合并前
  A B C
1 1 1 1
2 2 2 1
3 3 3 1
4 4 4 1

合并后
  A_C B
1 1.0 1
2 1.5 2
3 2.0 3
4 2.5 4

df <- data.frame(A=(1:4),B = (1:4),C = rep(1,4))

df1 <- as.data.frame(cbind(apply(df[,c(1,3)], 1, mean), df$B))

``` r
library(tidyverse, warn.conflicts = FALSE)

df <- data.frame(A = 1:4,
                 B = 1:4,
                 C = rep(1,4))
df %>%
  dplyr::mutate(A_C = (A+C) / 2, .keep = "unused")
#>   B A_C
#> 1 1 1.0
#> 2 2 1.5
#> 3 3 2.0
#> 4 4 2.5
```