理解foreach

library(doParallel)
library(foreach)
library(dplyr)

input <- data.frame(matrix(rnorm(200*200, 0, .5), ncol=200))
input[input <=0] =0
input['X201'] <- seq(from = 0, to = 20, length.out = 10)
input <- input %>% select(c(X201, 1:200))
input2 <- split(input, f= input$X201)

a = 0
b= 0
cl <- parallel::makeCluster(20)
doParallel::registerDoParallel(cl)
tm1 <- system.time(
  y <- 
    foreach (h = length(input2),.combine = 'cbind') %:%
    foreach (j = nrow(input2[[h]]),.combine = 'c',packages = 'foreach') %dopar%{
      a = input2[[h]][j,3]
      b = b + a
    } 

)
parallel::stopCluster(cl)
registerDoSEQ()
print("Cluster stopped.")

0.55 (精确值取决于生成的随机数1),即 input2[[10]][20,3] ,不是我想要的累积值。我查看了foreach包的手册,但仍然不确定我是否完全理解foreach函数的机制。