Lecture -3

Programing in Python
Instructor : AALWAHAB DHULFIQAR Advisor : Dr. Tejfel Mate

What you will learn:

Making decisions (if statement)
loops
logic and bitwise operations
lists in Python
Functions, tuples and dictionaries
Questions and answers Comparison: equality operator

Question: are two values equal?

Fortunately, computers know only two kinds of
answers: To ask this question, you use the == (equal equal) operator.
It is a binary operator with left-sided binding. It needs two
yes, this is true; arguments and checks if they are equal.
no, this is false.
Example : A==B or A!=B

Comparison operators: greater than

How to ask python a question

Question: are two values equal?

To ask questions, Python uses a set of very You can also ask a comparison question using the > (greater than)
special operators. operator.
black_sheep > white_sheep # Greater than
Comparison operators: greater than or equal to
Priority Operator

>= (greater than or equal to). 1 +, - unary

< (less than) operator . 2 **
<= (less than or equal to) 3 *, /, //, %
4 +, - binary
5 <, <=, >, >=
Making use of the answers 6 ==, !=

Conditions and conditional execution

if a condition is met, and not do it if it isn’t.

store it in a variable make a decision
about the future of if true_or_not:
A = B == D the program do_this_if_true
if sheep_counter >= 120:
make_a_bed()
Ex:
take_a_shower()
if the_weather_is_good:
sleep_and_dream()
go_for_a_walk()
feed_the_sheepdogs()
have_lunch()
Conditional execution: the if-else statement The elif statement

if the_weather_is_good:
go_for_a_walk()
if true_or_false_condition:
elif tickets_are_available:
perform_if_condition_true
go_to_the_theater()
else:
elif table_is_available:
perform_if_condition_false
go_for_lunch()
if the_weather_is_good:
else:
if the_weather_is_good: if nice_restaurant_is_found:
play_chess_at_home()
go_for_a_walk() have_lunch()
else: else:
go_to_a_theater() eat_a_sandwich()
have_lunch() else:
if tickets_are_available:
if the_weather_is_good: go_to_the_theater()
go_for_a_walk() else:
have_fun() go_shopping()
else:
go_to_a_theater()
enjoy_the_movie()
have_lunch()
Pseudocode and introduction to loops An infinite loop

How you think when you want to write a program?

1- largest_number = -999999999 while True:
2- number = int(input()) print("I'm stuck inside a loop.")
3- if number == -1:
4- print(largest_number)
5- exit()
6- if number > largest_number: # Store the current largest number here.
7- largest_number = number largest_number = -999999999
8- # Go to line 02
# Input the first value.
number = int(input("Enter a number or type -1 to stop: "))
Looping your code with while # If the number is not equal to -1, continue.
while number != -1:
while # Is number larger than largest_number?
if number > largest_number:
conditional_expression:
while there is something to do # Yes, update largest_number.
instruction_one largest_number = number
do it
instruction_two # Input the next number.
instruction_three number = int(input("Enter a number or type -1 to stop: "))
while conditional_expression:
:
instruction
: # Print the largest number.
instruction_n print("The largest number is:", largest_number)
Looping your code with for The break and continue statements

i=0 break - exits the loop immediately, and unconditionally ends the loop's
while i < 100: operation; the program begins to execute the nearest instruction after
# do_something() the loop's body;
i += 1 continue - behaves as if the program has suddenly reached the end of
-------------------------------------------------- the body; the next turn is started and the condition expression is tested
for i in range(100): immediately.
# do_something()
pass # break - example

print("The break instruction:")

for i in range(10): for i in range(1, 6):
if i == 3:
print("The value of i is currently", i) break
print("Inside the loop.", i)
print("Outside the loop.")
for i in range(2, 8):
print("The value of i is currently", i) # continue - example
Warning ! Donot do this Example:
for i in range(1, 1): power = 1 print("\nThe continue instruction:")
print("The value of i is currently", i) for expo in range(16): for i in range(1, 6):
Or this if i == 3:
print("2 to the power of", expo, "is", power) continue
for i in range(2, 1):
power *= 2 print("Inside the loop.", i)
print("The value of i is currently", i) print("Outside the loop.")
The while loop and the else branch Computer logic

i=1 Argument A Argument B A and B

1
while i < 5: False False False
2
print(i) False True False
i += 1 3
else: 4 True False False
print("else:", i) else: 5 True True True

