Brute-force algorithms

Communication 1

Please, in your answers to the various exercises online, if you have to write a Python code, be sure that the correct indent is preserved by previewing your post before to publish it

You can use the ``` environment for defining your Python code:

```
write your Python code here
```

Any question about the previous lecture?

Historic hero: Betty Holberton

She was one of the programmers of the earliest electronic and general-purpose computer, the ENIAC

She was involved in the development of several programming languages, such as COBOL and FORTRAN

She was the creator of the first statistical analysis tool

A huge part of her work about the development of algorithms for sorting the elements in a list

Why sorting is important

Sorting things is expensive, in particular if you have to order billions of items

However, having such items sorted is crucial for several additional kinds of tasks that we can perform

Library: books are clustered according to Dewey classification, and each cluster contains books ordered according to the authors' name and the book title

In this way a librarian can find a requested title avoiding to look all the billion books available one by one, thus saving a huge amount of time

Addressing computational problems

Problem-solving: the activity of creating an algorithm for solving some given computational problem, e.g. ordering alphabetically all the books in a library

Brute-force approach

Brute-force algorithm: a process that reaches the perfect solution of a problem by analysing all the possible candidates that may provide a solution to a certain computational problem, and then check if each candidate solves the problem question

Advantages: simple, it finds a solution always

Disadvantages: costly for large inputs

Suggestion: use brute-force algorithms when the problem size is small

Big solution spaces

Abstract strategy board games are computational problems that have a quite huge solution space

Develop a brute-force algorithm which is able to play appropriately Go means to consider all the possible legal moves that are available on the board

Number of all the possible legal moves in Go: 208168199381979984699478633344862770286522453884530548425639456820927419612738015378525648451698519643907259916015628128546089888314427129715319317557736620397247064840935

Avoid brute-force for these kinds of problems

Iteration

The usual way for solving computational problems by means of a brute-force approach is to iterate over a certain input or a block of instructions several times

for item in <collection>:
    # do something using the current item

while <condition>:
    # do something until the condition is true

Foreach: an example

def stack_from_list(input_list):
    output_stack = deque()
    
    for item in input_list:
        output_stack.append(item)
​
    return output_stack

While: an example

def run_forever():
    value = 0
​
    while value >= 0:
        value = value + 1

value = 0 1 2 3 4 ...

Linear search: description

Computational problem: find the position of the first occurrence of a value within a list

1. Iterate over the items in the input list

2. Check if each of them is equal to the value we are looking for

3. Once the value has been found, its position in the list is then returned

4. If the value is not contained in the list, no position is returned at all

Ancillary objects and algorithms

Tuple: sequence of values, specified in a precise order - they are different from lists since they cannot be modified

my_tuple = (1, "a", 2, ...)

my_list = list()
my_list.append("a")
my_list.append("b")
my_list.append("c")
enumerate(my_list)
# it will return the following enumeration of tuples:
# enumerate([(0, "a"), (1, "b"), (2, "c")])

Decoupling tuples in foreach

Python allows us to decouple the items in a tuple by specifying names for each item with variables created in the for statement on-the-fly

For instance,
for position, item in enumerate(my_list)
will assign (the number in the following list refers to the particular iteration of the foreach loop):

1. position = 0 and item = "a"

2. position = 1 and item = "b"

3. position = 2 and item = "c"

Linear search: algorithm

def linear_search(input_list, value_to_search):​  
    for ​position, item in enumerate(input_list):
        if item == value_to_search:
            return position

In Python, None (that means nothing) is returned if no return statement is executed

Useful functions

range(<stop_number>) returns the range (i.e. a kind of list) of all numbers from 0 to the one preceding the stop number

E.g.: range(3) returns range([0, 1, 2]), while range(0) returns an empty range

Get and insert items by position

<list>[<position>] returns the item in the list at that particular position

E.g.: considering my_list = list(["a", "b", "c"]), my_list[1] returns "b"

Insertion sort: algorithm

def insertion_sort(input_list):
    result = list()

    for item in input_list:
        insert_position = len(result)

        for prev_position in reversed(range(insert_position)):
            if item < result[prev_position]:
                insert_position = prev_position

        result.insert(insert_position, item)

    return result

Additional readings

From How To Code in Python:

• Chapter "How To Construct While Loops"
  All content
• Chapter "How To Construct For Loops"
  All content
• Chapter "Understanding Tuples"
  All content
• Chapter "Understanding Lists"
  Section "Constructing a List with List Items"
• Chapter "How To Use List Methods"
  Section "list.insert()