Generate random non repeating samples from an arra

Question

I made a battleships game and I now need to make sure that the computer doesn't attack at the same spot twice.

My idea of it is storing each shot's co-ordinates in a variable which gets added to whenever there is a new shot and then I just have to check and make it check if the current shot is in the variable.

This is the code I have for shooting:

if playerNumber == "1":

        eg.msgbox("Player " + str(playerNumber) + " your shot.")

        hit=False

        shotX=eg.enterbox("Enter the x-coordinate for your shot (1-5): ")
        shotY=eg.enterbox("Enter the y-coordinate for your shot (1-5): ")

    else:

        eg.msgbox("The computer will now attack!")

        hit=False
        shotX=str(random.randint(1,5))
        shotY=str(random.randint(1,5))

        eg.msgbox ("The computer shot at " + str(shotX) + ", " + str(shotY) + "")

My idea of making sure the computer chooses something that isn't in the list is using a loop where it keeps choosing a shot until the one it chooses hasn't been chosen already.

The actual code that makes the computer fire his shot is:

hit = haveShot("2", p1ship1, p1ship2 , player2Board)

    if hit:
        p2 = p2 + 1
        eg.msgbox("The Computer has " + str(p2) + " points and Player 1 has " + str(p1) + " points")
        if p2 == 2:    
            eg.msgbox("Unlucky, The computer won. Press enter to continue")
            if platform.system() == "Windows": # ONLY PLAY SOUNDS IF ON WINDOWS OS
                finish()
            break

Thanks!

Answer 1

U can use a random permutation of n targets and shoot a target from this randomly permuted list. Use random.shuffle for this like below. I have shown a sample output for non-repetitive randomly generated in range of 5. Use this shuffled list to target without having to check if you have already taken this shot.

>>> s=list(range(5))
>>> s
    [0, 1, 2, 3, 4]
>>> random.shuffle(s)
>>> s
    [3, 4, 2, 1, 0]
>>> s
    [3, 4, 2, 1, 0]
>>> random.shuffle(s)
>>> s
    [4, 0, 2, 1, 3]
>>> random.shuffle(s)
>>> s
    [2, 1, 3, 0, 4]

Answer 2

I first answer in a language agnostic way.

A very efficient way would be to keep a sequential list of all the possible positions (in your case 5x5=25 positions), use a single random choice in that list and then removes the position from the list. This saves you from the what to do when I hit a position I've already hitted

Python implementation :

First generate the list of positions :

positions = [ (i, j) for i in range(1,6) for j in range(1,6) ]

Then get a random position and remove it from the list

index = random.randrange(len(positions))
shotX, shotY = positions[index]
del positions[index]

Edit :

As a prove it should fork :

>>> import random
>>> positions = [ (i, j) for i in range(1,6) for j in range(1,6)]
>>> for i in range(25):
    index = random.randrange(len(positions))
    print positions[index]
    del positions[index]


(5, 5)
(5, 4)
(1, 4)
(1, 2)
(3, 3)
(1, 1)
(4, 1)
(4, 4)
(5, 1)
(4, 2)
(2, 2)
(2, 4)
(2, 3)
(2, 1)
(3, 2)
(3, 5)
(1, 5)
(5, 3)
(5, 2)
(4, 3)
(4, 5)
(1, 3)
(3, 4)
(2, 5)
(3, 1)
>>> print positions
[]

Answer 3

You could keep a log of all the shots fired:

# during initialization
fired = set()
# ... skipping to the interesting part ...
eg.msgbox("The computer will now attack!")
while True:
    shotX = random.randint(1,5)
    shotY = random.randint(1,5)
    if (shotX, shotY) not in fired:
        break # OK, no need to look any further
    # else try another shot...
eg.msgbox ("The computer shot at {0}, {1}".format(shotX, shotY))
fired.add((shotX, shotY))

I've also kept shotX and shotY as integers (you might need them that way later if you want to look up indices in a matrix, for example.