Argument A Argument B A or B
for i in range(5): False False False
print(i) False True True
else: 0
True False True
print("else:", i) 1
2 True True True
i = 111 3
for i in range(2, 1): 4
print(i) else: 4
else: else: 111 Argument not Argument
print("else:", i) False True
True False
Logical expressions

not (p and q) == (not p) or (not q)

not (p or q) == (not p) and (not q)

Bitwise operations (~)

Logical values vs. single bits
Argument ~ Argument
0 1
i=1 1 0
j = not not i

Bitwise operators

Bitwise operations (&, |, and ^)

Argument A Argument B A&B A|B A^B
0 0 0 0 0
0 1 0 1 1
1 0 0 1 1
1 1 1 1 0
 Logical vs. bit operations How to deal with single bits?

i = 15 i: 00000000000000000000000000001111 flag_register = 0x1234

j = 22 j: 00000000000000000000000000010110
flag_register = 0000000000000000000000000000x000
log = i and j logneg = not I
True False x&1=x
x&0=0 if flag_register & the_mask:
# My bit is set.
bit = i & j the_mask = 8 else:
i 00000000000000000000000000001111 # My bit is reset.
j 00000000000000000000000000010110 x|1=1
bit = i & j 00000000000000000000000000000110 x|0=x

x=x&y x &= y
x ^ 1 = ~x
x=x|y x |= y
x^0=x
x=x^y x ^= y
Binary left shift and binary right shift

var = 17
var_right = var >> 1
var_left = var << 2
print(var, var_left, var_right)
 The len() function
Why do we need lists?

var1 = int(input()) The length of a list may vary during execution. New elements may be added to the list, while
var2 = int(input()) others may be removed from it. This means that the list is a very dynamic entity.
var3 = int(input())
If you want to check the list's current length, you can use a function named len() (its name
var4 = int(input()) comes from length).
var5 = int(input())
var6 = int(input()) The function takes the list's name as an argument, and returns the number of elements currently
stored inside the list (in other words - the list's length).
:
:
list is a collection of elements, but each
element is a scalar.

numbers = [10, 5, 7, 2, 1]

How do you change the value of a chosen element in the list? Negative indices are legal

numbers = [10, 5, 7, 2, 1] Removing elements from a list numbers = [111, 7, 2, 1]

del numbers[1] print(numbers[-1]) 1
numbers[0] = 111 <- indexing print(numbers[-2]) 2
numbers[1] = numbers[4] You can't access an element which
print(numbers) doesn't exist
print(numbers[0]) print(numbers[4])
print("\nList length:", len(numbers)) numbers[4] = 1
Functions vs. methods
my_list = [] # Creating an empty list.
result = function(arg)  Function
result = data.method(arg)  Method for i in range(5):
my_list.append(i + 1)

Adding elements to a list: append() and insert() print(my_list)

numbers = [111, 7, 2, 1] my_list = [10, 1, 8, 3, 5]

print(len(numbers)) total = 0
print(numbers)
### for i in range(len(my_list)):
numbers.append(4) total += my_list[i]
list.append(value)
print(len(numbers))
list.insert(location, value) print(total)
print(numbers)
###
numbers.insert(0, 222)
my_list = [10, 1, 8, 3, 5]
print(len(numbers))
length = len(my_list)
print(numbers)
for i in range(length // 2):
my_list[i], my_list[length - i - 1] = my_list[length - i - 1],
my_list[i]
print(my_list)
Sorting a list The inner life of lists

my_list = [] list_1 = [1]

swapped = True list_2 = list_1
num = int(input("How many elements do you want to sort: ")) Output = 2?
list_1[0] = 2
print(list_2)
for i in range(num):
val = float(input("Enter a list element: "))
my_list.append(val) slices

while swapped: my_list[start:end]

swapped = False
for i in range(len(my_list) - 1): # Copying the entire list.
list_1 = [1]
if my_list[i] > my_list[i + 1]:
list_2 = list_1[:]
swapped = True
list_1[0] = 2
my_list[i], my_list[i + 1] = my_list[i + 1], my_list[i]
print(list_2)
# Copying some part of the list. my_list = [10, 8, 6, 4, 2]
print("\nSorted:")
my_list = [10, 8, 6, 4, 2] new_list = my_list[-1:1]
print(my_list)
new_list = my_list[1:3] print(new_list)
print(new_list)
my_list = [8, 10, 6, 2, 4] my_list = [10, 8, 6, 4, 2] []
my_list.sort() new_list = my_list[1:-1]
print(my_list) print(new_list)
The in and not in operators

