Hopping and Skipping

Here is a simple game: the user starts at position 1, and must advance to the last position. At each step he or she either "hops" (advances one step), or "skips" (advances two steps). One of the positions is "bad": if the user gets there, he or she has lost.

To make this work: we need to know

how many positions are there?
where the user is now
where is the bad position

We will tell the user up-front where the bad position is, so this is just a game of skill (very little skill), not luck. We also ask the user to enter h or s to hop or skip. Here is the code:

current_position = 1
field_length = 10
bad_position = 5
print 'Position', bad_position, 'is bad, avoid it!'

while True:
    print 'You are at position', current_position
    action = raw_input('Next? h to hop 1, s to skip 2: ')
    if action == 'h':
        current_position += 1
    else:
        current_position += 2

    if current_position == bad_position:
        print 'You lost!'
        break
    elif current_position == field_length:
        print 'You won!'
        break

Let's make the game more interesting, by adding two variations:

the bad position will be randomly chosen
sometimes (with 30% probability), the player will "slip" back to the beginning.

To pick a random number, we will import the random library. It has many functions, but will only use these:

random.random() returns a random float from 0.0 (inclusive) to 1.0 (not inclusive).
random.randrange(x) returns a random value from 0 (inclusive) to x (not inclusive). The value x must be an int. For example, random.randrange(5) may return 0, 1, 2, 3, or 4, at random, but will never return 5.

Here is our revised game:

import random

field_length = 10
current_position = 1
slip_probability = 0.3
bad_position = random.randrange(field_length - 1) + 1
print 'Position', bad_position, 'is bad, avoid it!'

while True:
    print 'You are at position', current_position
    action = raw_input('Next? h to hop 1, s to skip 2: ')
    if action == 'h':
        current_position += 1
    else:
        current_position += 2

    if random.random() < slip_probability:
        print 'You slipped!  Back to the beginning!'
        current_position = 1

    if current_position == bad_position:
        print 'You lost!'
        break
    elif current_position == field_length:
        print 'You won!'
        break

Notice line 6: bad_position = random.randrange(field_length - 2) + 2. What's going on here? Well, I don't want the "bad" position to ever be 1 or 10 (otherwise you can't win the game), so:

field_length is 10,
field_length-2 is 8,
random.randrange(field_length-2) is a number from 0 to 7, and so
random.randrange(field_length-2)+2 is a number from 2 to 9.

Notice also line 17:

if random.random() <
     slip_probability:

. What's going on here? Well, slip_probability is 0.3, and random.random() can return anywhere between 0.0 and 0.9999..., so the probability that it returns a number less than 0.3 is 30%, as desired.