First commit

exercises/ch02/ex1.lua

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+N = 8 -- board size
+
+-- check whether position (n,c) is free from attacks
+function isplaceok(a, n, c)
+    for i = 1, n - 1 do   -- for each queen already placed
+        if (a[i] == c) or -- same column?
+            (a[i] - i == c - n) or -- same diagonal?
+            (a[i] + i == c + n) then -- same diagonal?
+            return false  -- place can be attacked
+        end
+    end
+    return true -- no attacks; place is OK
+end
+
+-- print a board
+function printsolution(a)
+    for i = 1, N do -- for each row
+        for j = 1, N do -- and for each column
+            -- write "X" or "-" plus a space
+            io.write(a[i] == j and "X" or "-", " ")
+        end
+        io.write("\n")
+    end
+    io.write("\n")
+end
+
+-- add to board 'a' all queens from 'n' to 'N'
+function addqueen(a, n)
+    if n > N then -- all queens have been placed?
+        printsolution(a)
+        os.exit(0)
+    else       -- try to place n-th queen
+        for c = 1, N do
+            if isplaceok(a, n, c) then
+                a[n] = c -- place n-th queen at column 'c'
+                addqueen(a, n + 1)
+            end
+        end
+    end
+end
+
+-- run the program
+addqueen({}, 1)