Introductory Programming
Fall 2004

For Tuesday you should:

1) Start homework 6.

2) Read Chapter 8.


Evaluation 4 soln
-----------------

def all_upper(s):
    i = 0
    while i<len(s):
        c = s[i]
        if not is_upper(c):
            return False
        i = i + 1
    return True

Can we rewrite this using a for loop?


for loops
---------

s = 'allen'
for letter in s:
    print letter

This is equivalent to

letter = s[0]
print letter
letter = s[1]
print letter
letter = s[2]
and so on.

It is also equivalent to

i = 0
while i < len(s):
    print s[i]
    i = i + 1

An advantage of the for loop is that you don't
have to use indices to access the elements of the string.

The while statement is more general than the for loop,
because you can generate the sequence "on the fly":

while n != 1:
    print n,
    if n%2 == 0:
        n = n/2
    else:
        n = n*3+1

Can you express this as a for loop?



Common string operations
------------------------

The string module contains functions that are useful for
working with strings.

To use them, you have to import the module.

As usual, there are two ways to do that:

import string

and

from string import *

If you use "import string" then you have to call the functions
by their full names:

import string
s = 'allen'
print string.find(s, 'en')

If you use "from string import *" then you can use the shorthand:

from string import *
s = 'allen'
print find(s, 'en')


WARNING: many of the string functions _sound_ like they modify
strings, but they don't because

	 STRINGS ARE IMMUTABLE!!!

Actually, they create new strings that resemble existing strings!

from string import *
s = 'allen'
t = upper(s)
print t


In addition to function, the string module also defines
some variables:

string.uppercase
string.punctuation
string.whitespace
etc.

This are particularly useful in conjunction with the in operator...


The in operator
---------------

in checks whether an element appears in a sequence and returns
True if it does.

Try this:

'e' in 'allen'
'e' in 'banana'


So a short way to write has_e is

def has_e(s):
    if 'e' in s:
        return True
    else:
        return False


An even shorter way is

def has_e(s):
    return 'e' in s

The in operator produces a boolean, which is what
has_e is supposed to return, right?

Exercise: rewrite is_upper (from Eval 4) so that the
	  body only contains one line.