elem in my_list board = [[EMPTY for i in range(8)] for j in range(8)]

elem not in my_list
my_list = [0, 3, 12, 8, 2]

print(5 in my_list)
print(5 not in my_list) EMPTY = "-"
print(12 in my_list) ROOK = "ROOK"
board = []
for i in range(8):
Lists in lists row = [EMPTY for i in range(8)]
board.append(row)
row = [] board[0][0] = ROOK
for i in range(8): board[0][7] = ROOK
row.append(‘_____’) row = ["-------" for i in range(8)] board[7][0] = ROOK
print (row) board[7][7] = ROOK
print(board)

for i in board:
List comprehensive print (i)

squares = [x ** 2 for x in range(10)]

Functions: Why do we need functions?
It often happens that a particular piece of code is repeated many times in your program.
if a particular fragment of the code begins to appear in more than one place, consider the possibility of isolating it in the form
of a function

Decomposition

Your first function

def function_name():
function_body

def message():
print("Enter a value: ")
How functions work
Donot do these:

print("We start here.")

message()
print("We end here.")

def message():
print("Enter a value: ")

def message():
print("Enter a value: ")

message = 1

Parameterized functions

def function(parameter):
def message(number):
###
print("Enter a number:", number)
def message(number):
message()
print("Enter a number:", number)
Parametrized functions: Positional parameter passing
def my_function(a, b, c):
print(a, b, c)

my_function(1, 2, 3)

def introduction(first_name, last_name):

print("Hello, my name is", first_name, last_name)

introduction("Luke", "Skywalker")
introduction("Jesse", "Quick")
def message(what, number): introduction("Clark", "Kent")
print("Enter", what, "number", number)

message("telephone", 11) Keyword argument passing

message("price", 5)
message("number", "number") def introduction(first_name, last_name):
print("Hello, my name is", first_name, last_name)

introduction(first_name = "James", last_name = "Bond")

introduction(last_name = "Skywalker", first_name = "Luke")
Mixing positional and keyword arguments

positional argument passing keyword arguments

def adding(a, b, c):
print(a, "+", b, "+", c, "=", a + b + c)
adding(c = 1, a = 2, b = 3)
adding(1, 2, 3)

1+2+3=6 2+3+1=6

adding(3, c = 1, b = 2) 3+2+1=6

adding(3, a = 1, b = 2) 3+2+1=6

adding(4, 3, c = 2) TypeError: adding() got multiple values for

argument 'a'

def introduction(first_name, last_name="Smith"): introduction("James", "Doe")

introduction(first_name="William")
print("Hello, my name is", first_name, last_name)
introduction("Henry“)
 Effects and results: the return instruction

return without an expression

def happy_new_year(wishes = True):

print("Three...")
print("Two...")
print("One...")
if not wishes:
return
def boring_function():
print("Happy New Year!") print("'Boredom Mode' ON.")
return 123

print("This lesson is interesting!")

happy_new_year() boring_function()
Three... happy_new_year(False) print("This lesson is boring...")
Two... Three...
One... Two...
Happy New Year! One...
None lists and functions
None is a keyword
def strange_list_fun(n):
strange_list = []
value = None
if value is None: for i in range(0, n):
print("Sorry, you don't carry any value") strange_list.insert(0, i)
[4, 3, 2, 1, 0]

TypeError: unsupported operand return strange_list

print(None + 2)
type(s) for +: 'NoneType' and 'int'
print(strange_list_fun(5))
def strange_function(n):
True
if(n % 2 == 0):
return True Functions and scopes
print(strange_function(2))
None
print(strange_function(1))

def my_function(): def scope_test():

print("Do I know that x = 123
NameError: name 'x' is not defined
variable?", var) scope_test()
var = 1 print(x)
my_function()
print(var)
the global keyword simple functions: Fibonacci numbers

global name
global name1, name2, ...
def fib(n):
if n < 1:
return None
if n < 3:
def my_function(): return 1
global var
var = 2 elem_1 = elem_2 = 1
print("Do I know that variable?", var) the_sum = 0
for i in range(3, n + 1):
the_sum = elem_1 + elem_2
var = 1 elem_1, elem_2 = elem_2, the_sum
my_function() return the_sum
print(var)

for n in range(1, 10): # testing

print(n, "->", fib(n))
See you in the Lab ☺