✎ aa:lab:introduction-to-python [books]

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
====== Introduction to Python ======

==== Python basics ====

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:

<code python>
l = []
l.append("1")
l.append(0)
l.append([])

print l
</code>
**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

<code python>
for i in range(0,len(l)):
	print l[i]

print range(0,len(l))

for elem in l:
	print elem
</code>
**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 '':''

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

**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: [[https://docs.python.org/3/reference/datamodel.html | Python data model]])
  * ''+'' denotes list concatenation

**Exercise 1** Write a function which determines the maximum number from a list.
**Exercise 2** Write a function which prints EACH repeating character from a string. (Hint: strings are lists of characters).

==== Useful data structures: dictionaries ====

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).

<code python>
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"
</code>

**Exercise 3** Write a function returns the number of repetitions of each character from a text. (Hint: use dictionaries to store the number of repetitions.)

**Exercise 4** Write a function returns the number of unique characters from a list.

**Exercise 5** 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 6** Modify the previous implementation to use slicing.

Remark:
  * in Python we can use arbitrarily nested functions

**Exercise 7** Write a function which searches for a list of patterns in a text.
<code python>
def find_patterns (pattern_list, text):
	# checks if pattern is found at position index in text
	def inner_search (pattern,index):
</code>
For example, ''find_patterns(["ab","cd"], "abcdabcd")'' should print out "0,2,4,6".

Remark:
  * Matrices can be represented as lists of lists.
<code python>
matrix = [[1,2,3],[4,5,6],[7,8,9]]
</code>

**Exercise 8** Write a function which returns the elements from the principal diagonal of a matrix, as a list. Example: ''diag(matrix)=[1,5,9]''.

**Exercise 9** Write a function which adds two matrices.

**Exercise 10** Write a function which implements matrix transposition.

**Exercise 11** Write a function which multiplies two matrices.