r - How use (only) for-loop for this operation?

Question

I have this matrix:

A <- matrix(c(1,2,4,3,5,7,5,7,6,6,9,
              5.9,9,11,8,4.5,5.5,7.9,
              21,6.7,13.6,3.5,5,6,6,
              7.9,1,67,4,2), ncol=3, byrow=T)

and this vector:

B <- c(2 ,3, 4)

I would this expected result:

      X1   X2   X3
 [1,]  4.0 14.0 22.9   
 [2,]  8.0 21.0 26.9
 [3,] 11.0 27.0 27.8
 [4,] 15.0 25.5 35.4
 [5,] 13.5 23.2 35.5
 [6,] 25.5 17.2 28.5
 [7,] 24.5 19.6 22.6
 [8,]  9.5 16.9   NA
 [9,] 73.0   NA   NA
[10,]   NA   NA   NA

# the operation under the eventually code is:
col 1: 
    A[1,1]+A[2,1]=1+3=4 # first result for first column. 
    A[2,1]+A[3,1]=3+5=8 # second result for first column
    . . .   
    A[9,1]+A[10,1]=6+67= 73 #last expected result for first column

col 2 : 
            A[1,2]+A[2,2]+A[3,2]=2+5+7=14 # first result for second column. 
            A[2,2]+A[3,2]+A[4,2]=5+7+9=21 # second result for second column
             . . .
            A[8,2]+A[9,2]+A[10,2]=5+7.9+4=16.9 #last expected result for second column
and so on for the third columns of matrix A accordingly to the values of vector B.

I tried with this code:

res <- data.frame(matrix(ncol=ncol(A),nrow=nrow(A)))
res_tot <- data.frame(matrix(ncol=ncol(A),nrow=nrow(A)))
for (j in 1:ncol(A)){
    for(t in 1:nrow(A)){
      res <- A[t+B[j],j]
       res_tot[t,j] <- res
    }
  }

but the index are not correct. Now, I would, if is possible, the code only with for loop and use of index(like i,j,k etc...) without function like mapply,rollSum, etc.. from packages. Is it possible? please...

Answer 1

I would run a combination of sapply and a for loop. Of course you can use a nested sapply function as well.

res <- NULL
for (j in 1:3){
 tmp <-   sapply(1:nrow(A), function(i) ifelse((i+j > nrow(A), NA, sum(A[i:(i+j), j])))
 res <- cbind(res,tmp);colnames(res) <- NULL
}
res
      [,1] [,2] [,3]
 [1,]  4.0 14.0 22.9
 [2,]  8.0 21.0 26.9
 [3,] 11.0 27.0 27.8
 [4,] 15.0 25.5 35.4
 [5,] 13.5 23.2 35.5
 [6,] 25.5 17.2 28.5
 [7,] 24.5 19.6 22.6
 [8,]  9.5 16.9   NA
 [9,] 73.0   NA   NA
[10,]   NA   NA   NA

Edit: without sapply and with vector B, defining number of rows to sum up:

B <- c(2, 3, 4)
res <- NULL
for (j in 1:3){
  for(i in 1:nrow(A)){
  if(i == 1) tmp2 <- NULL
  tmp <-   ifelse(( i-1+B[j] >nrow(A)),NA, sum(A[i:(i-1+B[j]),j])) 
  tmp2 <- rbind(tmp2,tmp);rownames(tmp2) <- NULL
  }
  res <- cbind(res,tmp2);colnames(res) <- NULL
}