Week 4, Wednesday: indefinite (while) loops

review cs21/examples/lettercounts.py

def count(text, letter):
  """count up and return how many of letter are in text"""

  acc = 0
  for ch in text:
    if ch == letter:
      acc = acc + 1
  # return the result after the for loop
  return acc


def main():
  text = raw_input("text: ")
  for ch in "abcdefgh":
    result = count(text, ch)
    print("Number of %s's in that text: %d" % (ch, result))

using import

Libraries and modules are already-written python code you can use in your programs. Some, like math and random, come with python, but still need to be imported if you want to use them. Others, like graphics, which we will use next week, need to be downloaded from the web, then imported. We will also learn later how to write our own modules. This is another example of code reuse: why write code again if there is already a well-tested library or module you can use?

To use any common math functions, like sqrt(), sin(), cos(), or even constants like pi, you must first import the python math library:

>>> sqrt(16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'sqrt' is not defined
>>> from math import *
>>> sqrt(16)
4.0

Side Note: another way to import modules is just import math. If you do that, then you need to call any function in the math module with math. in front of the function:

>>> import math
>>> sqrt(16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'sqrt' is not defined
>>> math.sqrt(16)
4.0

You can import modules either way. In this class, we will probably always use the from math import * syntax. If you go out in the real world and write real programs, using import math is probably better...

pseudo random numbers

Random numbers and picking things randomly (like from a list) can make our programs more interesting. Python has a library of functions dealing with random numbers and random choices. Two functions we often use are randrange() and choice(). Here are examples of each:

>>> from random import *
>>> randrange(1,11)
2
>>> randrange(1,11)
2
>>> randrange(1,11)
1
>>> randrange(1,11)
10
>>> randrange(1,11)
3

>>> names = ["jeff","carlo","zach","charlotte","lauri","andy"]
>>> choice(names)
'lauri'
>>> choice(names)
'zach'
>>> choice(names)
'carlo'

randrange() is similar to range(), except it just chooses one number from the range of values. choice() picks randomly from a list or a string.

Here is a simple example that simulates flipping a 'coin':

>>> coin = ["heads","tails"]
>>> choice(coin)
'heads'
>>> choice(coin)
'tails'
>>> choice(coin)
'tails'

indefinite (while) loops

Suppose we wanted to keep flipping a coin, until we got 3-in-a-row.

$ python flip5heads.py 
 1. tails
 2. tails
 3. heads
 4. tails
 5. heads
 6. tails
 7. heads
...
...
44. heads
45. tails
46. heads
47. heads
48. heads
49. heads
50. heads

That time it took 50 flips. If I ran it again, it might take 7, or 96, or who knows how many... The point is, there is a loop in that program, but it is not a for loop, because we don't know ahead of time how many iterations are needed.

The syntax for the above loop is something like this:

inarow = 0
while inarow < 5:
  flip coin
  print results of flip
  keep track of how many in a row (update inarow)

This says, as long as inarow is less than 5, keep flipping the coin.

The official syntax for a while loop is:

while some_condition_is_true:
   do this
   and this
   and this

So the indented code block, which can be 1 or any number of lines, is executed if the condition is True. Then we go back to the top of the loop and see if the condition is still True. If it is, execute the code block again. If not, the loop is done.

your turn...write this game

$ python whileloopgame.py 
.......... (10)
char (u/d): u
........... (11) level UP
char (u/d): u
............ (12) level UP
char (u/d): d
........... (11) level down
char (u/d): d
.......... (10) level down
char (u/d): d
......... ( 9) level down
char (u/d): d
........ ( 8) level down
char (u/d): d
....... ( 7) level down
char (u/d): pony
Please type u or d!
char (u/d): donkey
You can read...right?
char (u/d): d
...... ( 6) level down

The game starts at level 10. If the user enters u, go up a level. If they enter d, go down a level. Keep going until they get to level zero (not a very exciting game...I know!). And only accept u and d from the user. If you have time, add random insults. :)