06-Strings

A Character Too Far
•
You will get a python error if you
attempt to index beyond the end
of a string.
•
So be careful when constructing
index values and slices
>>> zot = 'abc'
>>> print zot[5]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: string index out of
range
>>>
Strings Have Length
•
There is a built-in function len that
gives us the length of a string
>>> fruit = 'banana'
>>> print len(fruit)
6
0
b
1
a
2
n
3
a
4
n
5
a
Len Function
>>> fruit = 'banana'
>>> x = len(fruit)
>>> print x
6
len()
function
'banana'
(a string)
6
(a number)
A function is some stored
code that we use. A
function takes some input
and produces an output.
Guido wrote this code
Len Function
def len(inp):
blah
blah
for x in y:
blah
blah
A function is some stored
code that we use. A
function takes some input
and produces an output.
>>> fruit = 'banana'
>>> x = len(fruit)
>>> print x
6
'banana'
(a string)
6
(a number)
Looping Through Strings
•
Using a while statement and
an iteration variable, and the
len function, we can construct
a loop to look at each of the
letters in a string individually
index = 0
while index < len(fruit) :
letter = fruit[index]
print index, letter
index = index + 1
0 b
1 a
2 n
3 a
4 n
5 a
Looping Through Strings
•
A definite loop using a for
statement is much more
elegant
•
The iteration variable is
completely taken care of by
the for loop
b
a
n
a
n
a
for letter in fruit :
print letter
Looping Through Strings
•
A definite loop using a for
statement is much more
elegant
•
The iteration variable is
completely taken care of by
the for loop
index = 0
while index < len(fruit) :
letter = fruit[index]
print letter
index = index + 1
fruit = 'banana'
for letter in fruit :
print letter
b
a
n
a
n
a
Looping and Counting
•
This is a simple loop that
loops through each letter in a
string and counts the number
of times the loop encounters
the 'a' character.
word = 'banana'
count = 0
for letter in word :
if letter == 'a' :
count = count + 1
print count
Looking deeper into in
•
The iteration variable
“iterates” though the
sequence (ordered set)
•
The block (body) of code is
executed once for each
value in the sequence
•
The iteration variable
moves through all of the
values in the sequence
for letter in 'banana' :
print letter
Iteration variable
Six-character string
Done?
Yes
print letter
Advance letter
for letter in 'banana' :
print letter
b a n a n a
letter
The iteration variable “iterates” though the string and the block
(body) of code is executed once for each value in the sequence

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