sum cells of certain columns for each row

Question

I would like to calculate sums for certain columns and then apply this summation for every row. Unfortunately, I can only get to the first step. How do I now make it happen for each row? I know that R doesn't need loops; what are good approaches?

My matrix (zscore) looks like this:

   a    b    c    t   y 1  3    4    7    7   4  2  4    56   6    6   4  3  3    3    2    1   7  4  3    88   9    9   9 

Now I would want to calculate the row sum for each row, based on some of the columns. For one row it could look like this:

f1 <- sum(zscore[1,1:2], zscore[1,3], zscore[1,5]) 

How do I do that now for each row?

Answer 1

You could do something like this:

summed <- rowSums(zscore[, c(1, 2, 3, 5)]) 

Answer 2

The summation of all individual rows can also be done using the row-wise operations of dplyr (with col1, col2, col3 defining three selected columns for which the row-wise sum is calculated):

library(tidyverse)  df <- df %>%      rowwise() %>%      mutate(rowsum = sum(c(col1, col2,col3)))