#                                                    strings.py

# A string is an ordered set of characters.

# A string is a data structure and an integer is a data structure.  An
# integer is a very simple data structure whereas a string is a more 
# complex data structure.  Every time you encounter a new data structure
# in any new language that you learn, figure out four things:
#
#    1. How do you create an instance of that data structure and give
#       a name to it?
#
#       Ex. sports = "golf"
#
#    2. How do you view an element of the data structure?  This is what
#       we call a 'read access'.
#
#       Ex. letter = sports[0]        # to access the first letter
#           print sports[1]
#
#    3. How do you change an element in the data structure?
#       For example, if we want to modify the contents of sports so that
#       it holds "golg" instead of "golf", we might try to assign a new
#       letter in that location of the sports string by using an assignment
#       operator like this:
#
#           sports[3] = 'g'
#
#       but modifying an element in place in string is not allowed in
#       Python.  So, we call string an immutable data structure in Python.
#       The best you can do is to create a new string with a 'g' in that
#       place and reassign the whole string to be the new value of the
#       variable sports.  When we study lists later, we will try to modify
#       an element in a list in place.  Stay tuned.
#
#    4. How do you destroy the data structure when we are done using it.
#       In a language such as C or C++ we would have to worry about that,
#       but in Python, you don't need to.  So, actually there are only
#       3 things to worry about when you encounter a new data structure
#       in Python.  Sometimes, only 2 as with strings.


# Let's see some strings and related functions that we can use to manipulate
# strings.


# Define a variable with the string value 'banana" as its initial value
fruit = "banana"


# String is a compound value and we can access individual pieces in it.

# Here we are accessing the second character in fruit.
letter = fruit[1]

print "letter = ", letter

# index in Python is 0-based as in most other programming languages
print "fruit[0] = ", fruit[0]

# length of a string
print "len(fruit) = ", len(fruit)


# more looping using strings

print "printing fruit using a while loop"

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


# Exercise: Create a function that takes one string argument and prints the
#    letters backward.


# We have leaned one kind of loop in Python: the 'while' loop.  Well, there
# is another looping construct in Python: the 'for' loop.  As you can see 
# below, it has a very simple intuitive form.  This loop is often referred
# to as the 'for-each' loop because that is what it does.  So, the example
# below can be read as 'for each letter referred to as char in the string 
# fruit, print the char' thus printing each letter in the string to the 
# standard output, the screen.

print "printing fruit using a for loop"

for char in fruit:
    print char


# Here is a simple yet fun thing to do.  I think you can figure out what it
# does.

prefixes = "JKLMNOPQ"
suffix = "ack"
for letter in prefixes:
    print letter + suffix


# slicing a string - by using the ':' operator we can specify a range
# within the string.

s = "Peter, Paul, and Mary"

print "s[0:5] = ", s[0:5]
print "s[7:11] = ", s[7:11]
print "s[17:21] = ", s[17:21]

print

print "s[:3] = ", s[:3]
print "s[17:] = ", s[17:]


# comparing strings - we can use the usual comparison operators that we use
# for numbers to compare two strings.

word = "apple"
if word == "banana":
    print "Yes, we have bananas!"
else:
    print "No, we have no bananas!"

if word < "banana":
    print "Your word," + word + ", comes before banana."
elif word > "banana":
    print "Your word," + word + ", comes after banana."
else:
    print "Yes, we have bananas!"


# finding a letter in a string - we can define a function that does that
# like this:

def find(str, ch):
    index = 0
    while index < len(str):
        if str[index] == ch:
            return index
        index = index + 1
    return -1

print "find(\"apple\", 'l') = ", find("apple", 'l')


# counting - or count the occurrences of a letter in a string.

def count(str, ch):
    count = 0
    for char in str:
        if char == ch:
            count = count + 1
    return count

print "count(\"apple\", 'p') = ", count("apple", 'p')


print
print

# built-in string module

import string

# to see what things are included in the module:
#
print "dir(string) . . . . . . . . "
print dir(string)

# one of those is digits
#
print "string.digits = ", string.digits

print
print

# if you want to find the documentation on 'find', try this.  Similarly you
# can find documentation on other functions as well.
#
print string.find.__doc__

# based on the information obtained from the documentation, we can now
# call find:
#
print "string.find(\"banana\", \"na\") = ", string.find("banana", "na")


# Exercise 1:
#
# See Section 7.15 of [DEM] if you want to see some exercise problems.