Change matrix dimension

Question

Let's make a replicable example:

This is my initial matrix

d<- matrix(1:80,,5)
d
     [,1] [,2] [,3] [,4] [,5]
[1,]    1   17   33   49   65
[2,]    2   18   34   50   66
[3,]    3   19   35   51   67
[4,]    4   20   36   52   68
[5,]    5   21   37   53   69
[6,]    6   22   38   54   70
[7,]    7   23   39   55   71
[8,]    8   24   40   56   72
[9,]    9   25   41   57   73
[10,]   10   26   42   58   74
[11,]   11   27   43   59   75
[12,]   12   28   44   60   76
[13,]   13   29   45   61   77
[14,]   14   30   46   62   78
[15,]   15   31   47   63   79
[16,]   16   32   48   64   80

I would like to reshape my matrix cutting every colomn by 4 and taking the numbers up, as follow:

new.d
    [,1] [,2] [,3] [,4]
[1,]    1    5    9   13
[2,]    2    6   10   14
[3,]    3    7   11   15
[4,]    4    8   12   16
[5,]   17   21   25   29
[6,]   18   22   26   30
[7,]   19   23   27   31
[8,]   20   24   28   32
[9,]   33   37   41   45
[10,]   34   38   42   46
[11,]   35   39   43   47
[12,]   36   40   44   48
[13,]   49   53   57   61
[14,]   50   54   58   62
[15,]   51   55   59   63
[16,]   52   56   60   64
[17,]   65   69   73   77
[18,]   66   70   74   78
[19,]   67   71   75   79
[20,]   68   72   76   80

Any help? Thanks

Answer 1

Using matrix indexing

# number of new columns
cols <- 4
matrix(t(d), ncol=cols)[matrix(1:(length(d)/cols), ncol=ncol(d), byrow=TRUE), ]

---

# This almost gets us there but rows are not in correct order
matrix(t(d), ncol=cols)
 #      [,1] [,2] [,3] [,4]
 # [1,]    1    5    9   13
 # [2,]   17   21   25   29
 # [3,]   33   37   41   45
 # [4,]   49   53   57   61
 # [5,]   65   69   73   77
 # [6,]    2    6   10   14
 # [7,]   18   22   26   30
 # [8,]   34   38   42   46

# use this t index / group the relevant rows
c(matrix(1:(length(d)/cols), ncol=ncol(d), byrow=TRUE))
#  [1]  1  6 11 16  2  7 12 17  3  8 13 18  4  9 14 19  5 10 15 20