Shrink a 28 * 28 matrix to 14 * 14 in R

shrink_matrix.R

# Divide each 28*28 image into 2*2 non-overlapping blocks. 
shrink_img <- function(img){
  # input: 28 * 28 image
  # window size is 2 * 2, move horizontally -- fix row first, move across columns
  # output: 14 * 14 image
  shrinked_img = matrix(, nrow = 14, ncol = 14)
  
  for (i in c(0:13)) {
      for (j in c(0:13)){
        row = 1 + 2 * i
        col = 1 + 2 * j
        small_block = img[row:row+1, col:col+1]
        ave = sum(small_block)/4
        # store it
        shrinked_img[i+1, j+1] = ave
      }
    
  }
  return(shrinked_img)
}