Презентация Basic programming онлайн
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Слайды и текст к этой презентации:
№1 слайд
Содержание слайда: Tirgul 2
Basic programming
№2 слайд
Содержание слайда: Overview
Variables
Types : int, float, string
User input
Functions with input and output
The Boolean type and Boolean operations
Conditional operation (if…else…)
№3 слайд
Содержание слайда: Variables - Motivation
Write a program which calculates the difference in the areas of a square with side = 1.5 and the circle enclosed within it.
№4 слайд
Содержание слайда: Variables - Motivation
1.5*1.5 –3.14*(1.5/2 )**2
№5 слайд
Содержание слайда: Variables - Motivation
Two problems :
The expression 1.5*1.5 –3.14*(1.5/2 )**2
is really difficult to understand :
When you get back to it after one week
When debugging
When the side of the square changes. Should you have an expression per side-length?
Side=1.5 : 1.5*1.5 – 3.14*(1.5/2 )**2
Side=3.7 : 3.7*3.7 – 3.14*(3.7/2 )**2
Side=9 : 9*9 – 3.14*(9/2) )**2
№6 слайд
Содержание слайда: Variables - Motivation
Wouldn’t it be much more readable, modular, easy to modify in this format :
side = 1.5, PI = 3.14
square_area = side*side
radius = side/2
circle_area = PI*r2
answer = square_area – circle_area
№7 слайд
Содержание слайда: Variables
Variables let us define “memory units” which can “remember” values.
Variables have 2 main components :
name
value
№8 слайд
Содержание слайда: Variables
Variables have 2 main functionalities :
Set their value
number_of_apples = 3
Get their values
tomorrow_apples = number_of_apples + 1
№9 слайд
Содержание слайда: Variables – Naming conventions
Use lower case letter
number,apples
Separate multiple words with underscore
word_and_more_words
Use meaningful names for names (don’t be shy to open a dictionary)
z = x/y ???
words_per_page = words_in_book/number_of_pages
Use capitals for constants (variables which do not change their value after first initialization)
PI = 3.14, ERROR_MESSAGE = ‘You had an error’
№10 слайд
Содержание слайда: Types
Can we perform the following command ?
x = 3 + 5
And this one ?
x = 3 + “hello”
Why not? 3 and ‘hello’ are not of the same category. The name Python gives to the categories which differentiate between objects such as 3 and ‘hello’ are called type.
№11 слайд
Содержание слайда: Types
int (Integer) : represent an Integer number (מספר שלם).
E.g. 1024, 13, 92,0
float : represent a fractional number.
E.g. : 0.0, 15.62545, 3.14
str (String) : represent text, a list of characters. Defined between a couple of apostrophe or quotes (equivalent).
E.g. ‘hello’, “hello”, ‘13’
№12 слайд
Содержание слайда: Types
The type() function receives a value and return its type.
type(3) int
type(3.0) float
type('3.0') str
What happens when we mix types?
type(1 + 0.5) float
type(1 + 'some string') ?
№13 слайд
Содержание слайда: Types
What happens when we mix types?
type(1 + 'some string')
TypeError: unsupported operand type(s) for +: 'int' and 'str'
This is an error message which tells us we have tried to execute a command not supported by the language.
№14 слайд
Содержание слайда: Error message
Error messages are our friends, they help us detect bugs in our program and point out how to fix them.
When you get an error “keep calm and read the error message”.
