Euler 11

Given a grid - what is the greatest product of four adjacent numbers in the same direction (up,down,left,right, all diagonals)

Setting up input (not shown)

Matricer for subsetting the matrix

horis <- cbind(x = 0, y = 0:3)
verti <- cbind(x = 0:3, y = 0)
right <- cbind(x = 0:3, y = 0:3)
left <- cbind(x = -(0:3), y = 0:3)

Beginning at a given position in the matrix, calculate the maximum product in different directions. We do not need all of them - they will be covered if we begin at another position.

ny_max_prod <- function(i,j){
  position <- cbind(x = rep(i, 4), y =rep(j, 4))  
  liste <- list(position + horis, position - horis,
     position +verti, position - verti,
     position + right, position- right,
     position + left, position - left)
  liste <- liste[sapply(liste, function(mat) all(mat > 0))]
  liste <- liste[sapply(liste, function(mat) all(mat < 21))]

mapply(function(x) grid[x], liste) %>% 
  apply(2, prod) %>% 
  max()
}

Apply that function to every position in the grid.

answer <- expand.grid(x = 1:20, y = 1:20) %>% 
  rowwise() %>% 
  mutate(maks = map2(x,y, ny_max_prod)) %>% 
  pull(maks)  %>% 
  unlist() %>% 
  max()