R按条件行号选择多行

Question

I have a R dataframe like this one:

a<-c(1,2,3,4,5)
b<-c(6,7,8,9,10)
df<-data.frame(a,b)
colnames(df)<-c("a","b")

df
  a  b
1 1  6
2 2  7
3 3  8
4 4  9
5 5 10

I would like to get the 1st, 2nd, 3rd AND 5th row of the column a , so 1 2 3 5 , by selecting rows by their number.

I have tried df$a[1:3,5] but I get Error in df$a[1:3, 5] : incorrect number of dimensions . What DOES work is c(df$a[1:3],df$a[5]) but I was wondering if there was an easier way to achieve this with R?

Answer 1

Your data frame has two dimensions (rows and columns). When you use the square brackets to extract values, R expects everything prior to the comma to indicate the rows desired, and everything after the comma to indicate the columns desired (see: ?[ ). Hence, df[1:3,5] means rows 1 through 3, from column 5. To turn your desired rows into a single vector, you need to concatenate (ie, c(1:3,5) ). That would all go before the comma, the column indicator, 1 or "a" , would go after the comma. Thus, df[c(1:3,5), 1] is what you need.

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For alternative answer (that might be more appropriate to a dataframe with many more columns), df[c(1:3, 5), "a"] as suggested by @Mamoun Benghezal would also get it done!