№15 слайд
Содержание слайда: Error message - Example
>>> x = 49
>>> x/(49**0.5 - 7)
Traceback (most recent call last):
File "C:/my_python/test.py", line 9, in <module>
x/(49**0.5 - 7)
ZeroDivisionError: float division by zero
Remember - “keep calm and read the error message”
№16 слайд
Содержание слайда: Error message - Example
Traceback (most recent call last):
File "C:/my_python/test.py", line 2, in <module>
x/(49**0.5 - 7)
ZeroDivisionError: float division by zero
№17 слайд
Содержание слайда: Error message - Example
Traceback (most recent call last):
File "C:/my_python/test.py", line 2, in <module>
x/(49**0.5 - 7)
ZeroDivisionError: float division by zero
№18 слайд
Содержание слайда: Error message - Example
Traceback (most recent call last):
File "C:/my_python/test.py", line 2, in <module>
x/(49**0.5 - 7)
ZeroDivisionError: float division by zero
№19 слайд
Содержание слайда: Error message - Example
Traceback (most recent call last):
File "C:/my_python/test.py", line 2, in <module>
x/(49**0.5 - 7)
ZeroDivisionError: float division by zero
№20 слайд
Содержание слайда: Error message - Example
Traceback (most recent call last):
File "C:/my_python/test.py", line 2, in <module>
x/(49**0.5 - 7)
ZeroDivisionError: float division by zero
№21 слайд
Содержание слайда: Error message - Example
Traceback (most recent call last):
File "C:/my_python/test.py", line 2, in <module>
x/(49**0.5 - 7)
ZeroDivisionError: float division by zero
№22 слайд
Содержание слайда: But what if we do want to mix types?
my_apples = 3
print('I have ' + my_apples + ' apples')
TypeError: Can't convert 'int' object to str implicitly
The error message tells us we have tried to convert an int to a str but we cannot do this implicitly.
So let’s do it explicitly.
№23 слайд
Содержание слайда: Converting types (casting)
int, float and str are not only names of types but also names of functions which convert between types.
Example :
str(5) '5'
float(5) 5.0
int('5') 5
№24 слайд
Содержание слайда: Converting types – int(),float()
Converts string representing numbers to the represented numbers
int('5')5
Cannot convert strings not representing an int :
int('5.5'), int('Hello')
Converts float to int by rounding the number down.
int(5.9)5
Converts string representing numbers to the represented numbers
float('5.5')5.5
Cannot convert strings not representing a float:
float('Hello')
Converts int to float by treating it as a round number.
float(5)5.0
№25 слайд
Содержание слайда: User input
To make a program interactive we can ask the user for some inputs and act upon them.
The function input(s) :
Prints to the screen s
Halts the program execution and waits for the user to insert some input and press enter
Return a string representing the user’s input
№26 слайд
Содержание слайда: User input - Example
square_side = input('Insert side length: ')
# Wait for user …
3
- The value of square_side is 3
area = square_side * square_side
Will this work?
№27 слайд
Содержание слайда: User input - Example
area = square_side * square_side
'3'*'3'
TypeError: can't multiply sequence by non-int of type 'str'
Input returns a string, and we can’t multiply string by string. So what do we do? Convert types
№28 слайд
Содержание слайда: User input - Example
square_side = float(input('Insert side length: '))
# Wait for user …
3
area = square_side * square_side
The value of area is 9.0
№29 слайд
Содержание слайда: Functions with input
def function_name(param1, param2,…,paramN):
#indented code is here
#as usual
№30 слайд
Содержание слайда: Functions with input
When we call a function with input parameters, we can use the parameters’ value inside the function using their name.
№31 слайд
Содержание слайда: Functions with input
№32 слайд
Содержание слайда: Functions with input
№33 слайд
Содержание слайда: Functions with input
№34 слайд
Содержание слайда: A word about scopes
№35 слайд
Содержание слайда: A word about scopes
№36 слайд
Содержание слайда: A word about scopes
№37 слайд
Содержание слайда: A word about scopes
№38 слайд
Содержание слайда: A function with more than 1 input
def get_details(name, password):
print('Name is :' + name + ', Password is:' + password )
get_details('John', '1234')
№39 слайд
Содержание слайда: Functions’ parameters default value
Sometimes ….
A function has an obvious use case that will be utilized most of the time
You have prepared a good option for the user but don’t want to force her to use it
In such cases, you can define a default value to the function’s parameters. A value that will be used if no other value is specified.
