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R Data Analysis Cookbook - Second Edition by Kuntal Ganguly

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Using apply on a three-dimensional array

  1. Create a three-dimensional array:
> array.3d <- array( seq(100,69), dim = c(4,4,2)) 
> array.3d 
 
, , 1 
 
     [,1] [,2] [,3] [,4] 
[1,]  100   96   92   88 
[2,]   99   95   91   87 
[3,]   98   94   90   86 
[4,]   97   93   89   85 
 
, , 2 
 
     [,1] [,2] [,3] [,4] 
[1,]   84   80   76   72 
[2,]   83   79   75   71 
[3,]   82   78   74   70 
[4,]   81   77   73   69 
  1. Calculate the sum across the first and second dimensions. We get a one-dimensional array with two elements:
> apply(array.3d, 3, sum) 
[1] 1480 1224 
 
> # verify 
> sum(85:100) 
[1] 1480 
  1. Calculate the sum across the third dimension. We get a two-dimensional array:
> apply(array.3d,c(1,2),sum) 
     [,1] [,2] [,3] [,4] 
[1,]  184  176  168  160 
[2,]  182  174  166  158 
[3,]  180  172  164  156 
[4,]  178  170  162  154 

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