Introduction to Python

Python is an interpreted, dynamically typed language which is easy to use for scripting and prototyping applications.

C and Java programmers quickly adjust to the Python syntax. Unlike C, in Python, lists are predefined and usually more used than arrays:

l = []
l.append("1")
l.append(0)
l.append([])
 
print l

Remarks

• in Python we often use side-effects to modify data
• Python is not strongly-typed. As a consequence, lists may have different 'types' of elements

for i in range(0,len(l)):
	print l[i]
 
print range(0,len(l))
 
for elem in l:
	print elem

Remarks

• List traversal may be achieved using indexing. Indexes are integers taken from a range. range(0,len(l)) is a list in itself.
• Lists (and other data structures) can be iterated using in

Syntax

• unlike C or Java where we often use {…} for scoping, in Python we use tabs
• control instructions such as for, while, if do not require (…) for conditions but they must end with :

def func (l1, l2):
	l1.append(3)
	return l1 + l2
x = [1]
y = [2]
print func(x,y)
print x

Remarks

• although it is possible to add type annotations, in Python a function's signature only consists of the number of parameters and their names
• objects are generally are passed as reference (hence, when printing x, we see the list [1,3]) (for details see: Python data model)
• + denotes list concatenation

Exercise 1 Write a function which prints EACH repeating character from a string. (Hint: strings are lists of characters).

Dictionaries are another useful data structure. A dictionary is a <key> : <value> mapping. Unlike lists, keys may be of any type (integers, strings, or any other datatype).

d = {}
d["X"] = ["X"]
 
if "X" in d:
	print "d[X] is defined in the dictionary"
if not "Y" in d:
        print "d[Y] is not defined in the dictionary"

Exercise 2 Write a function returns the number of repetitions of each character from a text. (Hint: use dictionaries to store the number of repetitions.)

Exercise 3 Write a function returns the number of unique characters from a list.

Remark:

• a simpler way to ensure uniqueness is to use sets.
• a set may be created from a list: s = set([1,2,3,1])
• and in turn, a list may be created from a set: l = list(s)

Exercise 4 Write a function which takes a pattern and a text and prints all indexes where an occurrence of pattern in text are found.

Remark:

• lists can be sliced in Python using the following syntax: l[start_index:end_index] Test it to see how slicing is performed.

Exercise 5 Modify the previous implementation to use slicing.

Remark:

• in Python we can use arbitrarily nested functions

Exercise 6 Write a function which searches for a list of patterns in a text.

def find_patterns (pattern_list, text):
	# checks if pattern is found at position index in text
	def inner_search (pattern,index):

Remark:

• Python supports functional-style programming to some extent.

def plus1(x):
	return x + 1
print map(plus1,[1,2,3])
print map(lambda x:x+1, [1,2,3])