1

I try to append an array in a for loop. However at each iteration my loop gets overwriten. How can I solve that.

rnormRdn <- matrix() #init empty matrix    set.seed(1234)
set.seed(1234)
  for(i in 1:3){
    rnormRdn <- matrix(rnorm(n = 4), nrow = 2, ncol = 2)
    print("New random matrix that should be appended is:")
    print(rnormRdn)
    appendMat <- array(data = rnormRdn, dim = c(2,2,i))
    print("New random matrix not correctly apended on after first iteration:")
    print(appendMat)
    i <- i+1
  }

Result: 

[1] "New random matrix that should be appended is:"
           [,1]      [,2]
[1,] -1.2070657  1.084441
[2,]  0.2774292 -2.345698
[1] "New random matrix not correctly apended on after first iteration:"
, , 1

           [,1]      [,2]
[1,] -1.2070657  1.084441
[2,]  0.2774292 -2.345698

[1] "New random matrix that should be appended is:"
          [,1]       [,2]
[1,] 0.4291247 -0.5747400
[2,] 0.5060559 -0.5466319
[1] "New random matrix not correctly apended on after first iteration:"
, , 1

          [,1]       [,2]
[1,] 0.4291247 -0.5747400
[2,] 0.5060559 -0.5466319

, , 2

          [,1]       [,2]
[1,] 0.4291247 -0.5747400
[2,] 0.5060559 -0.5466319

[1] "New random matrix that should be appended is:"
           [,1]       [,2]
[1,] -0.5644520 -0.4771927
[2,] -0.8900378 -0.9983864
[1] "New random matrix not correctly apended on after first iteration:"
, , 1

           [,1]       [,2]
[1,] -0.5644520 -0.4771927
[2,] -0.8900378 -0.9983864

, , 2

           [,1]       [,2]
[1,] -0.5644520 -0.4771927
[2,] -0.8900378 -0.9983864

, , 3

           [,1]       [,2]
[1,] -0.5644520 -0.4771927
[2,] -0.8900378 -0.9983864

Expected result at last iteration:

[1] "New random matrix not correctly apended on after first iteration:"
, , 1

           [,1]      [,2]
[1,] -1.2070657  1.084441
[2,]  0.2774292 -2.345698

, , 2

           [,1]       [,2]
[1,] 0.4291247 -0.5747400
[2,] 0.5060559 -0.5466319

, , 3

           [,1]       [,2]
[1,] -0.5644520 -0.4771927
[2,] -0.8900378 -0.9983864
S12000
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  • You are saying you do not understand why assigning a new value to a variable causes the old value to disappear? Perhaps you should consider defining the array outside of the for loop (with sufficient dimensions to support all iterations), and do a subset assignment, such as `appendMat[,,i] <- ...`. There are perhaps more elegant ways to do it, but this should work. – r2evans Apr 27 '16 at 15:58
  • why append instead of having one go? `set.seed(1234); rows <- cols <- 2; z <- 3; array(rnorm(rows * cols * z), dim = c(rows, cols, z))` – rawr Apr 27 '16 at 16:12

1 Answers1

1

Try the following:

set.seed(1234)
# initialize array, giving dimensions
myArray <- array(0, dim=c(2,2,3))

for(i in 1:3){
  rnormRdn <- matrix(rnorm(n = 4), nrow = 2, ncol = 2)
  print("New random matrix that should be appended is:")
  print(rnormRdn)
  myArray[,,i] <- rnormRdn
  print("New random matrix not correctly apended on after first iteration:")
  print(myArray)
}

If you know the size of the array ahead of time it is much more efficient to preallocate the space for it as I did in the second line.

lmo
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