我是R的新手，我正在努力寻找当棋盘从角落开始时，骑士在棋盘周围移动所需的最少移动次数。
我使用了这个网站上的Python算法：https://www.geeksforgeeks.org/the-knights-tour-problem-backtracking-1/
我试着把它翻译成R。
然而，当我运行程序时，它的输出是：
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,] 0 -1 -1 -1 -1 -1 -1 -1
[2,] -1 -1 -1 -1 -1 -1 -1 -1
[3,] -1 -1 -1 -1 -1 -1 -1 -1
[4,] -1 -1 -1 -1 -1 -1 -1 -1
[5,] -1 -1 -1 -1 -1 -1 -1 -1
[6,] -1 -1 -1 -1 -1 -1 -1 -1
[7,] -1 -1 -1 -1 -1 -1 -1 -1
[8,] -1 -1 -1 -1 -1 -1 -1 -1
[1] "Minimum number of moves: -1"
我能做些什么来解决这个问题？
这是代码：
chess = rep(-1, times = 64)
board = matrix(data = chess, nrow = 8, ncol = 8, byrow = TRUE)
move_x = c(2, 1, -1, -2, -2, -1, 1, 2)
move_y = c(1, 2, 2, 1, -1, -2, -2, -1)
board[1, 1] = 0
pos = 1
valid_move <- function (x, y, board) {
if (x >= 1 && y >= 1 && x <= 8 && y <= 8 && board[x, y] == -1) {
return (T)
}
return (F)
}
solve <- function (board, curr_x, curr_y, move_x, move_y, pos) {
if (pos == 64) {
return (T)
}
for (i in seq(1, 8)) {
new_x = curr_x + move_x[i]
new_y = curr_y + move_y[i]
if (valid_move(new_x, new_y, board)) {
board[new_x, new_y] = pos
if (solve(board, new_x, new_y, move_x, move_y, (pos+1))) {
return (TRUE)
}
board[new_x, new_y] = -1
}
}
return (F)
}
main <- function() {
sims = 10
ctr = 0
number_of_moves = c()
solve(board, 1, 1, move_x, move_y, pos)
for (x in board) {
for (y in board) {
number_of_moves <- c(number_of_moves, board[x, y])
}
}
print(board)
print(paste("Minimum number of moves: ", min(number_of_moves)))
}
main()
在R中，当函数对其参数之一进行更改时，它只对本地副本进行更改，而不对原始变量进行更改。
例如，在这个R片段中，我们可以看到该函数实际上并没有修改变量l。
try_to_modify <- function(l) l[[1]] <- -100
l <- list(1)
try_to_modify(l)
l
#> [[1]]
#> [1] 1
与python相比，python实际上会修改l。
# (python code)
def try_to_modify(l):
l[0] = -100
l = [1]
try_to_modify(l)
l
#> [-100]
如果你想让一个函数向调用者传递一些信息，它要么需要修改一个全局变量(这通常不是最好的解决方案(，要么需要使用返回值。(也有一些例外，但通常都是这样(。
因此，您可以返回board或NULL，而不是返回TRUE或FALSE。
valid_move <- function (x, y, board) {
x >= 1 && y >= 1 && x <= 8 && y <= 8 && board[x, y] == -1
}
solve <- function (board, curr_x, curr_y, move_x, move_y, pos) {
if (pos == 64) {
return (board)
}
for (i in seq(1, 8)) {
new_x = curr_x + move_x[i]
new_y = curr_y + move_y[i]
if (valid_move(new_x, new_y, board)) {
board[new_x, new_y] = pos
result <- solve(board, new_x, new_y, move_x, move_y, (pos + 1))
if (!is.null(result)) {
return (result)
}
board[new_x, new_y] = -1
}
}
# Return NULL
# As this is the last result of the function, you don't need to write `return (NULL)`
NULL
}
final_board <- solve(
board = matrix(
c(0, rep_len(-1, 63)),
nrow = 8,
ncol = 8,
byrow = TRUE
),
curr_x = 1,
curr_y = 1,
move_x = c(2, 1,-1,-2,-2,-1, 1, 2),
move_y = c(1, 2, 2, 1,-1,-2,-2,-1),
pos = 1
)
final_board
#> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
#> [1,] 0 59 38 33 30 17 8 63
#> [2,] 37 34 31 60 9 62 29 16
#> [3,] 58 1 36 39 32 27 18 7
#> [4,] 35 48 41 26 61 10 15 28
#> [5,] 42 57 2 49 40 23 6 19
#> [6,] 47 50 45 54 25 20 11 14
#> [7,] 56 43 52 3 22 13 24 5
#> [8,] 51 46 55 44 53 4 21 12