沿行复制列的值

Question

给出以下数据框:

df1 <- data.table( V1=c(0,0,0,0),V2=c(0,0,0,0),V3=c(0,2,0,2))
df1
   V1 V2 V3
1:  0  0  0
2:  0  0  2
3:  0  0  0
4:  0  0  2

我试图在整行上复制V3的值,这样:

df2
   V1 V2 V3
1:  0  0  0
2:  2  2  2
3:  0  0  0
4:  2  2  2

我怎样才能做到这一点？

提前谢谢了.

Answer 1

您可以使用base-R语法:

# to overwrite
df1[] <- df1$V3 

# to make a new table
df2   <- copy(df1)
df2[] <- df1$V3 

我认为修改这么多列的最多data.table-ish方法是set:

# to overwrite
for (j in setdiff(names(df1),"V3")) set(df1, j = j, value = df1$V3)

# to make a new table -- simple extension

最后,还有来自@akrun,@ DavidArenburg和@VeerendraGadekar的其他几个好主意:

# to overwrite
df1[, (1:ncol(df1)) := V3] # David/Veerendra

# to make a new table
df2 <- setDT(rep(list(df1$V3), ncol(df1))) # akrun
df2 <- df1[, rep("V3",ncol(df1)), with = FALSE] # David