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--- lfa:lab01-python-intro [2021/09/03 09:44]
pdmatei
+++ lfa:lab01-python-intro [2021/10/07 11:32] (current)
ioana.georgescu [List comprehensions]
@@ Line 1: / Line 1: @@
-====== Introduction to Python ======
+====== 1. Introduction to Python ======
-==== Python basics ====
+Python3 is a multi-paradigm language, that combines the **imperative** style with the **functional** style. It also supports Object-Oriented programming, albeit in a limited way.
-<hidden The Pythonic way> This text will be hidden <code python> solution </code> </hidden>
+Python3 offers a few programming constructs that:
+  * make life real easy for the programmer,
+  * yield efficient programs,
+  * sum-up to a programming **style** called **pythonic** (a blend of imperative and functional features).
+This lab will introduce some of these constructs.
-Python is an interpreted, dynamically typed language which is easy to use for scripting and prototyping applications.
+==== List traversal ====
-C and Java programmers quickly adjust to the Python syntax. Unlike C, in Python, lists are predefined and usually more used than arrays:
+**Exercise 1.1.** Write a function ''f(l)'' which determines the maximum of a list ''l'' of integers.
+<hidden The pythonic way>
+Traversing a list using a ''for'' is done as follows:
 <code python>
-# Comments begin with a '#' and end at the end of the line
+for elem in collection:
-l = []  # comment
+    # do something
-l.append("1")
-l.append(0)
-l.append([])
-print(l)
 </code>
-**Remarks**
+This idiom can be used for any collection not just lists, as you will discover during the lab.
-  * Although Python is strongly-typed (the interpreter keeps track and verifies the type of objects), lists may have different 'types' of elements.
+</hidden>
-  * There exist two different major versions of Python: Python2 and Python3. Although similar, there are some incompatible differences between them. We will use Python3, so take care when researching documentation.
+**Exercise 1.2.** Modify the previous function to ''f(l,start,stop)'' and determine the maximum between positions ''start'' and ''stop''.
+<hidden The pythonic way>
+When we care about the indices of a list, we can use the function ''range(x,y)'' which returns **a list of integers** starting from ''x'' until ''y-1''.
 <code python>
-for i in range(0,len(l)):
+for i in range(0,len(collection)):
-    print(l[i])
+    # do something with collection[i]
+</code>
+</hidden>
-print(range(0,len(l)))
+==== Slicing lists ====
-for elem in l:
+**Exercise 1.3.** Find the **longest** sequence of positive integers from a list. E.g. for the list ''[1,3,-1,2,0,1,5,4,-2,4,5,-3,0,1,2]'' the answer is ''[2,0,1,5,4]''.
-    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 indentation levels (either a tab or 4 spaces)
-  * control instructions such as ''for'', ''while'', ''if'' do not require ''(...)'' for conditions but they must end with '':''
+<hidden The pythonic way>
+Python supports various list slicing operations, as well as "overloaded" indexing:
 <code python>
-def func(l1, l2):
+# the sublist from positions 0 to x of l:
-    l1.append(3)
+l[0:x]
-    return l1 + l2
+# the very same list:
+l[:x]
-x = [1]
+# the last element of a list:
-y = [2]
+l[-1]
-print(func(x,y))
+# the last three elements of a list:
-print(x)
+l[-3:]
+# the slice from n-3 to n-1, where n is the number of elements from the list
+l[-3:-1]
 </code>
+</hidden>
-**Remarks**
+**Exercise 1.4.** Write a pythonic function to check if a list is palindrome.
-  * 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 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**
+===== Other datatypes =====
-Write a function which prints EACH repeating character from a string. (Hint: strings are lists of characters).
-==== Useful data structures: dictionaries and sets ====
+The most widely used datatypes in Python are lists and dictionaries. Additionally, at LFA, we will also use: **sets**, **stacks**.
-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).
+==== Stacks ====
+In Python, **lists** are also used as stacks:
 <code python>
+l = []
+l.append(x)   # add x at the TOP of the stack (this will be the last element of the list)
+l[-1]         # this is the top of the stack
+x = l.pop()   # removes and returns an element from the top of the stack.
+</code>
+**Exercise 1.5.** Write a function which determines (and returns) the **longest** sequence of consecutive integers from a list.
+==== Dictionaries ====
+Python dictionaries are actually **hash maps** (or **hash tables**). The keys ''k'' are **dispersed** using a hash-function into buckets. Each bucket will hold its respective values. Some examples of dictionary usage:
+<code python>
+#create an empty dictionary:
 d = {}
-d["X"] = ["X"]
-if "X" in d:
+#add or modify a key-value:
-    print("d[X] is defined in the dictionary")
+d[k]=v
-if not "Y" in d:
+#search if a key k is defined in a dictionary d:
-    print("d[Y] is not defined in the dictionary")
+if k in d:
+  # do something
 </code>
-**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 1.6.** Write a function which determines the number of occurrences of each character in a string.
+<hidden The pythonic way>
+<code python>
+def count(l):
+   d = {}
+   for x in l:
+      if x in d:
+         d[x] += 1
+      else:
+         d[x] = 1
+   return d
+</code>
+</hidden>
-**Exercise 3** Write a function returns the number of unique characters from a list.
+==== Sets ====
-Remark:
+Sets are collections where each element is unique. Sets can be traversed and indexed exactly as lists. They are initialised using the constructor ''set'':
-  * a simpler way to ensure uniqueness is to use sets.
+<code python>
-  * a set may be created from a list:  ''s = set([1,2,3,1])''
+# the empty set:
-  * and in turn, a list may be created from a set: ''l = list(s)''
+s = set()
-**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.
