATE

Make sure that when you are using Chris' answer that your data type for "City" is "chr" otherwise you will pop up with an error message as I got originally before using the "as.character" function.

回答1:

I renamed your variable list to test, because list is a function name. You can do this, using data.table:

matdt <- as.data.table(mat)
setkey(matdt, City)
sapply(test, function(x) matdt[x, Number])
     x y a
[1,] 1 3 7
[2,] 2 4 8

回答2:

You need to pass the City names to the extraction function one by one. In this case sapply will deliver a matrix as you expect but if there were a varying number of results per city, the retruned object would be a list rather than a matrix:

sapply( list, function(city) mat[ mat$City %in% city, "Number"] )
     x y a
[1,] 1 3 7
[2,] 2 4 8

回答3:

Using dplyr and tidyr:

mat %>%
  filter(City %in% c("x", "y", "a")) %>%
  group_by(City) %>%
  mutate(Index = 1:n()) %>%
  spread(City, Number) %>%
  select(-Index)

来源：https://stackoverflow.com/questions/31278008/pull-specific-rows

标签

dataframe

extraction