№40 слайд
Содержание слайда: Functions’ parameters default value
def shoresh(number, root=2):
print(number ** (1/root))
The first parameter, number, has no default value. Hence every call to the function must indicate its value.
The second parameter, root, has a default value. Hence if we don’t indicate its value it will get the default declared value, 2.
№41 слайд
Содержание слайда: Functions’ parameters default value
def shoresh(number, root=2):
print(number ** (1/root))
shoresh(64) # Here we didn’t indicate the second variable, hence the default value was used
>> 8
shoresh(64, 3) # Here we indicated the second variable, hence its value was used and not the default
>> 4
№42 слайд
Содержание слайда: Function’s return value
Many times we want functions to not only perform some functionality, but also to return a result.
Using the return keyword, a function is able to return a value.
№43 слайд
Содержание слайда: Function’s return value
def always_return_5():
return 5
print('hi')
№44 слайд
Содержание слайда: Function’s return value
def always_return_5():
return 5
print('hi')
print(3 + always_return_5())
>>> 8
№45 слайд
Содержание слайда: Function calling a function
We can use the return value of one function as another function’s input.
def per_week(per_day=1):
return per_day * 7
def per_year(how_many_per_week):
return how_many_per_week * 52
print('Apples per year : ' + str(per_year(per_week())))
What happens here?
№46 слайд
Содержание слайда: Function calling a function
def per_week(per_day=1):
return per_day * 7 # return 7
def per_year(how_many_per_week):
return how_many_per_week * 52
print('Apples per year : ' + str(per_year(per_week())))
per_week()is called with no value and so gets the default value, 1, hence its return value is 7.
№47 слайд
Содержание слайда: Function calling a function
def per_week(per_day=1):
return per_day * 7 # return 7
def per_year(how_many_per_week):
return how_many_per_week * 52 # return 364
print('Apples per year : ' + str(per_year(7)))
per_year()is called with the value 7 and so returns the value 364
№48 слайд
Содержание слайда: Function calling a function
We can use the return value of one function as another function’s input.
def per_week(per_day =1):
return per_day * 7 # return 7
def per_year(how_many_per_week):
return how_many_per_week * 52 # return 364
print('Apples per year : ' + str(per_year(7)))
>>> Apples per year : 364
№49 слайд
Содержание слайда: Multiple outputs functions
To return more than one value, separate return values by comma
def diff_and_ratio(num1, num2):
return num1-num2, num1/num2
diff, ratio = diff_and_ratio(1, 5)
print(diff)
print(ratio)
№50 слайд
Содержание слайда: None
None is a special value which is used to represent absence of value.
Every function which does not return value explicitly, return None implicitly.
№51 слайд
Содержание слайда: None - example
def print_hi():
print('hi')
x = print_hi() # x is assigned the value None
print(x)
>>>hi
>>>None
№52 слайд
Содержание слайда: The Boolean type
Like int, str and float , Boolean is another Python type.
Boolean can get only one of two values :
True
False
type(True)
>>> <class 'bool'>
№53 слайд
Содержание слайда: Boolean expressions
Boolean expressions are expressions which use Boolean operators to evaluate a value of True or False.
For example > is a Boolean operator. Its Boolean evaluation is “Is the object on the right larger than the object on the left?”
5 > 7 is a Boolean expression because it uses a Boolean operator. Its value is False.
№54 слайд
Содержание слайда: Boolean operators
№55 слайд
Содержание слайда: Boolean expressions
7 == 4 ?
(7 != 2) == (5 > 4) ?
type(5 > 7) == type(8 < 3) ?
№56 слайд
Содержание слайда: Boolean expressions
7 == 4 False
(7 != 2) == (5 > 4) True
type(5 > 7) == type(8 < 3) True
№57 слайд
Содержание слайда: Complex Boolean operators
Take few Boolean operators and evaluate a new Boolean value from them.
and and or evaluate 2 Boolean expressions
not evaluates 1 Boolean expression
The return value of complex Boolean operators could be represented in a Truth table – a table that lists al the combination of truth value of input variables and their evaluated output
№58 слайд
Содержание слайда: Complex Boolean operators
Truth table
№59 слайд
Содержание слайда: Conditional operation
We do not always want to execute all the lines in our code. Sometimes we want to execute some lines only if a certain condition is maintained.