+# set constructed from a list:
+s = set([1,2,3])
-Remark:
+# adding a new element to the set:
-  * lists can be sliced in Python using the following syntax: ''l[start_index:end_index]'' Test it to see how slicing is performed.
+s.add(4)
+</code>
-**Exercise 5** Modify the previous implementation to use slicing.
+**Exercise 1.7.** Determine the list of characters which occur in a string. E.g. for "limbajeformale" we have: "limbajefor". (Hint 1: in Python there is no difference between a character and a singleton string and strings are just lists; Hint 2: to create a list from another object, use ''list()'').
-Remark:
+==== Pairs ====
-  * in Python we can use arbitrarily nested functions
-**Exercise 6** Write a function which searches for a list of patterns in a text.
+Pairs are very similar to lists and can be substituted by the latter in many cases:
 <code python>
-def find_patterns (pattern_list, text):
+p = (x, y)     # a pair where the first element is x and the second is y
-    # checks if pattern is found at position index in text
+t = (x, y, z)  # a tuple
-    def inner_search (pattern,index):
+fst = p[0]
+snd = p[1]
+for k in t:
+  # do something with element k of the tuple
 </code>
-Remark:
+===== The functional style =====
-  * Python supports functional-style programming to some extent.
+==== Higher-order functions and lambdas ====
+The most used higher-order functions in Python are **map** and **reduce**:
 <code python>
-def plus1(x):
+from functools import reduce
-    return x + 1
+# adds 1 to each element of a list
+def allplus1(l):
+   return map(lambda x:x+1,l)
+# computes the sum of elements of a list
+def sum_l (l):
+   # inner functions are very useful for defining local, reusable functionality
+   def plus(x,y):
+      return x + y
+   return reduce(plus,l) # reduce is a little different from Haskell folds: it does not use an initial value
-print(map(plus1,[1,2,3]))
+def short_sum(l):
-print(map(lambda x:x+1, [1,2,3]))
+   return reduce(lambda x,y:x+y,l) # observe the syntax for lambdas
 </code>
+==== List comprehensions ====
-**Exercise 8** Modify the previous implementation and instead of ''for'', use ''map'' (cast the return of ''map'' to ''list'': ''list(map(...))'')
+List comprehensions are widely used programming tools in Python. Usage examples:
-However, it is more common in Python to employ //list comprehensions// instead of ''map'':
 <code python>
-def plus1(x):
+# adding 1 to each element of a list
-    return x + 1
+l1 = [x+1 for x in [1,2,3]]
+# [2,3,4]
-print([plus1(x) for x in [1,2,3]])
-print([(x + 1) for x in [1,2,3]]))
+# packing elements into pairs
+l2 = [(x,x+1) for x in [1,2,3]]
+# [(1,2), (2,3), (3,4)]
+# unpacking pairs in the for notation
+l3 = [x+y for (x,y) in [(1,2), (2,3), (3,4)]]
+# [3,5,7]
+# combined list comprehensions
+l4 = [(x,y) for x in [1,2,3] for y in [4,5,6]]
+# [(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]
+# filters
+l5 = [x for x in [1,2,3,4] if x>2]
+# [3,4]
 </code>
-List comprehensions also support the functionality of ''filter'':
+**Exercise 1.8.** Write a function which takes a list of records //first name, last name, CNP// (encoded as tuples), and returns a list of **the last names** and **ages** of all females which are younger than the average of the entire list. E.g. ''[ ("Mary", "Smith", "2030602123456"), ("Anne", "Doe", "2121092123456"), ("Matei", "Dan", "1121202123456"), ("Maggie", "Byrne", "2121078123456")]'' yields ''[("Smith",19), ("Doe",29)]''. Maggie was born in '78, whereas Mary, Anne and Matei were born in '94, '92 and 2002, respectively.
+<hidden The pythonic way>
 <code python>
-print([(x+1) for x in [1,2,3,4,5,6] if (x % 2 == 0)])
+from functools import reduce
+def getYouth(l):
+	# this function computes the age of a given CNP
+    def age(cnp):
+        # conversion to integer of the two-character year code
+        if int(cnp[5:8]) <= 21:
+            return 2021 - int("20"+cnp[5:7])
+        else:
+            return 2021 - int("19"+cnp[5:7])
+    # computing the average ages (a map could have also been used)
+    avg = reduce(lambda a,b:a+b, [age(x[-1]) for x in l]) / len(l)
+    # we return the last name and the age of the filtered list l
+    return [(ln,age(cnp)) for (fn,ln,cnp) in l if cnp[0]=='2' and age(cnp) <= avg]
 </code>
+</hidden>
-**Exercise 9** Modify the previous implementation and instead of ''for'', use list comprehensions.
+/*
 ==== Classes and inheritance ====
@@ Line 195: / Line 279: @@
   * Extend the class example shown previously to include class ''Node'' which models non-empty trees. Implement methods ''size'' and ''contains''.
+*/
+===== Practice =====
+A labelled graph is encoded as a file where:
+  * **the first line** consists of the number of nodes
+  * **each subsequent line** is an edge ''<from> <label> <to>''
+Example:
+<code>
+X 1
+O 2
+X 3
+O 4
+X 1
+O 2
+</code>
+  * Suppose we encode streets as labelled graphs, where each label 'X' or 'O' denotes if a street is closed or open.
+  * Compute the set of accessible nodes from a given **source**, via open streets.
+  * (Hint1: google //Python read lines// to see how to read from a file; also, google ''split'' in Python)
+  * (Hint2: you will need a dictionary to store, for each node and label l, the list of its l-successors)
+===== Haskell practice =====
+Solve the same exercise, only build your own input as a string, instead of a file.