sum cells of certain columns for each row

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)))

You could do something like this:

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