For example : Divide 9 by user’s input.
We get the number from the user.
Only if the number is different than 0, we can divide 9 by it.
№60 слайд
Содержание слайда: Conditional operation - if
How do we implement this notion in Python?
if boolean_expression:
#Code to perform if the #boolean_expression is True
#(Note the indentation under the if
#block).
№61 слайд
Содержание слайда: Conditional operation - if
For example :
num = float(input('Insert a number‘))
if num != 0 :
print(9/num)
But what if the number does equal 0? We still want to let the user know.
№62 слайд
Содержание слайда: Conditional operation - if
num = float(input('Insert a number'))
if num != 0 :
print(9/num)
if num == 0 :
print('Cannot divide by 0')
This is not a natural way to present our intention. What we would usually say is : if the number is different than 0 divide, else print some message to the user.
Python lets us use such structure using the else keyword.
№63 слайд
Содержание слайда: Conditional operation - else
num = float(input('Insert a number'))
if num != 0 :
print(9/num)
else:
print('Cannot divide by 0')
else should appear directly under an if block with the same indention.
№64 слайд
Содержание слайда: Conditional operation - elif
And what if we had some more options to choose from?
If condition1 then result1,
if not, than if condition2 then result2
…
if not, than if conditionN then resultN
If none of the above then result_Final
Use elif! (=else if)
№65 слайд
Содержание слайда: Conditional operation - elif
if now == 'Morning':
print('Good morning!')
elif now == 'Noon':
print('Good noon')
else:
print('It must be evening')
The first elif should appear directly under an if block with the same indention.
As many elif’s as you wish can follow.
elif can be terminated by a single else, or not at all.
№66 слайд
Содержание слайда: Nested if
What operations could be included inside an if block? Any operations we like :
print
input
… and – another if!
An if inside another if is called nested if – it opens a new block with its own indentation.
№67 слайд
Содержание слайда: Nested if - example
if now == 'morning':
if 'y' == input('Are you hungry?'):
print('Bon appetit!')
else:
print('Some other time than')
elif now == 'Noon':
print('Good noon')
else:
print('Good night')
№68 слайд
Содержание слайда: Nested if - example
if now == 'morning':
if 'y' == input('Are you hungry?'):
print('Bon appetit!')
else:
print('Some other time than')
elif now == 'Noon':
print('Good noon')
else:
print('Good night')
№69 слайд
Содержание слайда: split()
The method split() returns a list of all the words in the string, using a given string as the separator (default is whitespace)
# a = 'hello'; b = 'world'
>>> a,b = 'hello world'.split()
# a = 'hell'; b = 'w'; c = 'rld'
>>> a,b,c = 'hello world'.split('o')
№70 слайд
Содержание слайда: Example
Calculate the circumference (היקף) of a circle or square according to user request.
Let’s break the problem into parts :
1. Get user input
2. Validate if is it either a circle or a rectangle
If it is not print an error message and do not continue
3(a). If it is a circle
Ask for the radius, calculate circumference
3 (b). If it is a square
Ask for the side’s length, calculate circumference
4. Report to user the calculated result
№71 слайд
Содержание слайда: Example – break it up into functions
calculate_circle_ circumference()
calculate_rectangle_ circumference()
is_valid_shape_choice(choice)
get_user_input()
calculater_user_choice_circumference()
error_safe_circumference()
Then call the function to run the program:
error_safe_circumference()
№72 слайд
Содержание слайда:
№73 слайд
Содержание слайда:
№74 слайд
Содержание слайда: Summary
Today we have learned :
How to use variable
What are types and how to convert between them
How to receive an input from a user
How to use functions which get input and return output
Conditional operation : if, elif, else
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