#                                                     lists.py

# Remember that a string is an ordered sequence of characters?
# Well, a list is an ordered sequence of values of any type.
# Both strings and lists and other things that behave like an ordered
# sequence are called sequences.

# Since a list is a data structure, think about four things that I talked
# about when I introduced strings earlier:
#
#   (1) How do you create one?
#   (2) How do you access an element in it?
#   (3) How do you modify an element in it in place if possible?
#   (4) How do you destroy it when we don't need it any more? (don't worry
#       about this one in Python)


# Let us see some lists and related functions that we can use to manipulate
# lists.  Also think about where lists would be useful as you write your
# programs.


# Create a list on the fly and print it to the standard output device:
#
print [10, 20, 30, 40, 50]

# Once more with different type of elements
#
print ["Let", "us", "stop", "spams!"]

# Once more with an element itself as a list
#
print ["hello", 2.0, 5, ["nested", 2, True]]


# Here is another way of creating a list
#
print range(1, 7)     # 1 inclusive, 7 exclusive


# From 0 to 9 inclusive
#
print range(10)

# 1 to 10 in 2 or 3 increments
#
print range(1, 10, 2)

print range(1, 10, 3)


# Let us see how we access elements in a list

# First create a list
#
numbers = [10, 20, 30, 40, 50]

# Make sure it is created as expected
#
print numbers

# Remember that strings are immutable?
# Well, lists are mutable, which means that we can modify an element in
# a list in place.  This is the syntax that you use to do it.
# 
numbers[2] = 3

# Let us see if indeed the modification was done correctly in place.
#
print numbers

# Use a for loop as we did with a string:
#
for n in numbers:
    print n


# Concatenating two lists together

a = [1, 2, 3]
b = [3, 4, 5]

print a + b   # note that neither a nor b has been changed.

c = a + b
print c       # note that neither a nor b has been changed. c is new.


# What would you expect this to do?  Well, verify your answer by running it.
#
print [0] * 4

# How about this one?  Again run it to see what it does.
#
print [1, 2, 3] * 3


# Slicing lists, much like we did with strings.
#
alist = ['a', 'b', 'c', 'd', 'e', 'f']

print alist[1:3]   # 1 inclusive, 3 exclusive

print alist[:4]    # 0 to 3 inclusive

print alist[3:]    # 3 and the rest

print alist[:]     # everything


# Deleting elements of a list in place
#
del alist[2]        # This is a destructive operation
print alist


del alist[2:4]      # 2 to 3 inclusive
print alist


# Inserting elements into a list - this is useful enough that we create
# a function for it.
#
def insert(u, v, i):
    u[i:i] = v

print "\nTesting insert . . ."

print numbers

insert(numbers, [111], 1)
print numbers

insert(numbers, [22, 33, 44], 2)
print numbers

insert(numbers, [[222, 333, 444]], 2)
print numbers

insert(numbers, range(1000, 2000, 500), 2)
print numbers

def sum(u):
    sum = 0
    for e in u:
        sum = sum + e
    return sum

def ave(u):
    return sum(u) / len(u)

def makeList(u):
    return [u, ave(u)]

insert(numbers, [makeList(range(1, 5))], 2)
print numbers


# Converting strings to lists - sometimes it is useful to convert a string
# into a list and manipulate it as a list.
#

print "\nConverting a string into a list. . . "

s1 = "Stop that!"
print list(s1)


# Some basic functions dealing with lists:
#

print "\nnow del_head . . ."

def del_head(u):
    del u[0]

aa = [11, 22, 33, 44, 55]

del_head(aa)

print aa



print "\nnow tail . . ."

def tail(u):
    return u[1:]

print tail(aa)

print aa

print "\nnow range . . . "

x = range(1, 11)

print x


print "\nWhat does this do?. . . "

# What does this do? 
#
# For each element in x double it and include them all in a list - neat, huh?
#
y = [j * 2 for j in x]

print y



# Matrix

# | 1 2 3 |
# | 4 5 6 |
# | 7 8 9 |

print "\nmatrix . . ."

m = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

print "m = ", m

print "m[1] = ", m[1]

m[1] = [40, 50, 60]

print "m = ", m


print "m[1][2] before = ", m[1][2]


m[1][2] = 300

print "m[1][2] after = ", m[1][2]

print


# We can print the 3 rows in order like this:
#
print "printing 3 rows . . ."

for r in m:
    print r


# We can print the 9 elements in order like this:
#
print "\nprinting 9 elements . . ."

for r in m:
    for e in r:
        print e


# This is a rather inefficient way to deal with matrices in Python.  Later,
# toward the end of this tutorial we will learn a more efficient way, using
# Numpy.



# Exercise 1:
#
# See Section 10.15 of [Downey] if you want to see some exercise problems.