如何返回R数据框中偶数行号的总和？

Question

I am very new to coding.

I'm looking to retrieve frequency values of rows in my data frame

I already know you can do this using:

df$col <- rowSums( data[,0:100] )

But I specifically want the sum of data from rows that are divisible by two, in other words, even rows up to a specific point in my data frame.

Perhaps you would need to incorporate an if else function? Something vaguely similar to this oversimplified code?

   if df$col[0:5]%%2
    print rowSum
   else:
    don't

Anyone have any ideas?

Much appreciated

Answer 1

使用逻辑向量和回收规则进行索引将给出很好的解决方案：

rowSums(cars[c(FALSE, TRUE), ])

Answer 2

st.mat <- matrix(1:100,ncol = 10, nrow =10)
for(i in 1:dim(st.mat)[1]){
 if(i %% 2 == 0){
  print(sum(st.mat[i,]))
 }
}

This would be a very simple way to do it.