Why it includes -1

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a = randint(0,len(board_in) - 1)
return a
i am currently doing an exercise on how to make a battleship game and when having to select a number randomly it tells me to include a negative 1 (-1), why?

The length of the board is 1 more than the highest index. If we generate a randint from 0 to len, it could overflow, so to prevent this, we subtract 1; but, doing so could put us at index -1, so we start randint with 1 instead of zero.

randint(1, len(board) - 1]

If we compare a zero indexed list to the a default range of the same length, they match up perfectly.

s = [3, 6, 9, 12, 15]
r = range(len(s))
print r
# [0, 1, 2, 3, 4]

Notice how the range lines up perfectly with the indices?

Now if we really want to generate a number that cannot overflow, then randrange fits that bill. It will produce a number that is in range, and does not need to be modified to suit.


The output will always match up to a list index.