############# Basic Notes on Tuples and Arrays

# Previously we learned that Python has a type of expression called "tuples", 
# which are just sequences of expressions grouped into a single expression.

m = (3,5,7,9)  # m is a tuple of integers
print len(m)   # prints 4, which is the length of the tuple.

# we can also access the individual members of the tuple m by specifying an
# *index*.  The first element of the tuple is the zeroth element, and the
# last element has index len(m)-1:

print m[0]  # prints 3, the first element
print m[3]  # prints 9, the last element.

### Arrays

# Arrays are similar to tuples with one big exception.  Elements of an
# array can CHANGE.  Syntactically, arrays are indicated by square brackets
# instead of parentheses.

p = ["abc",23]        # arrays (and tuples) can contain elements of different type.
n = [2,3,5,7,11]

# typical loop that visits each element of the array:
i = 0 # starting index
while i<=len(n)-1:  # same as i<len(n)
  print n[i]  # prints the ith element of the array
  i += 1
# while i

## Changing an element of the array:
p[0] = "hello"   # this is a new type of assignment statement
print p

i = 0
while i<len(n):
  n[i] = n[i] * n[i]   # square each element.
  i += 1
#while
print n

## We can also add new elements to the end of the array:
n.append(13)
print n

## We can test if some element is in the array using a BOOLEAN expression
#  of the form (x in n):
print (2 in n)   # prints False because n does not contain 2
print (13 in n)  # prints True because n contains 13


### To join two arrays together, use the EXPRESSION  A + B

A = [1,2,3]
print A + [6,7,8]   # prints [1,2,3,6,7,8]

### IMPORTANT: unlike append, this operation does NOT change A:
print A  # still [1,2,3].

A = A + [6,7,8]   # Now it's been changed.

### The difference here is similar to the difference between the
#   EXPRESSION x+1 and the STATEMENT x = x+1.

## A.append(2) and A = A+[2]  are equivalent STATEMENTS.

A.append([2,4,5])
# what do you expect A to be now?  Has it been changed?  To what?

###
##  We often do A = [] to initialize a variable to an empty array, then
#   append to A one element at a time:

## The following loop builds an array of powers of two.
P = []  # initialize array
i = 0   # for computing 2**i power
while i<=20:
  P.append(2**i)
  i += 1
# while
print P

## If you know how large an array must be beforehand, it is more efficient
# to create the entire array with some initial value (such as 0) for all
# positions.
A = [0]*10
# this will create the array [0,0,0,0,0,0,0,0,0,0].  


### Note to Java/C++ programmers: Python's arrays are much more flexible,
#   though not as efficiently managed compared to arrays in C++ and Java.
#   For example, you can have elements of different types in the same array.


#  Strings as Tuples:  a string can be used also as a tuple of individual
#  characters:

s = "abcd"
print len(s)  # prints the length of the string (4)
print s[0]    # prints the 1st character
i = len(s)-1  # initialize string index to last character
while i>=0:
  print s[i],
  i -= 1
# while loop will print s backwards

## But strings are not arrays because you can't change a character in a
## string:  s[0] = 'z'  will give error.

#### Note on for loops.
#A while loop can always be used to "traverse" an array.  However, there is
#another convenient form that is sometimes used:

A = [2,3,5,7,11]
for x in A:
   print x,
#for loop

# This will print 2 3 4 7 11
# Note that it is the CONTENTS of the array, not the indices, that are
# printed.  Therefore, there is no need to increment x (no x+=1) inside
# the body of the for loop.  The update part is automated by the for-loop.
# In contrast, what does the following print:
x = 0
while x<len(A)
  print x,
  x += 1
# ??

# it will print 0 1 2 3 4, which are the INDICES, not the contents.  Be sure
# not to confuse the indices of an array with their contents.

# A while loop is a much more flexible contruct than a for loop, which is
# just a convenience in certain situations.  For example, you cannot traverse
# an array backwards, nor can you decide to visit every other element in an
# array using a for loop. With while loops these tasks are easily accomplished.